AES/TG/09-29 Reservoir Geology and Modelling of Carboniferous Coal-bearing marginal Marine and Fluvial Deposits of Eastern Kentucky and Implications for Hydrocarbon Exploration and Development
October 2009
Patrick Were
Summary of the steps taken in Petrel to model & interpret facies from well logs
Title
: Reservoir Geology and Modelling of Carboniferous Coalbearing marginal Marine and Fluvial Deposits of Eastern Kentucky and Implications for Hydrocarbon Exploration and Development
Author(s)
: Patrick Were
Date Professor(s) Supervisor(s) TA Report number
: : : :
Postal Address
Telephone Telefax
: Section for Petroleum Geosciences Department of Applied Earth Sciences Delft University of Technology P.O. Box 5028 The Netherlands : (31) 15 2781328 (secretary) : (31) 15 2781189
Copyright ©2009
Section for Petroleum Geosciences
October 2009 Dr. Andrea Moscariello and Prof. Luthi Dr. Raik Bachmann and Dr. Michiel Dekker AES/TG/09-29
All rights reserved. No parts of this publication may be reproduced, Stored in a retrieval system, or transmitted, In any form or by any means, electronic, Mechanical, photocopying, recording, or otherwise, Without the prior written permission of the Section for Petroleum Geosciences
i
Table of contents
Abstract
(2)
1. General Introduction
(3)
1.1
Aims and objectives of the project
2. Regional Geology 2.1. Depositional settings and Facies 2.2. The Mississippian Paleogeography 2.3. The Pennsylvanian Paleogeography
3. Data and Methods
(4) (7) (12) (12) (13) (18)
3.1. Data collection and input into Petrel 3.1.1. Conversion of borehole data for Petrel 3.1.2. Utility of coal seams 3.1.3. Making stratigraphic surfaces 3.1.4. Thickness maps and Facies pie-charts
(18) (19) (22) (25) (26)
3.2. Facies modelling 3.2.1. Introduction 3.2.2. Procedure
(35) (35) (35)
4. Results
(37)
4.1. Overall delta stratigraphy & architecture in study area 4.1.1. Introduction 4.1.2. Analysis of cross-sections 4.1.3. Description & Interpretation of stratigraphy 4.1.3.1. The Coastal plain system 4.1.3.2. The Magoffin transgression 4.1.3.3. The Fluvial-Deltaic system
(37) (37) (37) (38) (39) (39) (40)
4.2. Detailed description & interpretation in Broad bottom 4.2.1. Stratigraphy & architecture in Broad bottom 4.2.1.1. The Lower coastal plain system 4.2.1.2. The Kendrick transgression
(44) (44) (45) (46)
ii 4.2.1.3. The Upper coastal plain system 4.2.1.4. The Magoffin transgression 4.2.1.5. The Fluvial-deltaic system
(46) (46) (46)
4.2.2. Description & interpretation of coastal-plain 4.2.2.1. Zone 0 4.2.2.2. Zone 1 4.2.2.3. Zone 2 4.2.2.4. Zone 3 4.2.2.5. Zone 4 4.2.2.6. Zone 5 4.2.2.7. Zone 6 4.2.2.8. Zone 7 4.2.2.9. Zone 8 4.2.2.10. Zone 9 4.2.2.11. Zone 10 4.2.2.12. Zone 11 4.2.2.13. Zone 12 4.2.2.14. Zone 13
(53) (54) (56) (58) (60) (62) (64) (66) (68) (70) (72) (74) (76) (78) (80)
4.2.3. Description & interpretation of fluvial-delta 4.2.3.1. Zone 14 4.2.3.2. Zone 15 4.2.3.3. Zone 16 4.2.3.4. Zone 17 4.2.3.5. Zone 18 4.2.3.6. Zone 19 4.2.3.7. Zone 20 4.2.3.8. Zone 21
(82) (82) (85) (87) (89) (91) (93) (95) (97)
5. Discussion 5.1. Evolution of the delta in Broad bottom and Its implications for Reservoir and Petroleum Geology 5.2. Reservoir characteristics 5.2.1. Flow barriers 5.2.2. Reservoir communication
6. Conclusions 7. Recommendations 8. References 9. Other relevant literatures
(99)
(99) (104) (104) (105) (106) (109) (111) (116)
iii
10. Appendices 10.1. Stratigraphic surfaces 10.2. Example of raw data from KGS
(124) (125) (148)
iv List of Figures Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig.
1.1. Location map of the study area 2.1. Appalachian basin, structures and cross-section 2.2. Tectonic loading and unloading 2.3. Elements of Foreland deformation 2.4. Paleogeography of basin in the Late Mississippian 2.5. Paleogeography of basin in Middle Pennsylvanian 2.6. Stratigraphy of Breathitt Group in Perry and Leslie 2.7. Stratigraphy of basin in Late Carboniferous period 3.1. Counties and quadrangles in study area 3.2. Location of wells in study area 3.3. Generation of sedimentological logs from well data 3.4. Sedimentological logs flattened on horizon 3.5. Generation of stratigraphic surfaces in Petrel 3.6. A pack of stratigraphic surfaces in Broad bottom 1 3.7. A pack of stratigraphic surfaces in Broad bottom 2 3.8. Sedimentological logs in Broad bottom 3.9. Cross-section A-B, in the NE-SW direction 3.10. Cross-section C-D, in the NW-SE direction 4.1. Stratigraphic framework of Pennsylvanian 4.2. Strike-section E-F in study area 4.3. Dip-section G-H in study area 4.4. Colour of legend to diagrams for correlated facies 4.5. Dip-section for correlated facies in the NW-SE 4.6. Strike-section for correlated facies in the NE-SW 4.7. Thickness map of the Breathitt Group in BRDBTTM 4.8. Colour of legend for the facies pie-charts 4.9. Composite map of isochore and pie-charts in zone 0 4.10. Composite map of isochore and pie-charts in zone 1 4.11. Composite map of isochore and pie-charts in zone 2 4.12. Composite map of isochore and pie-charts in zone 3 4.13. Composite map of isochore and pie-charts in zone 4 4.14. Composite map of isochore and pie-charts in zone 5 4.15. Composite map of isochore and pie-charts in zone 6 4.16. Composite map of isochore and pie-charts in zone 7 4.17. Composite map of isochore and pie-charts in zone 8 4.18. Composite map of isochore and pie-charts in zone 9 4.19. Composite map of isochore and pie-charts in zone10 4.20. Composite map of isochore and pie-charts in zone11 4.21. Composite map of isochore and pie-charts in zone12 4.22. Composite map of isochore and pie-charts in zone13 4.23. Composite map of isochore and pie-charts in zone14 4.24. Composite map of isochore and pie-charts in zone15
(6) (9) (10) (11) (14) (15) (16) (17) (19) (21) (24) (25) (26) (28) (29) (30) (31) (33) (41) (42) (43) (47) (48) (50) (52) (53) (54) (56) (58) (60) (62) (64) (66) (68) (70) (72) (74) (76) (78) (80) (84) (86)
v Fig. Fig. Fig. Fig. Fig. Fig. Fig.
4.25. Composite map of isochore and pie-charts 4.26. Composite map of isochore and pie-charts 4.27. Composite map of isochore and pie-charts 4.28. Composite map of isochore and pie-charts 4.29. Composite map of isochore and pie-charts 4.30. Composite map of isochore and pie-charts 5.1. Vertical log along well BRDBTTM004
in in in in in in
zone16 (88) zone17 (90) zone18 (92) zone19 (94) zone20 (96) zone21 (98) (100)
vi List of Tables Table 3.1. Well codes and quadrangles in study area Table 3.2. Names of Coal seams in study area
(18) (23)
vii
Acknowledgement This project was a grant from SGS to fund an internship position at Horizon Energy Partners in The Hague, I am indeed grateful for the technical and financial support that was accorded me for its implementation. I would like to thank my external supervisors, Dr. Huw Williams and Dr. Paul Davies, Reservoir Geology consultants UK, for their guidance in data acquisition and software instructions. The constructive comments, criticism and support from my referees, especially those of Dr. Raik Bachmann and Dr. Michiel Dekker, is greatly appreciated. Their time and patience with me is very much appreciated. Dr. Andrea Moscariello, the chief project co-odinator, is greatly thanked for the energy and time he sacrificed to make the project a success. I would like to thank all my lecturers and colleagues from the University Faculty of Civil Engineering and Geosciences, TU Delft for their support and co-operation.
1
Reservoir Geology and Modelling of Carboniferous coal-bearing marginal Marine and Fluvial Deposits of Eastern Kentucky and implications for Hydrocarbon Exploration and Development
By
Patrick Were
2
Abstract A considerable amount of geological information required for subsurface reservoir characterization of new and old hydrocarbon fields can be attained by analyzing critically the stratigraphic framework and facies architecture of analog outcrops and mature fields with a dense network of wells. This project makes use of the dense network of cored boreholes and outcrops to construct, using Petrel, a detailed conceptual model that illustrates facies architecture and stratigraphic framework of the Breathitt Group in the Central Appalachian Basin of Eastern Kentucky, with the aim of investigating the pattern of depositional environments and how they influenced the vertical and lateral distribution of facies in this part of the foreland basin. The model provides excellent guidelines as to the distribution of depositional energies in analogous subsurface coastal-plain and fluvial-deltaic reservoir sequences. The study is primarily based on cored borehole data from 12 quadrangles in Eastern Kentucky which was loaded in Petrel to generate vertical sedimentological logs of the subsurface. Based on extensive coal seams and marine flooding surfaces the logs were correlated to obtain the strike and dip stratigraphic sections of the foreland basin. Stratigraphic surfaces and composite maps of isochors and facies pie-charts were prepared for use in predicting the lateral and vertical distribution of facies, depositional energies and paleoenvironments in each zone. Basic principles of sequence stratigraphy were also applied to explain the evolution of the delta system in the basin. Correlations revealed two broad depositional systems caused by differential subsidence during alternating periods of active tectonics and quiescence. The upper system is predominantly composed of immature sediments derived from the thrust-fronts in the southeast and transported toward the northwest by high energy braided and meandering streams, whilst the lower system is composed of mature sediments whose deposition was mainly influenced by waves/storms and tides from the sea in the northwest that frequently transgressed the subsiding basin in periods of tectonic quiescence. Further evidence for tectonic influence on the distribution of facies is revealed by the presence of a series of small anticlinal and synclinal structures which may affect the dynamics of fluid flow in the basin. Facies analysis shows that the fluvial system in Broad bottom offers better reservoirs with good vertical and lateral connectivity than those in the coastal plain system, which are only connected in the lateral direction. Nevertheless, the coastal plain system could provide a good source region for the generation of hydrocarbons, because it has a high content of organic matter and its great depth of burial in the basin, could offer the kitchen (enough heat energy) for the generation of hydrocarbons. Isochores and pie-charts provide a quick method of reserve estimates in both mature and new hydrocarbon fields. The method yields important petrophyical parameters which can assist reservoir engineers to plan accurate flow simulation models required for well spacing, well numbers, well positioning, and enhanced oil recovery (EOR).
3
1.0. GENERAL INTRODUCTION Fluvial channel sandstones and particularly fluvial-deltaic deposits are important targets for petroleum exploration.The clastic coal-bearing marginal-marine and fluvial-deltaic deposits in the Central Appalachian Foreland Basin in the Eastern Kentucky Coal Field, USA, have been studied in an area of twelve 7.5-minute quadrangles to reconstruct a 3D geometry and architecture of a conceptual facies model based on a sequence stratigraphical framework of the Late Carboniferous formations belonging to the Breathitt Group. Significant effort has been focused upon understanding their sequence-stratigraphic framework or architecture and internal sandstone body geometries in order to facilitate the construction of appropriate conceptual facies/depositional models. Depositional facies are a significant control on the distribution of petrophysical properties in clastic reservoirs, largely influencing the reservoir’s capacity to store and produce hydrocarbons. As facies are one of the key controls on the distribution of petrophysical properties, facies models can be integrated into the reservoir modeling workflow by using them as a template for capturing the distribution of the petrophysical properties needed in fluid flow simulation. Sedimentological heterogeneities that affect hydrocarbon production occur at a variety of scales including sub-seismic and less than the typical development well spacing. Lack of data at these relatively detailed scales and the need for uncertainty analysis has led to the application of stochastic methods for capturing facies heterogeneity in reservoir models. However, to be useful and reliable, the use of stochastic facies modeling algorithms needs to be driven by appropriate conceptual depositional models, largely derived from studies of outcrop and subsurface analogues. Analogous systems are also used to provide certain key parameters such as sandstone body dimensions or shale bed length and to improve our understanding of modeling approaches and reservoir forecasting. The Late Carboniferous outcrops of clastic coal-bearing, marginal marine and fluvial deposits of eastern Kentucky in the United States of America constitute world famous stratigraphic successions used by many companies as a direct analogue to understand and solve stratigraphic problems in the subsurface coal mines and coal-bearing hydrocarbon reservoirs. This project applied a multistage modeling approach using a variety of different algorithms to address facies modeling at different stages. In order to obtain the required conceptual facies model the following working procedure was planned: (1) converting raw borehole data into a compatible form, appropriate for input into Petrel, (2) conversion of borehole input data into sedimentological logs, (3) correlation of equivalent stratigraphic marker horizons between well logs based mainly on regionally extensive coal seams, (4) conversion of correlated horizons into 3D stratigraphic surfaces and isochores for each zone, (5) preparation of composite thickness maps and facies pie-charts in each zone, and (6) the final step involved the manual preparation of the conceptual facies models in each zone based on well logs and facies pie-charts along cross-sections vertically cut through the thickness maps, in chosen directions.
4 The results of this working strategy were stratigraphic conceptual facies models chronologically arranged to enable the prediction of vertical and lateral distribution of facies and depositional environments in the study area. It was observed that the Breathitt Group is mainly composed of the following alternating facies associations: sandstones (Ss), heterolithics (Ht), conglomerates (Cnglmrt), shales (sh), coals (cl), limestones (ls), and some unknown facies (Unknwn). A coastal setting, including environments such as a shallow sea, a series of small deltas, tidal flats and estuaries, a coastal plain, fluvial channels, and alluvial plains, is envisaged for the deposition of the coal-bearing strata of the central Appalachian basin in eastern Kentucky. 1.1. Aims and objectives of the project In order to carry out earth resources exploration and estimation, it is essential that appropriate outcrop analogues are carefully selected in order to accurately supplement the sparse subsurface data with outcrop-derived measurements. Equally important is the role of sequence stratigraphy to provide certain key parameters such as sandstone body dimensions or shale bed length which can tremendously help to improve the understanding of modeling approaches and reservoir forecasting particularly for siliciclastic fluvial-deltaic deposits so as to optimize hydrocarbon recovery from the subsurface. The criteria suggested for appropriate analogue selection may include tectonic setting, geological age and subsidence rates. The Late Carboniferous outcrops of clastic coal-bearing, marginal marine and fluvial-deltaic deposits in the Central Appalachian Basin of eastern Kentucky provides an appropriate analogue equipped with world famous stratigraphic successions used by many companies to understand and solve stratigraphic problems in subsurface coal mines and coal-bearing hydrocarbon reservoirs. Large man-made road cuts including extensive subsurface and coal mine data, quarry excavations and a large number of cored borehole data make this region a highly interesting area to characterize and quantify sand body geometries, coal and shale extents and their overall 3D spatial distribution (architecture). To date no full integration of this data has been accomplished. It is therefore the aim of this project to collect and integrate a large diversity of stratigraphical and sedimentological data so as to reconstruct a 3D geometry and architecture based on a sequence stratigraphical framework of part of the Breathitt Group located along the US Highway 80 between the Towns of Hazard and Prestonsburg in eastern Kentucky (Figure 1.1, location map of study area). This study updates the geological model of the Breathitt Group in eastern Kentucky (including the area covered by the quadrangles of Martin, Harold, Broad bottom, Wayland, McDowell, Pikeville, Kite, Wheelwright, Dorton, Mayking, Jenkins West, and Jenkins East; Figure 3.1 and Table 3.1), and aims to gain a more detailed understanding of the facies distribution, stratigraphy and erosional events in this part of the Appalachian foreland basin. More specifically the aims are to: (1) Review, undertake, and update core descriptions, facies depositional model, sand-body architecture and sequence stratigraphy of the Breathitt Group in eastern Kentucky. (2) Investigate the thickness distribution of the stratigraphic
5 zones in the Breathitt Group. (3) Construct stratigraphic cross-sections using borehole and outcrop data in the study area which illustrate the distribution of facies and depositional environments in this part of the Appalachian foreland basin. The genetic processes, which led to the deposition of peat or coal formation and the highly variable carboniferous sediments in this part of the foreland basin, are not yet well understood. Applying the concepts of sequence stratigraphy to the facies model, however, gives considerable insights. With the help of the current geo-modelling computer software techniques (Petrel) cored borehole data was used to develop deterministic accurate conceptual facies models for the marginal marine and fluvial sequences and ultimately build a sequence stratigraphic-based deterministic geological model of the study area. The detailed facies models extend across the preserved portion of the foreland basin in the quadrangle of Broad-bottom (9.7 km long, 7.5 km wide and up to 330 m thick), which is comparable in scale to the reservoir systems typically resolved from 3D seismic data in the subsurface.
6
Fig. 1.1. Location map of the three major Paleozoic basins (Illinois, Michigan and the Appalachian) in the eastern interior of the USA. The basins are separated by a system of structural arches and domes including the Cincinnati arch. The study area (inset) is located in the Appalachian basin in the eastern part of Kentucky, USA.
7
2.0. REGIONAL GEOLOGY The clastic coal-bearing marginal marine and fluvial deposits of Eastern Kentucky are part of the Central Appalachian Basin, a typical Foreland Basin that has subsided episodically under the loads of successive thrust-sheets (Reed et al., 2005 and Tankard, 1986). It is separated from the Illinois Basin (an intercratonic basin) in the west and the Michigan Basin in the northwest by a system of arches and domes (Figures 1.1 and 2.1A). The eastern Kentucky coal field is bounded in the north by a system of basement faults that belongs to the Rome Trough, a Precambrian aulocogen, and to the west by the Cincinnati arch (Figure 2.1B). Unlike the Midcontinent cyclothems of Kansas and Michigan that were controlled by glacio-eustacy and the Illinois Basin cyclothems that were intermediate between tectonics and eustatic processes, the Appalachian cyclothems were predominantly controlled by flexural tectonics (Greb et al., 2004 and Heckel et al., 1998). The three basins were repeatedly decoupled and yoked together from CambroOrdovician until the Late Carboniferous times, due to episodic upwarping and downwarping of the arches or forebulge system (Tankard, 1986; Buter et al., 1991). The Appalachian Basin and its stratal deposition patterns is mainly a result of three major successive collisional tectonic phases or allocycles, namely: the Taconic, the Acadian and the Alleghenian thrust phases (Greb et al., 2002; Tankard, 1986). The three episodes of tectonism together with preconvergence deposition gave rise to the current basin geometry including a stratigraphy that consists of four major unconformity-bounded sequences in the Central Appalachian Basin. The Carboniferous structures and sedimentation in the Appalachian and Illinois Basins were intricately linked (Greb et al., 2002). Episodic thrust sheet loading on the eastern margin of the North American craton was inferred to have caused lithospheric flexure beneath the loads, with the consequent downwarp and subsidence of the lithosphere to form a Foreland basin, the Central Appalachian Basin, close to the orogene and a forebulge (the Cincinnati Arch) along the cratonward edge of the basin. The static tectonic load of the orogen and the dynamic loading due the viscous drag force of mantle corner flow are the primary subsidence mechanisms that control accommodation and sedimentation patterns in the foreland basin settings (DeCelles and Giles, 1996; Catuneanu, 2004). Tectonic loading alone provides the defining features of foreland systems, i.e. their partitioning into the flexural provinces: foredeep (which is the foreland basin), forebulge (which is the peripheral bulge) and the back-bulge (Figure 2.3). Along the flexural profile the uplift of the forebulge was virtually synchronous with the subsidence of the foredeep. This is caused by the rapid lateral displacement of the viscous mantle material as a result of lithospheric downwarp beneath the orogen and the adjacent foredeep (Catuneanu, 2004). Several renewals of this process forced the forebulge to migrate westward into the Illinois Basin (Dorsch et al., 1994; Tankard, 1986). Basement structures were reactivated by the increased load in the foreland, and those structures distal to the foreland were reactivated by the forebulge migration
8 (Eble and Grandy, 1990). Cyclothemic sedimentation within the transgressiveregressive units of the Appalachian foreland basin could possibly have resulted from laterally changing flexural deformation (Greb et al., 2002; VerStraeten and Brett, 2000). The Appalachian basin contains a Carboniferous stratigraphical succession whose depositional systems are attributed to deposition in a Foreland Basin that fluctuated between underfilled and overfilled conditions with facies that emphasize the sedimentary response to basin tectonic subsidence and peripheral upwarping (Einsele, 1992 and Tankard, 1986). The Appalachian orogenic belts were obducted across an earlier extensional passive-margin whose configuration and miogeoclinal wedge influenced the patterns of compressional tectonism and the structural levels (Howell and Pluijm, 1990). The thickest and most extensive part of the foreland basin, as observed from seismic studies in the southern Appalachians, occurs where overthrusting of the continental margin is greatest (Tankard, 1986). The passive-margin history was terminated by the Taconic orogeny during the Middle Ordovician. Taconic orogene was characterized by magmatic arc convergence and accretion of exotic terranes (Tankard, 1986). The early foredeep was about 700-2000 m deep and shale dominated. Transition from passive to convergent tectonics is marked by the Knox unconformity which was incised across a migrating forebulge. Tectonic loading of the progressively thicker crust resulted into shallowing of the foreland basin, emergence of the overthrust terranes and influx of coarser siliciclastics (Tankard, 1986). The Taconic orogeny persisted until the Early Silurian, when thick sequences of nonmarine sediments were deposited into the basin (Chesnut, 1980 and Tankard, 1986). Collision between the eastern North American portion of Laurentia and a landmass or series of terranes beginning in the late Silurian or Early Devonian resulted in the formation of the Acadian orogenic belt and subsidence of the adjacent Appalachian retroarc foreland basin (Haworth et al., 1988; VerStraeten and Brett, 2000; and Filer, 2003). Based on the stratal record in the foreland basin a model for the Acadian orogeny was proposed that recognized three to four tectonically active to quiescent tectophases between the Early Devonian and the Early Mississippian. It was intense during the Middle to Late Devonian and best developed in the northern Appalachians. The Acadian orogeny was characterized by voluminous granitic plutonism although less convergent than the earlier Taconic orogeny (Tankard, 1986). During this orogenesis the foreland basin was dominated by basinwide deposition of organic-rich shales or mudstone, especially in the distal part of the basin and prominent unconformities were incised along the upwarped margin of the basin. The Acadian orogenic cycle waned through the Mississippian until it was superseded by the Pennsylvanian orogenesis, also called the Alleghenian orogeny. The foreland thrust-belt of the Central Appalachian basin evolved mainly during the Pennsylvanian-Permian Alleghenian orogeny. Terrigenous sandstones of the Breathitt Formation in eastern Kentucky reflect derivation from this orogenic source.
9
Fig. 2.1. The Appalachian foreland basin resulted from Paleozoic thrusting and flexure (A) System of arches and domes which separates the Appalachian basin from Illinois and Michigan basin. (B) Major structural elements of the Appalachian basin. (C) NW-SE cross section of the basin between X and X* and the Rectangle in B is the study area (Redrawn from Tankard, 1986)
10
Fig. 2.2. Flexural response to orogenic loading and unloading. Repeated thrusting (loading) results in foredeep subsidence and forebulge uplift. The reverse occurs during stages of orogenic quiescence (erosional or extensional unloading) when the foredeep undergoes uplift as a result of isostatic rebound, compensated by the subsidence of the forebulge (Redrawn from Catuneanu, 2004).
The crinoid and ammonoid taxa from the Kendrick Shale are indicative of a Morrowan or early Middle Pennsylvanian age. Progradation of the Pennsylvanian molasse wedge is thus correlated with the early stages of the Alleghenian orogenesis (Chesnut, 1996). The Pine Mountain thrust plate, bounded to the northeast and southwest by tear faults, has formed a ramp upward across the incompetent Devonian-Mississippian shales and overthrust the Pennsylvanian section (Figure 2.1B). Alleghenian tectonism resulted in thrust-sheet loading of thick, unstretched lithosphere towards the centre of the hinge line. Due to the great flexural rigidity of the lithosphere the foreland basin became shallow and generally filled its depositional base-level by relatively coarse terrigenous clastics (Kusznir et al., 1985 and Tankard, 1986). The responses of the lithosphere to thrust-belt loading were modelled with three lithospheric types in an attempt to account for the rock record (Figure 2.3): elastic, uniform viscoelastic and temperature-dependent viscoelastic. It was suggested that the temperature-dependent viscosity model most satisfactorily accounts for the stratigraphy (Tankard, 1986; Filer, 2003). The initial response of the lithosphere to loading is elastic, and results in a downwarped flexural basin adjacent to the orogene and a forebulge along cratonward edge of the basin (Figure 2.2 and stage one in Figure 2.3). However, if the thrust load remained unchanged for long periods, relaxation of the plate-bending stress would result in deepening of the basin, as well as uplift of the forebulge and its contraction toward the load (stage two in Figure 2.3). Each new thrust-sheet advance repeats the entire process. The net result of long histories of thrusting would be the migration of the forebulge away from the load, its distance reflecting the effective elastic thickness. Thus thrust-sheet loading on a thick (strong) lithosphere would produce a wider and shallower basin than on a thin (weak) lithosphere (Catuneanu, 2004; Tankard, 1986).
11
Fig. 2.3. Elements of foreland deformation in which basin subsidence and peripheral upwarping are a response to thrust-belt loading. (1) Lithosphere responds elastically to initial loading. (2) Overthrust load remains in place for a long period of time and the lithosphere adjusts viscoelastically. The basin deepens while the forebulge undergoes accentuated upwarping and contracts toward load. (3) At renewed loading lithosphere again responds elastically forcing the forebulge to migrate ahead of the advancing load (Redrawn from Tankard, 1986).
12 The vertical movement of basin-margin arches may be relatively small, but episodic uplift may induce erosion and reworking within the surf zone (arch peaks or crests, including the immediate surrounding steep areas on the flanks). In eastern Kentucky the western margin of the Appalachian Basin comprises the broad Cincinnati arch including a smaller Waverly arch (Figure 2.1B). Foreland deformation also reactivated the old listric normal faults in a strike-slip sense as exhibited by the Kentucky River Fault System, which offsets the Waverly arch. 2.1. Depositional settings and facies Distribution of the Carboniferous depositional systems in eastern Kentucky was largely controlled by the foreland basin dynamics (Tankard, 1986). The Middle to Upper Pennsylvanian Breathitt Group (~ 950 m thick) comprises delta plain facies of siltstone, claystone, sandstone, bituminous coal and rare ironstone and limestone, deposited in a foreland basin setting (Aitken and Flint (1996). The Breathitt depositional systems reflect periods of deposition in broad embayments when the basin was underfilled, and alluvial plain deposition at times when the basin was overfilled. The foreland basin dynamics is reflected by the regional persistence of these depositional systems (Tankard, 1986). The first attempt to study the lateral variation of the depositional environments in the coal-bearing rocks in the Appalachian Basin was based on Weller’s cyclothem model for characterizing vertical depositional sequences of rock types from the Western Interior Basin, through Illinois, to the Appalachian Plateau. A steady reduction of marine strata in an eastward or landward direction and an increase in the number of coal beds possibly arising by the splitting of a major coal bed was observed (Ferm and Weisenfluh, 1989; Greb and Weisenfluh, 1996; Greb and Popp, 1999). These coals tend to occur in zones and are prone to lateral splitting because of foreland tectonic and sedimentation influences (Eble et al., 1999; Greb et al., 2002; Hower et al., 1989 and Hower et al., 1994). 2.2. The Mississippian paleogeography During the Mississippian (late Acadian) the Appalachian and Illinois Basins were widely yoked together and most stratigraphic units were regionally persistent. There was a widespread deposition of shales, implying a decaying orogene and an underfilled foreland basin (Tankard, 1986). Episodic uplift along the arch resulted in shoaling and wave-current reworking of the stratigraphic units and deposition of siliciclastics (such as the Berea and Carter Caves Sandstones deposited as elongate sand bars or barrier islands) and bioclastic Slade limestone containing numerous unconformities in the basin. As a result of progressive uplift and erosion the western flank of the arch the Slade limestone was punctuated by merging wedges of unconformities which migrated eastward, overstepping each other toward the arch axis (Tankard, 1986). Uplift and erosion along the arch and basement zone is also the main reason for the varied distribution and thickness of the Slade intervals (Tankard, 1986; Greb et al., 2002). This was a time of inactive orogene when the basin was relatively starved
13 of terrigenous sediment although the limestones do become argillaceous basinward (Tankard, 1986). As a result of rejuvenation of the orogenic source terrane the shallow marine limestone was eventually covered by a westerly prograding sandy mud sheet (the Paragon Formation). The arch was at this time relatively deflated and the two basins (Appalachian and Illinois) yoked together allowing lateral persistence and minor thickness variation of the paragon shales, mudstones and sandstones. Deposition of the Carter Caves Sandstones is attributed to tidal channel processes based on the observation of its mature composition and channeling, reactivation and emergence-runoff structures. The paleogeography of this sandstone is thus regarded as an area of shoal-water reworking and dissection by shallow tidal channels, with emergent barrier sand bodies forming in places (Tankard, 1986). The positive relief of the arch formed the major shoreline and effectively dampened the wave and tidal energy. Marine reworking was very rare within the subsiding foreland basin behind this arch. The Mississippian was formed late in the Acadian cycle at the time when the orogenic quiescence and terrigenous sediment starvation coincided with the uplift of the basin-margin arch system and its migration toward the orogene. This tectono-stratigraphic history supports a viscoelastic model of the lithosphere (Figure 2.3). 2.3. The Pennsylvanian paleogeography A regional unconformity occurs between Mississippian and Pennsylvanian strata along the basin-margin arch system and marks the termination of the Acadian deformation (Martino and Sanderson, 1993; Tankard, 1986). During the earliest Pennsylvanian time widespread erosion took place in association with cratonic emergence in eastern Kentucky and vicinity causing topography of paleovalleys with notably large relief. This eroded surface was onlapped by successions of the Lee and Breathitt Formations during the Early and Middle Pennsylvanian. These formations are mostly comprised of sandstone, mudstone, and coal lithologies. A southwest transport direction for riverine systems prevailed in eastern Kentucky occasionally being transgressed with an interior sea along the basin axis from the southwest during Early Pennsylvanian (Figure 2.4). Quartz-rich sandstone bodies of the Lee Formation were deposited by braided rivers flowing in the northeastsouthwest direction (Martino and Sanderson, 1993; Tankard, 1986). Various facies of the Breathitt Formation have been interpreted as deposits of lower delta plain, strand plain, back-barrier lagoon, estuarine, fluvial channel, and swampy environments (Martino and Sanderson, 1993; Tankard, 1986). During the Middle Pennsylvanian, clastic wedges from the Appalachian Orogen prograded northwest across the Pocahontas Basin (Figure 2.5). Generally, the Breathitt depositional systems reflect two main periods of sedimentation: (1) broad embayments when the basin was underfilled and (2) alluvial plain deposition at times when the basin was overfilled (Tankard, 1986).
14 Accordingly, Tankard divided the Breathitt Group into three main depositional systems (Figure 2.6 and 2.7) based on the distribution of facies, coal seams and the regional persistence of the Kendrick and Magoffin marine zones in the foreland basin: • The Lower Breathitt coastal plain system • The Magoffin transgression and • The Upper Breathitt fluvial-deltaic system. The Magoffin transgression is a marine unit of Middle Pennsylvanian age, with a basal limestone deposition that indicates a rapid transgressive flooding period of coal swamps and a regressive top indicating the return of rapid clastic influx into the basin by fluvial streams (Bennington, 1996; Greb and Chenut, Jr., 1992).
N
Fig. 2.4. Paleogeographic map of the Appalachian basin during the Late Mississippian to Early Pennsylvanian time, when a southwest transport direction for the riverine system prevailed in eastern Kentucky, occasionally being transgressed with an interior sea along the basin axis from the southwest (redrawn from Martino and Sanderson, 1993).
15
N
Fig. 2.5. Paleogeographic map of the Appalachian Foreland Basin during the Middle Pennsylvanian time, when clastic wedges from the Appalachian Orogen prograded NW across the basin (redrawn from Martino and Sanderson, 1993).
16
STONEY FORK TRANSGRESSION Hz9
FLUVIAL-DELTAIC SYSTEM 200 m
Hz8 Fluvial channels, subordinate bay-fill and splay deposits
Hz7
150 m
HZD
HDX M AGOFFIN TRANSGRESSION
Bayhead deltas
100 m
HM L
COASTAL PLAIN SYSTEM
FCR
Bay-fill sedimentation with coals Subordinate fluvial and tidal deposits
FCL WHI 50 m
KENDRICK TRANSGRESSION
COASTAL PLAIN SYSTEM Bayhead deltas
0m
AM B
Fig. 2.6. Composite stratigraphic column for the Breathitt Formation in Perry and Leslie counties (study by Tankard, 1986). Bay-fill deposition dominated coastal plain system. In contrast, succeeding fluvial-deltaic system is dominated fluvial sandstones. Arrows point in direction of decreasing grain size. Redrawn from Tankard, 1986.
17
Fig. 2.7. Upper Mississippian and Middle Pennsylvanian stratigraphy of the Appalachian basin in eastern Kentucky. Redrawn from Tankard, 1986.
18
3.0. DATA AND METHODS 3.1. Data collection and input into Petrel Cored borehole data from 12 quadrangles in the counties of Knott, Letcher, Pike and Floyd in Eastern Kentucky (Figure 3.1 and Table 3.1) were obtained from the Kentucky Geological Survey (KGS) website for Databases and Publications (www.uky.edu/KGS). The quadrangles are arranged in such a way that data from N55 (the quadrangle Martin) is furthest way from the thrust front in the northwest and Q57 (quadrangle Jenkins East) is directly in front of the basal thrust. Table 3.1 lists the names of the quadrangles and codes used for naming the boreholes in the study area. Table 3.1. KGS numbers and codes for the quadrangles in the study area. Abbreviations are used for borehole codes.
In some quadrangles with a massive number of wells the search was narrowed to well records longer than 300 ft or to those wells that cover the stratigraphic interval of interest, i.e. the Four Corners Formation.
19 A multi-stage modeling approach with Petrel was engaged using a variety of different algorithms to address modeling at different stages. The stages involved included: (1) converting the raw borehole data into a compatible form, appropriate for input into Petrel, (2) conversion of the input data into sedimentological logs, (3) correlation of equivalent surfaces between well logs based on coal seams, (4) preparation of stratigraphic surfaces and isochores in 3D between different zones, (5) preparation of composite thickness maps and facies pie-charts for each zone, and (6) manual preparation of conceptual facies models in each zone based on well logs and facies pie-charts along crosssections cut through the thickness maps, in chosen directions. 3.1.1. Conversion of raw borehole data for input into Petrel For every individual well three types of information (raw data in digital format) were extracted and stored in different Excel files before importation into Petrel. This information includes: • The borehole header • The lithological data and codes • The coal seam data. The data was treated and adapted for input into Petrel to create sedimentological logs for subsequent geological analysis and interpretations.
g
s in nk J e st e w
Study area
n rt o o
in nk Je st a e
d
D
le v il ke
i el
in
Pi
s
tu c
ky
k ay
d oa m B r ott o b
Co al F
l e he t W ri gh w
e K it
M
ld ro
cD ow el l
nd la ay W
Ha
M
rtin Ma
P L
Ea s te
rn
Ke n
KENTUCKY
F K
Fig. 3.1. Location map of the counties and quadrangles in the study area. In the rectangle F, K, L and P stands for the county names Floyd, Knott, Letcher and Pike, respectively.
The borehole header report from the KGS provided the input data for creating well header files in Petrel. The well head for each borehole is composed of a name, the X and Y positions, surface elevation for which the Kelly bushing (KB) position was used and total depth (TD). To convert the borehole log information
20 into sedimentological logs four basic logs were created in Petrel and combined to yield the required logs. These preliminary logs include: • Ferm Facies (the litho-log obtained from KGS database) • FaciesCurves (derived from the Ferm Facies logs) • Facies (derived from the Ferm Facies logs) • Coals logs (these are comment logs obtained from the KGS seam reports) Figure 3.3 is an example of a sedimentological log correlation in Petrel using data input from four wells in the quadrangle Mayking: MYKNG076, MYKNG061, MYKNG001 and MYKNG065. Altogether 286 wells were imported in Petrel for the total study area covering about 1900 square kilometers of which about 73 square kilometers in the northwester edge were selected for detailed study. Figure 3.2 is a map of the total study area created in Petrel to display all boreholes and locations of the cross-sections made to determine the lateral and vertical distribution of depositional facies as a result of the paleoflow of fluvial and marine systems during the Middle and Late Carboniferous. The cross-sections EF and G-H trend in the NNE-SSW and NNW-SSE directions, respectively, and represent the general distribution of facies in this part of the foreland basin. Cross-sections A-B and C-D (inset Figure 3.2) trend in the NE-SW and NW-SE directions, respectively and represent in detail the distribution of facies in the quadrangle Broad-bottom during the Pennsylvanian times. Note the crosssections are zigzag lines that do not consider only those wells falling on the straight lines as depicted in Figure 3.2, but also consider wells within a range of about 500 metres from the line. The well logs along each cross-section were correlated using the extensive coal seams and shale layers as maker horizons, to obtain a chronological stratigraphy in the study area (see Figure 3.3). Most horizons can be correlated throughout the entire study area. However, a few are observed to pinch out midfield at some locations. Altogether, 26 horizons were observed in cross-sections E-F and G-H, and only 22 in the cross-sections A-B and C-D in the quadrangle Broad-bottom.
21 F D
B
H
C
A MRTN
HRLD
WYLND
McDWLL
KT
MYKNG
E
BRDBTTM
PKVLL
DRTN
WHLWRGHT
JNKNSWST
JNKNSEST
G
Fig. 3.2.The study area composed of 12 quadrangles, represented by rectangular blocks. Locations of the studied cored wells are indicated by the small circles. Inset in the quadrangle Broad-bottom, the area selected for detailed study. The lines A-B, C-D, E-F and G-H are locations for geological cross-sections explained further in the text.
22 3.1.2. Utility of coal seams During the periods of Langsettian, Lower Pennsylvanian, to Duckmantian and Middle Pennsylvanian, paleoclimates in Eastern Kentucky were favorable for peat doming, allowing numerous low-sulfur coals to accumulate in zones and prone to lateral splitting because of foreland tectonic and sedimentation influences, Greb et al. (2002). The thick regionally extensive coal seams have significance in relation to baselevel changes and can be used as genetic stratigraphic sequence boundaries in nonmarine and marginal basins (Aitken, 1995). They have, therefore, been a useful guide in correlations within this project. Data for the coal seams has been used to provide input for well tops in Petrel for correlating the stratigraphic surfaces between well logs. For wells without coal seams fictious well tops were interpreted (based on the similarities of log shapes for the lithofacies in the neighboring sedimentological logs) to facilitate correlations between well logs. However, Aitken (1995) urged that the use of coal seams as genetic stratigraphic sequence boundaries is contentious for the following reasons: (1) they are not single surfaces, (2) they are not necessarily maximum flooding surfaces, and (3) systematic variations in accommodation space are not properly accounted for. Hence, coal seams, although readily identifiable and easily correlatable, do not fulfill the criteria for strict, genetic stratigraphic sequence boundary definition, but rather represent a genetic sequence boundary zone (Aitken, 1995). To obtain a clear view of the stratigraphic trends along a given cross-section a datum horizon was chosen along which other horizons were flattened (Figures 3.3 and 3.4, illustrate the correlated well logs before and after flattening on a horizon, respectively). The criterion for choosing such a surface is that it should be relatively thick and regionally extensive. Hence, a coal seam that is considered to have been processed during the transgressive period of maximum flooding. Flattening on a horizon makes it practically easy to illustrate the changes in thickness of all the other zones involved. The Fire Clay Coal seam (FCL B) was considered suitable for this purpose. It is relatively thick and regionally extensive in the Appalachian basin. The Fire Clay Coal bed is one of the major coal-producing bed in the States. It has a Flint Clay parting, the Fire Clay tonstein with a volcanic fall origin (Chenut, 1979). It provides a timecorrelative datum throughout the basin (Andrew, Jr. et. al., 1994). To facilitate the description and interpretation of stratigraphic features in the foreland basin some key stratigraphic surfaces (coal seams) similar to the Fire Clay Coal seam, which proved to be extensive across the basin have been used for the stratigraphic nomenclature (Table 3.2). By means of the available coal seams in the study area the middle Pennsylvanian rocks (the Breathitt Group) have been divided into three broad units formally ranked as formations and three marine units formally ranked as members, and are, in descending order, the Four corners Formation, Magoffin Shale Member, Hyden Formation, Kendrick Shale Member, Pikeville Formation, and the Betsie Shale Member. The Formations were further subdivided into smaller units, which together with the Shale Members add up to 21 units (Table 3.2).
23 Table 3.2. Nomenclature scheme used for identifying and correlating key stratigraphic coal seams (surfaces) within this project. The table shows the units and formation names identified in the study area
surface
unit
Formation
BRS B BRS A PCH B PCH I PCH A HZ7 B HZD A HDX A HML B HML A FC R A FC L B WHI B WHI A AMB B AMB A UE3 B UE3 A UE2 A UE1 B LEK B CLN B
21 20 19 18
Four corners
17 16 15 14
Magoffin Member
13 12 11
Hyden
10 9 8
Kendrick Member
7 6 5
Pikeville 4 3 2 1
Betsie Member
24
Fig. 3.3. Well logs with correlations based on coal seams before flattening on a horizon. The logs also illustrate the four most important information required for the data input into Petrel: the FermFacies, the FaciesCurves, the Facies and the Coals, all combined to form a complete sedimentological log.
25
Fig. 3.4. The same well logs as in Figure 3.3, but now with the corelations based on a datum layer (FCL B). The pattern of the stratigraphic units in this case can easily be observed and predicted between wells.
3.1.3. Making stratigraphic surfaces Having acquired satisfactory stratigraphic cross-sections in the study area, the next step was to construct stratigraphic surfaces in Petrel for each horizon, using well tops provided by the upper surfaces of the coal seams. The well top FCL B for the fire clay coal seam was used as the reference surface upon which the surfaces immediately below and above were constructed using the calculator function in Petrel. To create succeeding and preceding surfaces above and below the FCL B isochore points were calculated and isochore thickness maps for each zone were made, and then subtracted from the previous surface. Altogether 25 surfaces were generated in the entire study area and 23 in the
26 quadrangle Broad-bottom. This marked the first step towards making conceptual geological and facies models in the study area. At this stage the study was narrowed to an area of about 73 square kilometers in the northwestern corner of the study area, in the quadrangle Broad-bottom (Figure 3.2).
HZ8 B
HDX A
FCL B
UE1 B
Fig. 3.5. Stratigraphic surfaces in 3D obtained using well tops (top surfaces of the coal seams) as input data. The well top FCL B (Fire Clay B coal seam) was used as the reference surface.
3.1.4. Preparation of composite thickness maps and facies pie-charts The first step toward facies correlation a composite of thickness maps and lithofacies pie-charts created in Petrel with the aim of facilitating the estimation of the percentage of lithofacies, including their lateral and vertical distribution, connectivity and stacking patterns in each stratigraphic zone. The procedure for creating pie-charts is explained in detail in the next section. Finally, two special cross-sections (A-B and C-D) were prepared in Petrel by cutting through the thickness maps in the quadrangle Broad-bottom (Figures 3.6 and 3.7). Two further sections (E-F and G-H) were prepared in a similar way to illustrate the general stratigraphic trends in the total study area Well logs along or in close proximity (~ 500 m) to the intersection planes were also included in the crosssections to facilitate facies correlations and interpretations between wells. The
27 stratigraphic sections thus obtained were used as templates for vertically and laterally correlating facies in the study area. This was done manually with the help of the composite maps for zone thickness and facies pie-charts (Figures 4.6 to 4.27) and the preliminary stratigraphic sections correlated in Petrel. Individual facies available in the area have been assigned different colours (Figure 4.4). This gave the required conceptual facies models, which can also be used as geological stratigraphic models, representing the distribution and evolution of sediments, facies, lithotypes in this part of the Appalachian foreland basin in eastern Kentucky (see Figures 4.5 and 4.6).
28
B
A
Intersection Plane
Fig. 3.6. Correlated stratigraphic surfaces in the subsurface of Broad-bottom stacked together and cut through vertically by Petrel in preparation for the NE-SW cross-section A-B.
29
Intersection plane
D C
N
Fig. 3.7. Correlated stratigraphic surfaces in the subsurface of Broad-bottom stacked together and cut through vertically by Petrel in preparation for the NW-SE cross-section C-D.
30
BRS B BRS A
PCH B PCH I PCH A Hz7 B HZD A HDX A HML B HML A FCR A FCL B WHI B WHI A AMB B AMB A Ue3 B Ue3 A Ue2 A Ue1 B LEK B
CLN B BLR B
PKV007
PKV010 Bb029
011
028
010
012
020
005
004
076
073
072 HRLD033 Bb023
015
024
014
HRLD069
Fig.3.8. Detailed log cross-section linking all wells in the quadrangle Broad bottom and a few others from Pikeville and Harold.
31
SSW
Fig. 3.9. Detailed cross-section A-B (see Figure 3.1 for location)
NNE
32
SSW
Fig. 3.9. continued.
NNE
33
NW
Fig. 3.10. Detailed cross-section C-D (see Figure 3.1 for location
SE
34
NW
Fig. 3.10. continued
SE
35 3.2. Facies modelling 3.2.1. Introduction To acquire a better knowledge and understanding of the stratigraphic framework and facies distribution and architecture of the Breathitt Group of formations in eastern Kentucky, a detailed study was carried out in a single quadrangle (Broad bottom) northeast of the study area. This part of the field was chosen because it has a relatively sufficient number of wells (17) evenly distributed and covering almost the entire stratigraphic intervals of the Breathitt Group, including the Four Corners Formation, the Magoffin Member, the Hyden Formation, the Kendrick Member, the Pikeville Formation and the Betsie Member. The Four corners formation has been eroded in most of the southern parts of the study area, but still exists in areas further from the thrust-front. In the quadrangle Broad bottom there exist a few wells that avail data about the Four Corners Formation. In this study thickness maps and lithofacies pie-charts have been used to construct the basin stratigraphy. Thickness variations in a given zone can be due to a variety of stratigraphic and structural causes including tectonics, subsidence and differential compaction during and after deposition. Therefore, thickness maps are a valuable guide for structural and stratigraphic interpretation. 3.2.2. The procedure The lithological information obtained from the cored wells in the area was used as input data into Petrel. A zigzag cross-section connecting all existing wells in the quadrangle Broad-bottom, as well as two from the neighboring quadrangle Harold, was constructed in Petrel (Figure 3.8). Next the stratigraphic surfaces were correlated based on the extensive coal seams and marine flooding surfaces in the area dividing the basin into genetic stratigraphic units. Thickness maps (Isochores) were generated in Petrel for the zones between successive surfaces. The isochore maps thus derived are representative of time equivalent depositional zones or sequences within the Breathitt stratigraphic interval. Piecharts were created in each isochore or zone displaying the percentage volume of each lithofacies along those wells that have log information on the entire thickness interval of each zone. The procedure for creating pie-charts in Petrel can be summarized as follows: Open a new 2D window. Select the polygon for the area Broad bottom and display the thickness map for any desired stratigraphic interval by selecting it from the list of isochores in the lower part of the input pane. Insert the wells that actually have data in this interval. Using the general log correlation cross-section for the area Broad bottom, those wells that wholly penetrate the selected interval can be selected. In the upper part of the input pane right click “attributes” and select the option “insert new attributes”. Under new attributes select and click the option “continuous”, which then leads one to the “attribute operations”. Select
36 “zone to level 1”, facies, %, SSTV, in that order and eventually “run” the settings. Under facies choose either a single facies at a time from the list of 14 lithofacies coded for this project or select all of them at once to save time for several runs. Petrel will draw a pie-chart for each borehole showing the proportions of each lithofacies logged along the well bore in that stratigraphic interval. This procedure is repeated for all zones. For each stratigraphic zone pie-charts and isochore maps were used to critically analyse the thickness, and lateral distributions of each facies, including facies associations and proportions, and grain-size trends, with the aim of deriving quantitative and qualitative information about the vertical and lateral distribution of depositional energies and environments in the unit. This information is crucial at the initial stage of any scheme for geological reservoir modelling. The facies volumes, obtained from the pie-charts, and their patterns identified in each well were then used to predict or interpolate the facies vertical and lateral distribution (and hence the depositional environments) between wells. Thus, pie-charts provided a statistical database for the quantity of each facies in a given zone. To facilitate the task of facies description and interpretation process a composite of contoured isochore map and pie-charts were generated in each zone (see Figures 4.6 to 4.27). The subsequent architectural and sequence stratigraphical analysis was based on cross-sections A-B and C-D. The main elements of these cross-sections include: (1) the vertical stacking patterns of the sedimentary bodies (lithofacies) along each well that partially or wholly penetrate the mapped interval of the Breathitt Group in the area of Broad bottom, (2) the stratal surfaces (in principal these are the coal seams), (3) the stratigraphic units or zones composed of various facies associations and bounded above and below by the stratal surfaces. Each lithofacies was given a specific colour and the bounding surfaces are named according to the Nomenclature scheme adopted from Chesnut (1996) (Table 3.2). Stratigraphic correlation sections A-B was oriented parallel to the strike of the basin structures and axis and section C-D was oriented parallel to the dip direction of the deposited facies in the basin (Figures 3.2, 3.9, 3.10, 4.4 and 4.5). A simple lithofacies subdivision scheme was used in the cross-sections, consisting of only seven different types which, nevertheless, highlight the main lithological heterogeneity in the formations (Figure 4.4). They include: (1) Conglomerates (Cngl 12), (2) Sandstones (Ss 10), (3) Heterolithics (Ht 8), (4) Shale (Sh 6), (5) Coal (Co 2), (6) Limestone (Ls 13), and (8) Unknown (Un 0). The numbers attached to the facies abbreviation are litho-codes used by the KGS database centre.
37
4.0. RESULTS 4.1. Overall stratigraphy in the entire study area 4.1.1. Introduction A reliable geological sequence stratigraphic framework and facies architecture are necessary in order to investigate coal and other hydrocarbon resources in any exploration basin and the central Appalachian basin of Eastern Kentucky, in particular, so as to understand the depositional mechanisms (tectonics, eustatic, subsidence, paleoclimates, etc) that were involved in controlling the distribution of the deposited facies in the basin. It is also necessary to develop a usable stratigraphic framework that accurately reflects the present knowledge of the coal-bearing strata. Two cross-sections, E-F and G-H, were constructed across the study area (based on stratigraphic surfaces generated by in Petrel through the sedimentologic logs that were obtained from 286 cored boreholes in the area of twelve quadrangles in Eastern Kentucky) in order to examine the stratigraphic and structural framework of the coal-bearing rocks in the basin (Figures 3.2, 4.1, 4.2, and 4.3). The SSE-NNW cross-section is oriented parallel to the dip direction of the stratal units, whereas the SSW-NNE cross-section is oriented parallel to the strike direction (Figure 4.2 and 4.3). Table 3.2 is a modified version of the nomenclature scheme used by Chesnut (1996) to describe the geological stratigraphic framework for the coal-bearing rocks of the Central Appalachian Basin (Figure 4.1). 4.1.2. Analysis of cross-sections (E-F and G-H) Generally, the two cross-sections reveal a regional characteristic thickness trend in the stratigraphic strata of the Breathitt Group in the study area. The strata mainly tend to thin laterally across the basin from the thrust-front in the southeast toward the basin margin in the northwest (Figure 4.2 and 4.3), demonstrating the geometry of a typical foreland basin. The strike cross-section E-F (Figure 4.2) is thicker in the SSW direction (~ 1200 ft) near the basin axis and thinner in the NNE (~ 800 ft), the plunge direction. Furthermore, the strike-section reveals a significant proportion of a broad domal (anticlinal) structure in the foreland basin onto which all the mapped strata are superimposed. The section also shows a series of small folds around the peak region of the domal structure in the SSW and a broad synclinal structure midfield. These tectonic distortions however tend to wane out toward the NNE direction. The trending axes for all these structures are oriented in the NNW-SSE direction. Similarly, the dip cross-section G-H (Figure 4.2) is thicker in the SSE (~ 1320 ft) near the thrust-front and thinner in the NNW (~ 740 ft) toward the basin margin. Unlike the strike-section the small folds are uniformly spread onto the entire flank of the large domal structure in the field with a tendency to increase the frequency and amplitude toward the basin margin (forebulge) and waning out in the direction of the thrust-front. These structures have their axes trending in the
38 NNE-SSW direction. Also to be observed in this section is the degree of stratal inclination, which decreases down the stratigraphic column. Thus the strata in the Four Corners Formation are inclined most and those in the Pikeville Formation least. Both sections show that the major transgressive marine units are relatively thicker than other strata in the basin. This implies they are zones of major tectonic subsidence and sea level rise which out balanced the rate of sediment deposition. The Breathitt Group is observed to contain many strata that are aerally extensive across the basin, indicating basin- or larger-scale control over their deposition. This may be explained by two mechanisms: (1) tectonics was of a basin-scale and (2) eustatic controls were of great extent. Tectonic mechanisms can be used to explain the transgressive-regressive cycles observed in the strata, as thrustblock emplacement caused these foreland basin-scale features. Pennsylvanian glaciation in the Southern Hemisphere and its consequent glacioeustatic control over coastal sedimentation of the Central Appalachian Basin has been suggested (Chesnut, 1996; Aitken and Flint, 1996). A coastal setting, including environments such as a shallow sea, a series of small deltas, tidal flats and estuaries, a coastal plain, fluvial channels and alluvial plains, is envisaged for the deposition of the coal-bearing rocks of the Central Appalachian Basin (Chesnut, 1996). Sea-level changes on the order of several tens of metres would have had drastic effects on Pennsylvanian coastal settings and transgressions, whether of eustatic or tectonic origin and would have extended inland for several hundreds of kilometers in such lowland settings (Aitken and Flint, 1996). Generally the Breathitt Group in the study area is mainly composed of alternating litho-facies of coals, shales, heterolithics, conglomerates and sandstones. A detailed explanation of facies and their distribution in the Breathitt fluvial-deltaic strata in eastern Kentucky will be given in section 4.2 for the quadrangle Broadbottom. The following is a brief description and interpretation of the stratigraphy in the entire study area. 4.1.3. General description and interpretation of the stratigraphy in study area. Similar to the results of previous studies (e.g. Tankard, 1986), the stratigraphy of the Breathitt Group in the study area may be broadly divided into three zones (FC, H, and Pk) separated by two major marine transgressive zones, Magoffin and Kendrick (Figures 4.2 and 4.3). The uppermost zone (FC) is a fluvial-deltaic group of formations known as the Four corners formations. The middle zone (H) is a coastal plain system of formations known as the Hyden formations. It is separated from the uppermost and the preceding zones by the Magoffin and Kendrick transgressive systems, respectively. The lowermost zone (Pk) is also belongs to the coastal plain system.
39 4.1.3.1. The Coastal Plain System (H and Pk) The lower coastal plain system (Pk) occupies the stratigraphic interval between the Kendrick and the Betsie transgression systems. Bay-fill and bayhead delta facies, and locally incised channels are present but less abundant than in the Hyden system of formations. Subordinate fluvial sandstones comprise about 1020% of this succession and form thin, isolated bodies interspersed in transgressive deposits composed predominantly of heterolithic lithofacies. Coals are thick and regionally extensive. The upper coastal plain depositional system (H) occupies the stratigraphic interval between the Amburgy (AMB) and the Haddix (HDX) coal zones, sandwiched between two major transgressive sequences: the Kendrick Shale member at the bottom and the Magoffin Shale member at the top. The deposited lithofacies in this interval include sandstones and conglomerates, shales, heterolithics, and coals. These facies were predominantly deposited in various environments including Lower Delta Plain, Strand Plain, Back Barrier Lagoon, Estuarine Channel, and Swamp. Stratigraphic relationships show overall thinning toward the basin margin, the Cincinnati-Waverly arch complex. Bay-fill and bayhead delta facies, locally incised by channels are predominant. The coals are thick and regionally persistent. The preceding major transgressive interval, the Kendrick Shale, is similar in all aspects to the Magoffin Shale Member. 4.1.3.2. The Magoffin transgression Magoffin transgression is a record of marine flooding in the foreland basin as it subsided beneath the loads of the advancing thrust-sheet complex in the southeast. In the study area the Magoffin exhibits foreland basin geometry (i.e. asymmetric prism) and is sandwiched between the stratigraphic surfaces HDX (at the top) and HML B (at the bottom). It is about 300 ft thick in the southeast (in the area of the quadrangle Mayking) and about 75 ft in the northeast, in the quadrangle Broad-bottom (Figure 4.1). It is dominated by heterolithics and shale facies with minor sandstones in the middle and top zones. It has an overall upward coarsening trend in grain size. Three major facies tracts can be observed along well BRDBTTM024 in the Magoffin zone: a basal transgressive interval with shale and heterolithic facies, a bay-margin progradation interval with sandstone facies, and an upper marine interval with shale facies. Magoffin sedimentation terminates with peat swamp accumulation and in some sections erosion (e.g. along well BRDBTTM015) by fluvial channels and incised valley fills (e.g. conglomerates along well BRDBTTM004). Magoffin is a regionally persistent marginal marine interval that demonstrates a major tectonic control that caused overdeepening of the foreland basin. The rich invertebrate fauna in this zone records an early Atakon age (Tankard, 1986). Deposition of the Magoffin during the Atokan was contemporaneous with overthrusting in the Ouachita orogenic belt, indicating that basin subsidence and
40 transgression resulted from overthrust loading. Furthermore this rapid response of the lithosphere to overthrust loading implies an initial elastic behaviour (Tankard, 1986). Similar rock types, paleoenvironmental settings, and fauna characterize the Kendrick and Magoffin paralic systems (Chesnut, 1996 and Tankard, 1986) 4.1.3.3. The fluvial-deltaic system (FC) The Upper fluvial-deltaic system (identified as FC in Figures 4.2 and 4.3) is the main theme of this thesis and will be described and interpreted in detail in the next section using thickness maps and cross-sections. The transition from the transgressive system (Magoffin) to the Breathitt fluvial-deltaic system (the Four Corners Formations) is very abrupt, as observed from the presence of basal conglomerates and sandstones bodies in the lower sequences (Figure 4.5 and 4.6). This marks the entry of major rivers into the foreland basin. It is about 660 ft in the southeast and about 500 ft in the northwest (Figure 4.3). It is characterized by a framework of multistory channel sandstone bodies that coalesce along strike to form relatively continuous sandstone units measuring several kilometers in width (see Figure 4.5a). Channel fill sandstones usually comprise more than 70 % of the entire stratigraphic column. Braided and coarse-grained meandering stream deposits are very common in this system with subordinate amounts of paleovalley fills. Subordinate facies include heterolithics (which are possibly overbank mudstones) and coals, as well as shales which may possibly belong to the interdistributary bay and crevasse splay lithologies. Overall the fluvial system consists of progradational lowstand deposits. The rapid change from the transgressive marine sedimentation in the Magoffin zone to sand-dominated alluvial plain deposition was probably due the tectonic reactivation of the source terrane in the southeast by orogenic uplift. Coarse clastics were shed from the orogene faster than the subsiding foreland basin could accommodate it, resulting in an overfilled basin (Tankard, 1986). The most important result for this study is the observation of the positive and negative structural elements (anticlines and synclines) in the basin, which may have significantly influenced the drainage pattern in the Four Corners Formation. The fluvial-deltaic system terminates with an extensive coal unit and a regional marine deposition, the Stoney Fork Shale Member.
41
Fig. 4.1. A stratigraphic framework of Pennsylvanian rocks in the Central Appalachian Basin, redrawn from Chesnut, 1996.
42
NNE
SSW 2400 Vertical elevation in ft. a.s.l.
FC
1200
Mag offin Sha le M emb er
H Kendrick Shale Me mb er
Pk 1300 Betsie Shal e Member
500
Fig. 4.2. Strike section (E-F) of the Breathitt Group in the study area in Eastern Kentucky. It was constructed in Petrel from the intersection plane through 22 stratal surfaces (see Figure 3.2 for location). FC stands for Four Corners group of Formation; H stands for Hyden group of Formation, and Pk stands for the Pikeville group of Formations.
43
NNW
SSE
FC
o Mag
H
er e mb M e h al ffin S
Me Sha le k c i r Ke nd
r mbe
1440
Vertical elevation in ft. a.s.l.
2760
1440 Pk
M Sh ale B etsie
ember
700
Fig. 4.3. Dip section (G-H) of the Breathitt Group in the study area in Eastern Kentucky. It illustrates the overall foreland basin geometry (i.e. thickening toward the thrust-front). See further explanation in the text.
44 4.2. Description and Interpretation of the Stratigraphy, architecture and facies distribution in the quadrangle Broad bottom 4.2.1. Overall stratigraphy and architecture in Broad bottom The work reported in this section forms a small part of the total study area covering about 73 square kilometers, with a stratigraphic control provided by 17 wells, which have been used to define and map 21 units/zones. The result of facies analysis of each individual zone along the two cross-sections, A-B oriented in the NE-SW and C-D oriented in the NW-SE directions, gave rise to the general stratigraphy illustrated in Figures 4.5 and 4.6. These cross-sections demonstrate that each statal unit is characterized by: (1) a facies association with a vertical thickness in the order of tens of metres, (2) a lateral extent of several hundreds of metres to a few kilometres, and (3) a length of several kilometres. The stratigraphy is mainly dominated by seven lithofacies: sandstones (Ss), heterolithics (Ht), conglomerates (Cnglmrt), shales (sh), coals (cl), limestones (ls), and some unknown lithofacies (Unknwn) which in this project has been interpreted to be sandstone. The conglomerate and most sandstone lithofacies belong to fluvial-channels and incised valley fills. Other depositional environments include mouth-bars, estuaries, prodeltas, distributary and Interdistributary facies deposits. The heterolithic and shale lithofacies mainly represent sea transgressional facies and depositional facies that flanked the fluvial channel banks and are therefore composed of a mixed assortment of accumulated fine-grained sediments. They may therefore occur as channel-lobe transition or frontal splay deposits. Limestone is most likely to have precipitated from a marine environment while coals are a result of peat generation in swampy and marshy environments. Both limestone and coals could be indicators of maximum marine flooding surfaces. Broadly the stratigraphy in Broad bottom may be divided into five parts, similar to those observed in the stratigraphy for the total study area (Figures 4.2 and 4.3). In a descending order these zones/units may include: (1) Units 21 to 15 (the fluvial-deltaic system) (2) Unit 14 (the Magoffin transgressive system) (3) Units 13 to 7 (the upper coastal plain system) (4) Unit 6 (the Kendrick transgressive system (5) Units 5 to 0 (the lower coastal plain system. Considering all the units together, the overall thickness trend that was observed in the general stratigraphy (i.e. stratal thinning toward the NW or NE, Figures 4.2 and 4.3) still holds true for the stratigraphy in Broad-bottom. However, this may not be true for some sections (units 0 through 6) in the lower parts of the stratigraphy. This may be partly due to the differences in the magnitudes of the depositional controls (including tectonism, differential subsidence, and eustasy)
45 along the stratigraphic column. The upper parts, for instance, were deposited at the time of the Atokan orogenic activity, causing a significant inclination to the deposited stratas in the southeast. The inverse relationship between the thicknesses of some neighbouring stratigraphic units has also been observed. Nevertheless the overall stratigraphic thickness map for the Breathitt Group in Broad-bottom clearly demonstrates the thickness trend (Figure 4.7). Although less obvious, the general tendency for the units to plunge in the NE and dip in the NW can still be observed in Broad bottom. However, one of the series of anticlines and synclines that were observed in the general stratigraphy for the entire study area is now exaggerated in the dip-section (Figure 4.5). The synclinal structure (asymmetrical in shape) tends to increase its curvature (straining power) with depth through the fluvial system until the Magoffin transgression when its curvature begins to relax with depth, through the Kendrick transgression until the Betsie transgression when its small anticlinal lobe to the NW of the axis totally disappears. At the top of the fluvial system the structure has one anticlinal lobe to the NW of its axis (along wells BRDBTTM023 and 015), but begins to develop its second and broader lobe on the SE (along wells BRDBTTM024, 005, and 004) with increasing depth. However, at greater depths, in unit 2, for instance, when the NW lobe is almost completely attenuated, that on the SE side of the axis can still be observed, although very much diminished in shape. The strike section also exhibits a small anticlinal structure (along wells BRDBTTM029, 011, and 028) which is much less in magnitude than those observed in the dip-section. The age of the correlated part of the Breathitt Group has been estimated be between 306 Ma (top of unit 21) and 315 Ma (top of unit 0), based on the assumption by Greb et al. (2002) that the individual coal-clastic cycles had durations of approximately 400 ka. Further, the coal-clastic cycles are assumed to be eustatically controlled fourth-order sequences, grouped into sequence sets to form third-order sequences (Aitken and Howell, 1996 ; Miall, 1991). Generally the following observations were made: (1) Stratal thickness increases in the SE direction toward the axis of the foreland basin, (2) the intensity of folding (tectonism) attenuates with depth, (3) the proportion of marine (shales) and heterolithic lithofacies increases with depth, (4) conglomerates are mostly observed in the upper coastal plain and the lower fluvial system, deposited at the base but sometimes in the middle of the stratal units, and (5) the proportion of sandstone lithofacies decreases with depth. 4.2.1.1. The Lower coastal plain system Generally, the lower coastal plain system (units 5, 4, 3, 2, 1 and 0) has a very low proportion of sandstone facies and is predominantly composed of shales and heterolithics. In some locations, however, e.g. in unit 5 along well BRDBTTM004, conglomerate facies may be observed. These are likely to be a result of forced regressions deposited by the oscillating sea-level, at sea level fall. The deposited sediments of this system, like the rest of the coastal system, are retrogradational and generally tend to coarsen upwards.
46 4.2.1.2. The Kendrick transgression The Kendrick transgression (unit 6), like the Magoffin, thickens toward the basin axis, with the grain-size of its sediments coarsening upwards. It is predominantly composed of heterolithics and shales, with some minor sandstone facies in the middle and upper tracts. 4.2.1.3. The Upper coastal plain system In the upper coastal plain system (units 13 to 6), the pairs of units 14+13, 12+11, and 10+9 are inversely related in their thickness. The last two units (8 and 7) in this system, however, exhibit the true foreland basin geometry, i.e. they tend to thicken toward the thrust-front. This system has an equal proportion of sandstones and heterolithics plus shales. The strike cross section shows a large number of sandstone bodies and conglomerates laterally stacked together but separated by thin shales between units. The proportion of sandstones tends to increase towards the NE (strike section, Figure 4.6). 4.2.1.4. The Magoffin transgression Unit 14, the Magoffin transgression, as before shows a thickness pattern typical of the foreland basin geometry, i.e. thickening in the directions toward the thrustfront load or basin axis. It shows a tendency for its grain size to coarsen upward. It is predominantly composed of marine facies (shales) and heterolithics. 4.2.1.5. The Fluvial-deltaic system In the fluvial-deltaic system (units 21 to 14) the unit pairs 21 and 20, 17and 16, are inversely related in thickness, while the rest (units 19, 18 and 15) show constant thickness along the dip-section (Figure 4.5). The system shows an overall tendency for its grain size to thin upward and its deposits are mostly composed of sandstone and conglomerates (> 75 %), with the rest being shales and heterolithics lithofacies.
47
Fig. 4.4. A Colour legend to Figures 4.5 and 4.6
48
Fig. 4.5. (a) A schematic dip-section, illustrating the sequence stratigraphic framework and Depositional System Tracts for the facies in the upper part of the Breathitt Group (units 11 to 21). Correlations were done in Petrel based on the extensive coal seams and shales observed in the sedimentological logs. The logs were constructed in Petrel using cored borehole data along the cross-section C-D in the quadrangle Broad bottom (see Fig.3.2 for location). The numbers and codes at the top are identities for the reference wells and those on the sides are the names of major coal seams used for correlations and sequence stratigraphic abbreviations for the depositional processes. See Table 3.2 for nomenclature of the coal seams.
49
Fig. 4.5. (b)Sequence stratigraphic framework and Depositional System Tracts for facies in the lower part of the Breathitt Group (units 2 to 10) along the same cross-section as in (a). The diagram is as well a conceptual facies model showing the lateral and vertical distribution of facies in the Breathitt group of Formations between the stratigraphic surfaces BRS B and LEK B. LST = Lowstand System Tract, TST = Transgressive System Tract, HST = Highstand System Tract, and FSST = Falling Stage System Tract. SB = Sequence Boundary, FS = Flooding Surface. The approximate age of deposition is also indicated on the right. The arrows point in the direction of fining of the sediment grain-size. See Figure 3.2 and Table 3.2, respectively, for the location of the section and nomenclature of the coal seams.
50
Fig. 4.6. (a) A schematic strike-section showing the sequence stratigraphic framework and Depositional System Tracts for the facies in the upper part of the Breathitt Group (units 11 through 21). Correlations were done based on the extensive coal seams and marine shales observable in the sedimentological logs, constructed from cored borehole data along cross-section A-B in quadrangle Broad bottom, see Fig.3.2 for location. The numbers at the top are names for the reference wells used in correlation. See Figure 4.5 for the definition of the sequence stratigraphic abbreviations to the right and Table 3.2 for the names of the coal seams.
51
Fig. 4.6. (b) Lower part of the stratigraphic section A-B (units 1 through 10), for the lower part of the Breathitt Group.
52
Fig. 4.7. Thickness map of the Breathitt Group (21 units) in the quadrangle Broad-bottom. Thickness is given in ft and indicated by the colour legend and the contours. In the midfield thickness is influenced by the synclinal structure oriented NE-SW. The map shows a general progradation of the delta from the SE (thickest part) toward the NW (thinnest part of the basin).
53 4.2.2. Description and Interpretation of coastal plain facies The rest of this chapter will try to describe and interpret facies in each stratigraphic unit/zone mapped in the quadrangle Broad-bottom. The zones are numerically arranged from bottom to the top (1 to 21). The descriptions and interpretations attached to these units are based on the general characteristic features observed in terms of unit thickness, and distribution of facies or depositional energy as illustrated by the composite maps of isochors and lithofacies pie-charts. The maps together with the two stratigraphic crosssections (Figures 4.5 and 4.6) form the basis for the descriptions and interpretations. In general five dominating lithofacies associations are observed in the area of Broad bottom. They include sandstones, heterolithics, shales, conglomerates, and coals. Other lithologies present however, in minor quantities include ironstone and limestones. As a rule in geology descriptions and interpretations of the stratigraphic units are done starting from the bottom (0) to the top (21) of the stratigraphic units. Thickness maps for each unit have been drawn equipped with lithofacies piecharts at locations of cored boreholes that wholly or almost wholly penetrate the interval. Such composite maps give a general idea about sediment transport directions (paleoflow directions) including the distribution of the depositional energies and facies in each stratigraphic unit. The principles of sequence stratigraphy, where appropriate, have been applied to interpret the environments of deposition and facies associations in the zones. Figure 4.8 shows the colour legend used for the nomenclature of the facies quantified in the pie-charts.
Fig. 4.8. A Colour legend for the pie-charts in Figures 4.9 to 4.30
54 4.2.2.1. Zone 0 (BLR B – CLN B)
D B
C
A
Fig. 4.9. A composite of stratigraphic thickness map and lithofacies pie-charts used for the description and interpretation of facies and environments of deposition in zone 0, between surfaces BLR B and CLN B. Note that the map is contoured in ft.
55 Zone description The unit bounded below and above by the stratigraphic surfaces BLR B and CLN B, respectively (see Figures 10.1 and 10.2 in the Appendix). The unit shows the geometry of a foreland basin, i.e. thickest (110 ft.) in the southwest (the direction of the thrust-front) and thinnest the northwest and northeast, to a minimum of about 55 ft in some locations. Facies distribution as observed from the pie-charts in the unit shows a steady decline in depositional energy toward the northeast. Facies present in a decreasing order of magnitude include shales, heterolithics and sandstones, sandwiched in thin coals at the bottom and top of the unit. There is a tendency for the proportion of sandstone and heterolithics to decrease in the northern direction, while shales increase, see thickness map and the detailed log correlation section for Broad-bottom and the neighboring areas (Figures 3.8, 3.9 and 4.9). The logs in the extreme southeast (see wells PKVLL007 and PKVLL010) outside the southern borders to the quadrangle Broad bottom show a gradual upward coarsening trend for the sediment grainsize, while wells in the centre (BRDBTTM010 and BRDBTTM020) show an abrupt upward coarsening trend of the sediment grain-size (Figures 3.8, 3.9 and 4.9). As observed from the contour pattern in the thickness map, there is a synclinal structure oriented in the NE-SW direction, parallel to the strike direction of the stratigraphic unit. The unit generally dips in the northwest and strikes in the northeast. Also to be observed from the contour pattern is a series of depressions in the middle region trending in the NE-SW direction. However, there is not enough borehole data in this unit to enable the construction of crosssections that could possibly assist in explaining and interpreting these depressions in detail, but from the overall stratigraphy observed above, they can be interpreted as a series of minor anticlines and synclines trending NW-SE. Zone interpretation This is possibly a mixed fluvial and marine environment in the lower delta plain. The decrease in the proportion of sandstone toward the north is an indication of a system that was continuously losing energy in the northern direction and thereby progressively precipitating its suspension load of fine sediments (shales) probably into a large open body of water (a large lake or sea). The sandstones are most probably mouth-bars characteristic of a lower delta plain environment. The midfield structure trending in the northeast-southwest direction is most probably a syncline that controlled the distribution of depositional energy in the area. The northern half of the zone beyond this structure is predominantly marine, deposited with shale facies. Alternatively, the southern parts of the unit were deposited in a wave/storm dominated lower-middle shoreface or delta-front environment. The reduction in energy in the central and northern parts of the unit may suggest that these areas were protected from the storms and waves and therefore indicative of a tidally-influenced deposition environment. Therefore the northern parts can be interpreted as areas where tidal energies dominated over storm/wave energies.
56 4.2.2.2. Zone 1 (CLN B – LEK B)
D B
C A
Fig. 4.10. Composite of thickness map and lithofacies pie-charts for the zone CLN B-LEK B. A-B and C-D are locations for the NE-SW and NW-SE cross-sections, respectively.
57 Zone description Overall the unit is relatively thick and slightly inclined to the northwest (~ 175 ft in the SE, ~ 150 ft in the middle and ~ 160 ft in the NW). Depositional energy is observed to decline from the northwest to the southwest. Similarly, the proportion of sandstone decreases, while that of heterolithics, shale, and coals increases in a SE direction. Like the previous unit, a fold structure can be seen in the middle regions trending in the NE-SW direction. The zone is divided into three channellike lobes filled with thick sediments and spreading out from the centre into the northwest, southwest and eastern directions, with areas of relatively thin sediments in between. The eastern lobe is funnel shaped and the most extensive. Cored borehole logs in the unit show an overall upward-coarsening trend of sediment grain-size distribution. Thin and discontinuous coals occur everywhere in the zone with a tendency to increase in the southern direction. The strike-section (Figure 4.6b, unit 1) shows the zone is thickest with most deposits of sandstones between wells BRDBTTM011 and 004. The sandstones split into thin sheet-like layers between shales and heterolithics in the NE and SW directions. Zone interpretation The distribution pattern of depositional energy and sediment grain-size trends suggest that deposition was predominantly transgressive, with sea waves/storms giving rise to environments ranging from delta-front to strandplain. The sheet-like sandstone layers observed in wells BRDBTTM004, 010, 011, 014, and 020 could be mouthbars and distributary channels deposited in a wave/storm-dominated basin. Other distributary mouth bars and channels in the zone are represented by the coarse grained sandstones and conglomerates, as observed in well BRDBTTM072, in the north. These were deposited either by subaqueous cohesionless debris flows generated by oversteepening of the mouth bars or as reworked sandstone deposits in the estuarine during backfilling of the distributary channels. The presence of more coals and heterolithics in the landward direction may imply a relatively stable mixed fluvial/marine environment in the south. Facies associations and distribution in this unit give evidence that the deposition was done in a transgressive system when the sea level was rapidly rising to create more accommodation space. In the middle of the strike-section the coarse sandstones that laterally splits into sheets encapsulated in shales indicate the boundary (or bay-line) between the fluvial and marine systems, which migrated landward when the sea level rose. The synclinal structure midfield played a big role in the distribution of depositional energies in the area.
58 4.2.2.3. Zone 2 (LEK B – UE1 B)
D B
C
A
Fig. 4.11. A composite of thickness map and lithofacies pie-charts for the zone LEK B-UE1 B
59 Zone description The normal thickness trend for deposition in a foreland basin seems to be reversed in this zone. It is thickest (~ 98 ft) in the north and thinnest (~ 72 ft) in the south. The zone shows the depositional energy to be greatest in the midfield and least in the southeast. Accordingly there is more sandstone in the midfield with its proportion gradually declining in the northwest direction. The south is predominantly deposited with shales and heterolithics. The narrow diagonal region mid-field is occupied with a large proportion of sandstones and little shales and coal in the extreme NE and SW, with the central region being predominantly heterolithic. The northwest is predominantly occupied with two lithofacies, heterolithics (~ 75 %) and sandstone (~ 25 %). The middle and southwest regions are predominantly heterolithic with the proportion of shales increasing toward the southwest. This description is confirmed and clearly illustrated by the two cross-sections (Figures 4.5b and 4.6b). Half of the entire zone, from middle to the southeast was deposited with shales, while the other half was predominantly occupied with heterolithics and minor traces of sandstone lithofacies (Figure 4.5a, the dip-section). In the strike-section, more than threequarters of the zone toward the southwest direction is covered by heterolithics, while the remaining third is occupied by shales and minor sandstones. Zone interpretation The zone may be subdivided into three regions of different depositional energies: Facies associations in the northwest show predominance of wave/storm environment with the deposition of heterolithics and minor mouthbar sandstones. In the southeast, however, the facies association reveals a low energy environment of deposition, probably influenced by tides, at the time immediately following the peak period of sea transgression, when there was maximum flooding. The more coarse-grained region with mouthbar sandstones midfield was either formed by the action of wave reworking or by flood events in a still active distributary system. In this zone again the synclinal structure midfield influenced the distribution of depositional energies.
60 4.2.2.4. Zone 3 (UE1 B – UE2 A)
D
B
C
A
Fig. 4.12. Composite of thickness map and lithofacies pie-charts for the zone UE1 B-UE2 A
61 Zone description This zone has a broad region of thick sediments across the middle, with a maximum thickness of about 40 ft. From the middle region the unit gradually thins northwards to attain a minimum thickness of about 25 ft. However, from the middle southwards the zone rapidly thins out to attain a minimum thickness of about 2 ft predominantly composed of coals (~ 75 %) and shales (~ 25 %). Coals gradually decrease toward the northeast direction to be replaced by shales, heterolithics and sandstones. The strike-section (Figure 4.6b) shows that the zone becomes thicker towards the northeast, deposited with a variety of facies associations, including basal sandstones, heterolithics and shales, with the overall sediment grain-size tending to thin upward. The zone becomes thinner in the southwest and is predominantly occupied by coals. The dip-section (Figure 4.5b), however, depicts the zone to be almost horizontally deposited, uniformly thick and slightly folded in the middle region. The southeast is predominantly occupied with shales and the northwest with sandstone and heterolithics. Zone interpretation The widespread presence of thick coals and shales in the southern portion of the unit indicates that the interfluves were constantly flooded causing poor drainage conditions that favored the widespread generation of peat in swamps. The sandstones and heterolithics observed in the northern portion of the zone are probably estuarine point bars (channels and tidal flats) deposited during the high stand system tract. The large coal content in south could also imply a relatively stable mixed fluvial-marine environment tidally influenced.
62 4.2.2.5. Zone 4 (UE2 A – UE3 A)
D B
C
A
Fig. 4.13. Composite of thickness map and lithofacies pie-charts for the zone UE2 A-UE3 A
63 Zone description This zone is inversely related in thickness to the preceding zone. The unit has foreland basin geometry, thickest in the southwest (~ 75 ft) and thinnest in the north (~ 15 ft). There is also a reversal in the distribution of energy as well. The southeast is deposited with more sandstones and heterolithics facies than in the north. Overall the unit is deposited with a large proportion of heterolithics except in the northwest, where the proportion of shale is higher. Sandstone decreases gradually and laterally from the southeast and the northwest toward the centre, while the shales increase in the reverse directions. Coals are thin and discontinuous, but occur everywhere in the zone. The strike-section (Figure 4.6b) shows some thin and discontinuous sandstone in the northeast with a tendency to thin upward. The region from the center toward the southwest is predominantly occupied with heterolithics and subordinate thin and sheet-like shales (Figure 4.6b). The dip-section, however, shows a deposit of sandstones and heterolithics in the southeast and sandstones and shales in the northwest (Figure 4.5b). The central region is slightly folded and predominantly deposited with heterolithics. Zone interpretation This zone comprises of sediments deposited during the last phase of the long marine transgression period when the sea-level or base-level was beginning to fall. The fluvial system may now be observed as beginning to cut through the thick heterolithic sediments from the south east. Coals and shales though thin are observed adjacent to the incising channel overbanks where the system possibly offers a relatively stable environment. In terms of sequence stratigraphy, this zone may be at the transition phase from the highstand system tract to the beginning of the lowstand system tract when the sea-level begins to fall.
64 4.2.2.6. Zone 5 (UE3 A – UE3 B)
D
B
C
A
Fig. 4.14. Composite of thickness map and lithofacies pie-charts for the zone UE3 A-UE3 B
65 Zone description This zone is diagonally divided into two parts, with the upper part thicker than the lower half. A structure of regional extent lies between the two portions trending northeast-southwest. An Incised valley fill is also observed in a distributary channel flowing through the lower half of the zone from the southeast and depositing its sediment load into the upper zone across the midfield structure. The channel fill consists of a huge proportion of conglomerates, sandstones and coals (well BRDBTTM004). The channel fill is also thicker than the adjacent areas overbank which are the flood plains for the eroding channel when it spills its banks. They contain a large proportion of coals and shales than the upper region. The upper region however contains more sandstone in the north than in the west. Zone interpretation The presence of conglomerates, logged midfield in well BRDBTTM004, is a clear indication of an eroding channel or distributary stream during the lowstand system tract, flowing from the southeast incising and depositing its sediment load into the shallow open body of water in the northwest. This also led to the formation of a lower delta system in the upper part of the zone. Well logs in this region are blocky and show a coarsening-upwards trend with a high proportion of sandstones, heterolithics and subordinate shales, typical of mouth bars in a marginal marine environment. Overbank facies or Crevasse splay facies predominantly composed of fine sediment materials (shales and heterolithics) are observed in the thin flood plains adjacent to the relatively thick fluvial incised valley fill. The channel flood plains provides a stable swampy environment that is quite suitable for the peat generation, as supported by the presence of thick coals logged in wells BDRBTTM005 and 076. The unknown lithology logged in well BRDBTTM020 could possibly be more-or-less shales and heterolithics with thick coals since is located at the boundary between the swampy flood plains and the lower delta plain with mixed fresh and saline waters, a marginal marine-fluvial environment. In terms of sequence stratigraphy, this zone may be at a lowstand system tract when the sea-level was falling.
66 4.2.2.7. Zone 6 (UE3 B - AMB A)
D
B
C
A
Fig. 4.15. Composite of thickness map and lithofacies pie-charts for the zone UE3 B-AMB A
67 Zone description Generally, the zone is uniformly thick (~ 75 ft) with an equal spread of low energy in the entire field and consists of a large proportion of shales and heterolithics. Limestone has been logged in well HRLD033, in the northwest. The zone is terminated with a sharp contact at the top by the overlying sandstones of the next stratigraphic unit. Coals in this unit are very thin and hardly noticeable. A good proportion of sandstone lithofacies were deposited midfield within the synclinal structure. Zone interpretation This is a regional transgressive zone starting with a flooding surface at the bottom and ending sharply with a sequence boundary at the top. The abundance of shales indicates precipitation of the suspended fine sediments from a large body of water. Limestone observed in the north is a clear indication of a depositional environment in which the water depth remained relatively deep and clear allowing marine faunas to thrive. Carbonates could then precipitate as the rate of sediment supply was very low (Figure 4.5b). The sea-level continued to rise after the precipitation of limestone until the maximum phase of transgression came to the end, giving rise to a thick deposition of shales at the top of the sequence. This zone was truncated by an unconformity as supported by the sharp erosional contact between the topmost shale and the overlying sandstones and conglomerates that belong to the next sequence. The dip-section shows the zone to contain two system tracts, the transgressive system tract that starts from the bottom to the middle, followed by the highstand system tract up to the top, when it was truncated by the overlying sandstone unit at lowstand (Figures 4.5b and 4.6b). The unit has been identified as the Kendrich Shale Member. The synclinal structure in the middle influenced the distribution of the high energy sediments in this unit.
68 4.2.2.8. Zone 7 (AMB A - AMB B)
D B
C
A
Fig. 4.16. Composite of thickness map and lithofacies pie-charts for the zone AMB A-AMB B
69 Zone description This zone has a broad progradational lobe of thick sediments that tapers off towards the northwest (Figures 4.5b and 4.16). The lobe is thickest (~ 100 ft) in the southeast and gradually thins out (~ 15 ft) towards the northwest. The zone starts abruptly with a sharp base composed of blocky sandstones (Figure 4.5b). In the midfield region some unidentified lithofacies were logged in the prograding lobe. Coals only appear in the north and northwest. Some conglomerates have been logged in the central east of the field as well. Zone interpretation The blocky nature of the logged sandstones in the prograding lobe may be an indication of rapid fluvial deposition by braided streams flowing from the south east towards the northwest. The broad and thick prograding lobe of sediments scored the underlying strata of the preceding sequence depositing sandstones and in some places conglomerates. This could possibly imply that two processes, i.e. Eustace and Tectonics, took place simultaneously. The oscillating sea-level was falling at the time when terrain in the southwest was being uplifted by the tectonics. The appearance of coals in the north indicates a reduction in the stream energy which allowed the development of a stable swampy environment required for the growth of thick tropical forests, consequently generating the coals.
70 4.2.2.9. Zone 8 (AMB B - WHI A)
D
B
C
A
Fig. 4.17. Composite of thickness map and lithofacies pie-charts for the zone AMB B-WHI A
71 Zone description The zone has an equal cumulative sediment thickness in the south and north decreasing towards the central western direction. Logged wells in the centre east show the presence of conglomerates and more sandstones than in the regions to the south and north, where fine sediments tend to show a considerable increase. In general the logs show a blocky character gradually changing into a coarsening-upwards tendency in the northern direction. Zone interpretation The zone seem to have been divided into three regions with the middle region occupied by a strong erosive stream flowing from the east towards the sea in the west, as evidenced by the logged incised valley fills (conglomerate lithofacies) in wells BRDBTTM004, 005 and 076. The regions bounding the incised valley fill to the north and south with the thickest blocky sediments were possibly occupied by braided rivers and crevasse splay environments.
72 4.2.2.10. Zone 9 (WHI A – WHI B)
D B
C A
Fig. 4.18. Composite of thickness map and lithofacies pie-charts for the zone WHI A-WHI B
73 Zone description The zone is considerably thinner than the previous one although still divided into three regions. However, there seems to be a reversed flow direction in the middle region from the west to the east. The lithofacies in the middle strip are now entirely composed of heterolithics and shales. In the adjacent areas sandstones show a considerable increase towards the north and south. The coal facies has also increased in areas close to the middle region.
Zone interpretation There is transgression from the sea in the west towards land in the east, affecting only the middle region with the consequent deposition of fine sediments, including heterolithics and finally shales. The regions to the north and south as before still have a braided river system.
74 4.2.2.11. Zone 10 (WHI B – FCL B)
D B
C A
Fig. 4.19. Composite of thickness map and lithofacies pie-charts for the zone WHI B-FCL B
75 Zone description The former situation in zone WHI A - AMB B is now reversed. The middle region is now broad and thicker than the adjacent regions to the north and south. Also to be observed is the uniform, wide spread existence of the coals in the zone, although on a gradual decrease in the north-western direction. Its content is relatively higher than in any previous zone. Well logs in the southwest and northeast show a coarsening upward trend, whereas those in the middle region show a fining-upwards trend. The sand content increases in all directions away from the central region of this zone.
Zone interpretation The presence of thick coals uniformly distributed on top of the entire zone implies a flooded swampy but relatively stable environment in the region. The different trends of sediment fining or coarsening in the region imply a system with mixed environments, including marginal marine or lower deltaic, braided and meandering river environment. The great lateral extent of coals may also indicate very low overall rates detrital influx and abandonment of the depositional landscape.
76 4.2.2.12. Zone 11 (FCL B – FCR A)
D B
C A
Fig. 4.20. Composite of thickness map and lithofacies pie-charts for the zone FCL B-FCR A
77 Zone description The middle field of the zone is very thin and deeply eroded/incised to form a curved valley across the entire region. The adjacent cliffs are thicker and steeper to the southeast than those to the northwest of the central valley. There seem to be another eroding stream in the southwest that joins the central stream in the Far East. The proportion of fine sediments and coals increases with a decrease in the gradient of the zone. Well logs (e.g. well BRDBTTM005) in the centre of the eroded valley show a predominance of conglomeratic facies. Zone interpretation The two incising streams in the zone are an indication of a remarkable fall in sealevel. Stream flows seem to be structurally controlled perpendicular to the gradient. There is a gradual decrease in energy from the southeast to the northwest and from the central region towards the northeast and southwest as indicated by the decrease in the amounts of deposited sandstone facies logged in the wells. In addition to incised valley fills the environment is predominantly braided stream as indicated by the blocky trends of the well logs and meandering streams (fining-upwards trend) in the extreme north. The environment could also be coastal plain with the incising distributaries meandering along the coastline.
78 4.2.2.13. Zone 12 (FCR A – HML A)
D
B
C
A
Fig. 4.21. Composite of thickness map and lithofacies pie-charts for the zone FCR A-HML A
79 Zone description The zone is inhomogeneous in thickness with the centre being thinner than the corner regions. Logged wells show the predominance of condensed phases in the entire field, although conglomerates and sandstone lithofacies have been logged in some wells. Coals appear discontinuous although locally very thick, as seen in some wells. Zone interpretation This is probably a zone of mixed marine and fluvial systems. The predominance of condensed phases and thick coals in some locations indicates the zone to be a transgressive sequence in a relatively low energy and stable setting. The conglomerates are probably the result of sediment reworking by the strong sea currents and waves during transgression. The sandstones are deposited by meandering streams that are being pushed back by the sea currents.
80 4.2.2.14. Zone 13 (HML A – HML B)
D
B
C
A
Fig. 4.22. Composite of thickness map and lithofacies pie-charts for the zone HML A-HML B
81 Zone description Midfield the zone is predominantly blocky sandstones and conglomerates, with shales and heterolithics in the adjacent regions to the southeast and north west. The coals are thin and discontinuous except in one well where an extremely thick coal has been logged mid-sequence between sandstones (see Figure x). Conglomerates are logged in wells adjacent to the blocky sandstones midfield. In the extreme northwest only marine shales and heterolithics have been logged. Zone interpretation Here the depositional environment may be predicted to be mostly lower delta plain grading into marine. Conglomerates logged in well adjacent to the blocky sandstones midfield could imply incised valley erosion and fills in the lower delta plain by fluvial streams at falling sea-level. The blocky sandstones logged seawards can be interpreted as barrier-island deposits (called barrier sandstone) reproducing most clearly the arrangement of the lower delta and barrier portion of the Allegheny model as explained by Ferm and Weisenfluh, 1988. Deposits seaward of these barrier sandstones are open water marine shales.
82 4.2.3. The Fluvial-deltaic system (Four Corners Formation) The Four Corners Formation in Broad bottom is comprised of eight stratigraphic units, ranging from the Magoffin transgression (unit 14 in this study) at the bottom to unit 21 at the top. The descriptions and interpretations that follow are based on the thickness maps and pie-charts for the stratigraphic units that were prepared in Petrel to serve this purpose. 4.2.3.1. The Magoffin Member Unit/Zone 14 (HML B – HDX A) Zone description This zone thickens basinwards from 50 ft in the northwest to about 90 ft in the southeast (Figures 4.5, 4.6, and 4.23). There is a predominance of two lithofacies (shales and heterolithics) with sandstones occupying the middle intervals of the vertical logs in most parts of the zone. Considering the vertical stacking sequence, the unit begins with shale lithofacies at the bottom, followed by heterolithics, sandstones, and shales, in that order, and terminating with thin and discontinuous coals and conglomerates at some locations of erosion surfaces by fluvial channels. The map shows two high energy zones in the southeast and northwest, separated by a relatively low energy region in between and oriented diagonally in the southwest-northeast direction. There two thick lobes of sediments in the south are inclined in the northwest direction. The cross-sections constructed in this unit reveal that the top of the Magoffin unit is commonly incised by thick multistory, multilateral, braided channel complexes (e.g. along well BRDBTTM004, in the dip section, Figure 4.5 and along wells BRDBTTM028 and 004, in the strike section, Figure 4.6) by the succeeding unit above. Also to be observed from the logs is the overall gradual grain-size coarsening upward trends. The coal bed (HDX A) at the top of this unit has been eroded in several locations by the sandstones of the overlying unit (15). A synclinal structure with a NE-SW trend axis is observed in the middle of the dip-section. The section also reveals that the proportion of shales in this unit increases in the NW direction from the syncline, while SE of the syncline is predominately heterolithic. Zone interpretation This zone records a period of marine inundation in the foreland basin as it subsided beneath the load of an advancing thrust sheet complex in the southeast. The predominance of the condensed phase and heterolithics in the middle region is an indication a relatively low energy setting (possibly a tidallyinfluenced deposition environment, protected from storms and waves). This region, however, was probably not stable enough for thick coals to be generated. The predominance of sandstones further northwest could probably be due to sediment reworking by the strong sea currents and waves during transgression. The shales and heterolithics at the bottom of the unit could be interpreted as
83 basin and prodelta facies. The shales are marine deposits due the transgression at the base of the Magoffin Member. The unit coarsens upward into a distal mouthbar deposit. The middle sandstones observed in the logs are possibly mouth bar sands but, could also be the distributary channel facies, popularly known as Magoffin feeder channels. The two thick tongues of sediments observed in the south could be delta lobes prograding in the northwest direction. Considering the distribution of depositional energy in the area, the unknown facies in wells BRDBTTM072 and 029 are possibly mouthbar sandstones, as both wells seem to be located in high energy areas. The deposition of the HDX A coals on top of the marine Magoffin Member is interpreted as a basinward shift of the delta plain, implying overall progradation from the base of Magoffin until the top of this coal bed.
84
D A
C
A
Fig. 4.23. Composite of thickness map and lithofacies pie-charts for the zone HML B-HDX A
85 4.2.3.2. Zone 15 (HDX A – HZD A) Zone description A broad area of the field stretching from the southwest, through the southeast to the northeast is mainly composed of conglomerate and sandstone lithofacies. The northwest is mainly occupied by heterolithics and the middle region by shales and split coals. The unit has an erosive base with the sandstones and conglomerates locally cutting down deep into the underlying coals and Magoffin Member facies. The unit is shows a foreland basin geometry. It is thickest (~ 50 ft) in the southeast and thinnest in the northwest, furthest from the thrust-front (Figure 4.24). The unit shows overall grain-size coarsening downward trends. The strike-section shows the unit to be uniformly thick with heterolithic deposits in centre, sandstones and conglomerates to the southwest and northeast. However, the sandstones and conglomerates in the northwest are deposited in a sheet like layering style (Figure 4.6). The dip-section shows a tight fold structure in the middle region with shales predominately deposited in the synclinal axis and sandstones on the anticline flank/limb to the northwest. The conglomerates are observed to the southeast of the syncline deposited further away from the axis of the stressed region. The unit thins out on both limbs of the syncline, with the SE limb being the thinnest. Zone interpretation The widespread sandstones and conglomerates (belonging to distributary channel facies) are erossively based and cut down deeply into the underlying units. This incising behaviour is thought to be as a result of the widespread lowering of the base level. The unit shows a mixed marine and fluvial deposition environments (i.e. wave/storm dominated in the northwest and fluvial in the southeast).The presence of condensed phases including thick coals and shales in the middle regions indicates a stable and low energy region protected from the waves/storm from the north and the strongly incising fluvial currents from the south. This allowed a significant amount coals to generate. The high proportion of shales in this region implies it was tidally influenced.
86
D B
C
A
Fig. 4.24. Composite of thickness map and lithofacies pie-charts for the zone HDX A-HZD A
87 4.2.3.3. Zone 16 (HZD A – HZ7 B) Zone description The unit has a similar geometric shape to the preceding unit (15) with the same thickness (~ 20 ft) in the centre and east but much thicker (~ 80 ft in the SE and ~ 50 ft in the NW) in all other directions (Figure 4.25). Deposition energy and sediment thickness, are distributed in a similar way as in unit 15 with the exception that the central region has more shales and coals than there is in unit 15. The unit has little conglomerates but more sandstone (blocky logs) than unit 15. The dip-section shows the unit has been eroded in some locations to the SE of the synclinal structure by the basal conglomerates of the overlying unit 17 (Figure 4.5). The section further shows the tendency for the grain-size, and therefore depositional energy to increase in both directions away from the synclinal axis. The strike-section, however, shows a uniform thickness of the unit (~ 65 ft) with basal conglomerates in the SW and an erosive top in the NE (Figure 4.6). The unit shows overall grain-size thinning upward trends. Zone interpretation The blocky nature for the logs in the SW, SE and NE could imply a prograding fluvial system of braided streams that flowed in the northwest direction and gradually lost their energy in the central region, just before the flanks of the synclinal structure. The region around the synclinal structure shows a mixed marine (tidal) and fluvial environment which allowed the deposition of the suspended fine sediments and generation of coals. The sandstones deposits in the NW are estuarine facies interpreted as reworked mouthbar deposits. These estuarine facies were overlain by shales (here referred to marine or basin facies)
88
D B
C
A
Fig. 4.25. Composite of thickness map and lithofacies pie-charts for the zone HZD A-HZ7 B
89 4.2.3.4. Zone 17 (HZ7 B – PCH A) Zone description In the middle of this zone is a thick round body of sediments composed predominantly of blocky sandstones and some heterolithics sandwiched in coals above and below the sequence and laterally surrounded by deposits of condensed phases and heterolithics. Thick coals occur almost everywhere in the zone deposited on top of the sequence. Zone interpretation The middle section of the zone is probably a point of emergence where two strongly eroding systems (estuarine and fluvial) approached one another from opposite directions, blending and reworking their sediment loads. This interaction could possibly only allow deposition of sandstones and heterolithics in the high energy environment that prevailed at the time and the fine facies laterally increased further away from the centre where the environment was relatively calm. At the peak of the transgressive period (possibly a break or end of sealevel rise) the overall environment became stable allowing the fine sediments still in suspension to precipitate out of the shallow fresh/saline water mixture. This in turn gave rise to the creation of huge swamps, hence the generation of thick coals observed on top of the sequence. The situation in the middle region of this zone is comparable to what Ferm and Weisenfluh (1988), in their Allegheny model, described as mouthbar sands between the Upper and Lower Delta plains.
90
D
B
C
A
Fig. 4.26. Composite of thickness map and lithofacies pie-charts for the zone HZ7 B-PCH A
91 4.2.3.5. Zone 18 (PCH A – PCH I) Zone description The opposite situation as regard to thickness is to be observed here compared to the previous zone. The previous fluvial and estuarine erosive surfaces to the east and west of the region have been backfilled by loads of fine sediments and now appear to be thicker than the midfield area. There are more shaly facies in the middle than sandstones. There seems to be a weaker fluvial stream flowing through the middle region from the south towards the north. The proportion of sandstone and heterolithics are observed to increase or decrease laterally in opposing directions, whereas the former decreases northwards and the latter southwards. Like in the previous zone a considerable proportion of coal exists, thick at the bottom and relatively thin and discontinuous at the top of the sequence. Zone interpretation This seems to have been rejuvenation of the transgressive period (sea-level continued to rise) but with a change in flow directions for both the fluvial and estuarine systems. The estuarine flowed in from the north towards the south and the fluvial from the south towards the north. Both systems are observed to have laterally and continually lost energy in their flow direction as indicated by the decreasing amounts of sand and increasing heterolithic components in the northern direction. The conditions still favoured the generation of coals but were less stable than in the previous sequence.
92
D
B
C
A
Fig. 4.27. Composite of thickness map and lithofacies pie-charts for the zone PCH A-PCH I
93 4.2.3.6. Zone 19 (PCH I – PCH B) Zone description The zone is more-or-less homogeneous in thickness (~ 45 – 40 ft). Well logs in the south exhibit a blocky trend and are predominantly composed of sandstones. Borehole core logs for wells located in the northwest show a coarsening-upwards trend and begin the sequence with thin coals at the bottom and end with limestones at the top, as seen from the log for well BRDBTTM033. Midfield the sediments show a coarsening-upwards trend, with coals at the bottom and top of the sequence. Shales and heterolithics are proportionately higher than sandstones. Zone interpretation This zone can be divided into three depositional environments, i.e. a purely fluvial environment in the south, a mixed marine/fluvial environment in the middle, and a purely shallow marine environment in the north. The fluvial environment in the south is energetically high with braided steams flowing in from the south (as indicated by the blocky well-log with sandstones only) and prograding northwards into the deltaic system, midfield. The depositional environment seems to have been relatively stable midfield and northwards as indicated by the presence of high proportions of the condensed phase, the thick coals, and the limestones in well logs. These logs also show a coarsening-upwards trend that is typical of sediment logs in a lower delta plain. The presence of Ironstone nodules logged in well BRDBTTM015 implies the logged sandstones midfields are mouth-bar sandstones. This interpretation is also in agreement with the Allegheny model of Ferm and Weisenfluh (1988), which explains the situation where the sandstones are found capped by ironstone or limestone. In such a depositional environment it is urged that the water depth remained relatively deep and never attained a sufficiently shallow level to allow plants to take root and peat accumulation to begin. Instead marine and freshwater faunas can occupy the abandoned platforms, and in cases where sediment supply is sufficiently low carbonates will be precipitated (Ferm and Weisenfluh, 1988).
94
D
B
C
A
Fig. 4.28. Composite of thickness map and lithofacies pie-charts for the zone PCH I-PCH B
95 4.2.3.7. Zone 20 (PCH B - BRS A) Zone description The depositional process in this unit seems to have been retrogradational, i.e. the deposited sediments were supplied by sea waves and storms. The zone is thickest in the northwest (~ 100 ft) and thinnest in the southeast (~ 30 ft). The high energy sediment facies increase in the northeast direction, while the coals and shales increase in the reverse direction. Heterolithics and shales were deposited in equal and relatively significant proportions around the core of the syncline structure midfield. Zone interpretation The high energy sands in the northwest could possibly be a result of reworked sediments by sea waves and storm currents, forming a highstand system tract. The environment became relatively calmer toward the southeast, possibly influenced with tides, allowing the development of peat and precipitation of shales. The generation of coals also suggests there was some minimal fluvial influence from the southeast that created a mixed shallow saline/fresh water environment required for the generation of peat.
96
D
B
C
A
Fig. 4.29. Composite of thickness map and lithofacies pie-charts for the zone PCH B-BRS A
97 4.2.3.8. Zone 21 (BRS A - BRS B) Zone description The zone is predominantly composed of sandstone facies, which gradationally decreases in the northwest direction after the anticline structure midfield, being steadily replaced by the shale facies. Well HRLD033, in the extreme northwest shows a predominance of shale deposits (Figures 4.5 and 4.30). The zone is thickest (~70 ft) in the southeast and thinnest (~ 40 ft) in the southwest. Well logs are primarily blocky in shape. Coals are thickest midfield around the anticline and tend to be thin and discontinuous toward the northern. In the dip-section, the stratigraphy shows a tight synclinal structure in the middle of this zone, flanked with relatively broad anticlines in the southeast and northwest directions. Along this section the unit shows a significant inclination or slope in the NW (Figure 4.5a). The strike cross-section, however, the unit shows a broad and gentle dome/anticline structure gently dipping toward the NE (Figure 4.6a). Along this section the zone gradually thickens toward the NE. Zone interpretation The syncline and anticline structures midfield influenced the distribution of depositional energy in the area. The depositional environment to the right of the syncline is entirely continental with fluvial sandstone deposits by braided streams that prograded from the southeast. The depositional environment to the left of the syncline is mixed fluvial and marine. After the peak of the anticline the fluvial streams seem to have lost the power to transport the high energy sediments on meeting marine tides and started to precipitate the condensed phases (Figure 4.5a). The existence of coals around the anticline region is an indication of a reduction in the rate of sedimentation that caused a stable climate suitable for the development of peat. The lateral decrease in the proportion of the logged sandstones and increase in the condensed facies after the anticline implies a continuous attenuation of stream energy in the direction of progradation towards the sea.
98
D
B
C
A
Fig. 4.30. Composite of thickness map and lithofacies pie-charts for the zone BRS A-BRS B
99
5.0. DISCUSSION 5.1. Evolution of the delta in the quadrangle Broad bottom and its Implications for Reservoir geology and Petroleum geology The mapped stratigraphic interval in Broad bottom clearly illustrates the three broad divisions of the Upper Breathitt Group in this part of the foreland basin. The evolution of the delta will be explained using the stratigraphic framework of the deposited lithofacies logged vertically along well BRDBTTM004 (Fig.5.1), which almost wholly penetrates the entire thickness of the Breathitt Group. The lowest division (the Pikeville Formation, units 0 to 5, Figures 4.5b, 4.6b, and 5.1a) was deposited during the lower half of the Westphalian B (~ 315 to ~ 312.6 Ma). It is comprised of 6 predominantly transgressive zones with a maximum preserved thickness of about 405 ft in the southeast and 395 ft in the northwest, as calculated between the stratal surfaces BLR B and UE3 B (Figures 5.1a, 10.1 and 10.7). The interval is predominantly marine and maintains a constant thickness in the area of Broad bottom. Deposition most probably took place under conditions of an overall sea-level rise, initially yielding a series of cyclic parasequence deposits of Transgressive System Tracts (TST) predominantly stacked with transgressive deposits of shallow marine bay-fill (shales and heterolithics), with minor estuarine sandstones in units 0, 1, and 2. The upper units of this interval (units 3, 4 and 5) show an increase of fluvial influence in an environment that was predominantly marine, as seen from the increasing frequency and thickness of the coal seams and sandstone facies. This indicates an oscillating sea level that caused alternations in the deposit of fluvial sandstones by meandering streams from land and marine shales due to sea tides, forming a series of prograding High System Tracts (HST) and ultimately depositing, in some localities, conglomerates in the Falling Stage System Tact (FSST), see Figure 5.1a. The deposits in the lower part of this interval may imply the basin was underfilled possibly due to thrust loading in the southeast that caused the basin to deepen faster than the rate of sedimentation. The thick widespread coals in the upper parts of this interval imply periods of sediment starvation; with ever-wet paleoclimates (Figure 4.6b, units 3, 4 and 5; and Figures 4.12, 4.14 and 5.1a). In terms of reservoir characterization, the predominantly marginal marine environment of this division acted as host and preserved a large diversity of organic matter. The dip-section (see Figure 4.5b) shows deposition in the lower proximal end of this interval to have been tidally influenced, while deposition in the distal end was dominantly influenced by storms and waves. Consequently the interval has thin, but laterally continuous reservoir sandstone facies deposited in the northwest and predominantly non-reservoir facies of shales and thick coals in the southeast. The great depth of burial of this interval ensured the optimal temperatures required to convert the fossilized organic matter into liquid hydrocarbons and coals of high quality. The interval may thus provide a suitable source rock for the generation of hydrocarbons. The stratas have a very low gradient and are slightly deflected in the middle to imply a small and therefore
100 negligible tectonic (structural) influence, which possibly happened long after the deposition and compaction of the interval and has no impact on fluid dynamics in this interval.
Fig. 5.1. (a) Vertical log (along well BRDBTTM004) illustrating the Sequence Stratigraphic framework of the Pikeville Formation (the lowermost interval of the Breathitt Group) in the lower coastal plains of the central areas of Broad bottom. Note the predominance of the transgressive marine deposits in the lower parts and the alternating marine and fluvial deposits in the upper parts of the interval. Arrows point in the direction of decreasing grain size, the numbers to the left of the logs (not in bold) are stratigraphic elevations above mean sea level. The numbers written in bold are the individual stratal zones. The sequence stratigraphic abbreviations TST = Transgressive System Tract, HST = Highstand System Tract, FSST = Falling Stage System Tract, LST = Lowstand System Tract, SB = Sequence Boundary and FS = Flooding Surface. The abbreviations to the extreme right of the logs are names of the correlatable extensive coal seams (see Table 3.2). Refer to the color chart in Figure 4.4 for the identity of the lithofacies in the logs.
101 The middle division (the Hyden Formation, units 6 to 13, see Figures 5.1b, 4.5 and 4.6) was deposited during the second half of Westphalian B (~ 312.6 to ~ 309.4 Ma), under oscillating conditions of sea level giving rise to alternating marine and sandstone deposits. The pronounced sandstones are probably forced regression sequences with coarse grained sediments deposited during periods of sea level fall and sandwiched between marine shales, which were deposited during periods of seal-level rise. The thick laterally interconnected sands of units 7, 8, 10 and 11 were either deposited as a result of the transgressing sea waves/storms reworking and incising unconformities from the forebulge arch in the northwest or by rapidly flowing braided streams prograding from the thrust fronts in the southeast during lowstand. These sandstones are relatively thick, laterally extensively and contain in several locations large bodies of conglomerates (Figures 4.5 and 4.6). They hence qualify to be high quality reservoirs, although the extensive transgressive deposits of shales between them may act as barriers to vertical flow of hydrocarbons.
Fig. 5.1. (b) Vertical log (along well BRDBTTM004) through the Upper Coastal plain system, the Hyden Formation, Note the high frequency of alternating sandstones and marine facies. The facies in unit 7 was labeled unknown in the drillers log, but has been interpreted as a sandstone facies in this study (see Figure 4.5b and 4.6b).
102 The uppermost division of the Breathitt Group in Broad bottom (also called the Four Corners Formation) including units 14 to 21 was deposited during the Westphalian C, between 306 to about 308.4 Ma (i.e. over a period of 2.4 million years). The Four Corners Formation has a maximum preserved thickness of approximately 420 ft in the southeast and 390 ft in the northwest. Tectonics and subsidence seem to have played a major role in the deposition of this interval. Initially, the high rate of overthrusting and subsidence in the southeast gave rise to the transgressive deposits of the Magoffin, which is predominantly marine (with shale and heterolithics lithofacies) at the base. This was followed by a period of rapid transport and depositing of high frequency sequences composed of conglomerates and homogenous immature sediments (due to a rapid drop in base level) as the sea-level rapidly oscillated giving rise to huge amounts of forced regressive deposits or incised valley-fills by fluvial braided streams (unit 15 and 16). The sea-level suddenly began to rise contemporaneously with a high rate of subsidence causing widespread marine inundations and transgressive deposits of shales and heterolithics (units 17 and 18). Subsequently rapid fluvial sedimentation, eroded from the growing orogeny in the southeast, overwhelmed the rate of subsidence causing the basin to overfill with immature sediments (units 19 to 21, Figures 4.5a, 4.6a and 5.1c). The vertical log along the well BRDBTTM004 shows alternations of thick fluvial sandstones, conglomerates, shales and coals. However, as in the middle coastal interval, the shales and coals may act as barriers to vertical flow. Unlike the middle interval, the sandstones facies tend to be vertically and laterally stacked forming very extensive and thick reservoir units in several places (Figure 4.6a). The synclinal structure is more pronounced in the Four Corners Formation, with the stratal units stretched thin along the limbs of the syncline. This implies that the tectonics responsible for the formation of this structure is contemporaneous with the deposition of the existing lithofacies, and the structure may have influenced the distribution of the depositional energy in the area. It is clearly evident that the tectonic energy of depression declined with depth, affecting the stratal units in the Four Corners Formation most whilst hardly noticeable in the lower Pikeville Formation (Figure 4.5). The stratal gradients are also observed to increase vertically upward through the stratigraphic column of the Breathitt Group, with the stratas of the Four Corners Formation being the most inclined and those of the Pikeville Formation the least inclined. All this evidence appears to confirm the revival of tectonic activity in the area around the period of Westphalian C, the time when the Four Corners Formation was being deposited. Renewed orogenesis lifted the foreland basin in the southeast causing rapid erosion and sedimentation of coarse grained material by fluvial braided streams toward the northwest. The deposition of coarse grained clastic sediments by braided streams in the Four Corners Formation implies that the reservoirs formed have increased permeability to fluid flow. Furthermore, increased inclination of the reservoirs causes increased free flow of fluids by gravity, which may lead to a significant reduction in production costs in the early development phase. However, care
103 must be taken to avoid a rapid depletion of the reservoir or early water breakthrough during production.
Fig. 5.1. (c). Vertical log (Well BRDBTTM004) through the Four Corners Formation in the quadrangle Broad bottom. Note the alternating nature of marine and fluvial deposits. In this system marine units include: units 14 (the Magoffin), 17, 18, and the Stone Fork Shale Member above surface BRS B. Units 15, 16 and 17 contain conglomerates, an indication of rapid transport of fluvial sediments at a lowered base level and uplift of the mountain thrusts in the southeast by the tectonics.
104 The Delta evolution in Broad bottom can be summarized as follows: (1) a stratigraphy divisible into two broad parts; a lower division (including the Hyden and Pikeville Formation) which is overall retrogradational and was deposited predominantly by Transgressive Systems Tracts (TST) during the age of Westphalian B, and the upper part which is progradational/aggradational and was predominantly deposited by Lowstand System Tracts (LST) during the age of Westphalian C. (2) differential accommodation is demonstrated by the lateral increase in thickness of the individual units from the basin margin (distal end) toward the basin axis (proximal end). This is a strong indication that tectonics, subsidence and eustasy were all responsible, although in different proportions, for the creation and destruction of accommodation space in the foreland basin. (3) The synclinal structure (oriented approximately NE-SW) influenced the thickness of the deposited stratas in the upper formations of the Breathitt Group. Stratal thickness changes appear more pronounced across the axis of the syncline, with the units tending to thin onto the flanks of this structure. This effect is more pronounced in the fluvial/upper parts of the stratigraphic column, the Four Corners Formation (Figure 4.5), where the tectonic movements had the greatest impact during the Atokan orogeny. (4) Tidal and estuarine facies are widespread in the lower half of the stratigraphy, while the upper half is mainly dominated by coarse grained fluvial sandstone facies.
5.2. Reservoir characteristics The reservoir properties for the various lithofacies have been discussed in terms of the following aspects: (1) geometry (including thickness and lateral extent); (2) orientation (within the basin); (3) internal architecture; and (4) lateral and vertical heterogeneity. However, reservoir quality was not adequately covered, owing to a general lack of porosity and permeability data. Results of the facies analysis along the Breathitt Group stratigraphic column in Broad bottom show there is a considerable decline in reservoir quality with depth. The lowest division can only be categorized as a source zone, with hardly any significant reservoir. The middle division can be both a source and a reservoir zone for hydrocarbons. The reservoirs here are a result of wave reworking and are separated by thin but extensive shales. The upper division offers reservoirs of high quality. The thick sandstone bodies are laterally connected and vertically amalgamated. The reasons for these hard conclusions have already been given in the preceding section. 5.2.1. Flow barriers Shales are known to form the most common types of flow barriers in clastic reservoirs. However, as observed from the general stratigraphy, shale lithofacies in Broad bottom vary in thickness and lateral extent. In the coastal plain systems and the transgression members (zones 0 to 14) thick and thin laterally continuous and discontinuous shales are commonly observed within or between the stratigraphic units, enclosing the clean sandstone reservoir
105 bodies. These shales may constitute barriers to fluid flow, let alone forming pressure seals between reservoirs. Discontinuous shales too may cause a significant increase in the tortuosity of fluid flow paths. The situation is somehow different in the fluvial system, where mostly thick and thin discontinuous shales exist. Here the degree of connectivity or stacking between sandstone reservoir units is so high that vertical and horizontal fluid flow may not be adversely reduced. Thick and laterally continuous coals may also form fluid flow barriers, mainly in a vertical direction. In Broad bottom however, such coals are not commonly observed between reservoirs. The thick coals in the fluvial system were eroded by the incising coarse grained debris flow (the conglomerates) enabling perfect vertical connectivity of sand bodies between succeeding stratigraphic units. 5.2.2. Sand body interconnectness (Reservoir communication) Reservoir Internal communication, most often referred to as the plumbing system of the reservoir, is a factor of critical importance in the production of hydrocarbon. The sandstone units in the fluvial system show a high degree of connectivity both laterally and vertically. The shales in this system are equally important in terms of providing the sealing mechanisms for the accumulated hydrocarbons and helping to keep the reservoir pressure constant during production. The transgressive marine strata (unit 17), however, divides the fluvial system into two broad reservoirs: (1) units 18 to 21 form the upper reservoir, whilst (2) units 15 and 16 form the lower reservoir. The lower reservoir is however punctured in several locations along the strike with conglomerates, as incised valley fills. These are areas of extremely high permeability and may turn out to be thief zones during drilling and production. The upper fluvial reservoir is however the best one can ever find in a foreland basin. The sandstone reservoirs here are thick, and able to communicate in all directions. They are optimally inclined to enable production by gravity drainage. The upper coastal plain (units 7 to 13) has its sandstone bodies well connected but separated in the vertical direction by laterally extensive shales. In this case a regional fluid flow is only possible in the horizontal direction along the strata, being hindered in the vertical direction by the shales. In this system too patches of conglomerates exist, possibly deposited as forced regressions or slope channel deposits. The sandstone reservoirs in this section have no vertical communication but may nevertheless form deepwater reservoirs, which todate are considered to be important targets for petroleum exploration. The lower coastal plain is predominantly marine and composed of shales and heterolithics. A good reservoir can hardly be obtained here, but the system, with its great depth of burial and abundance of preserved organic matter, may provide a high quality source for the generation of hydrocarbons.
106
6.0. CONCLUSIONS Well correlations in the study area revealed the basin geometry to be typical of foreland basins (i.e. thickest toward the thrust front and thinnest toward the forebulge in the direction of the sea. Based on the presence of extensive coal seams and marine transgressions in Broad-bottom, 22 stratigraphic surfaces (sequences) were recognized and correlated (using Petrel) in the constituent Pikeville, Hyden and Four Corners Formations. Thickness maps between these surfaces have been constructed (in Petrel) equipped with facies pie-charts at locations of boreholes that wholly penetrate the individual units to facilitate the description and interpretation of the distribution of depositional environments, facies associations and energies in each stratigraphic unit. Facies architecture has been interpreted based on the systematic variations in processes that occurred throughout each zone including the evolution of depositional environments. This is because better predictions of earth resources (petroleum reserves, coal resources, groundwater aquifers and sedimentary mineral deposits) require an improved understanding on the fundamental controls (accommodation space and the processes inherent to depositional systems) on the stratigraphic architecture. The use of composite maps of isochores and pie-charts for facies modeling allows a better prediction of vertical and lateral geometrical dimensions and distribution of reservoirs and non-reservoir bodies particularly in mature fields with lots of data between production wells. The method represents one of the new industrial contributions to the latest developments in stratigraphy toward problem solving in reservoirs, using 3D computer graphics to display and analyse geology. The facies pie-charts are practically reliable in determining locations for infill wells in areas of unexploited reservoirs or bypassed oil in old fields, so as to maximize production. In addition the method allows the determination of the following: (1) direction of paleoflow in the depositional systems and (2) the lateral distribution of physical energy in the depositional systems, which in turn allows the various environments of deposition in the area of investigation to be established. The Breathitt Group in Eastern Kentucky is progrational, and can be divided into three broad formation-systems: the Four Corners (fluvial-deltaic), the Hyden (coastal plain) and the Pikeville (coastal/marginal marine). The three formations are stratigraphically separated by marine transgression shale members including the Betsie Shale Member at the base of the Pikeville Formation, the Kendrick Shale Member, between the Hyden and Pikeville Formations, the Magoffin Shale Member between the Hyden and Four Corners Formation, and the Stoney Fork Member on top of the Four Corners Formation.
107 The pie charts clearly demonstrate that the Breathitt Group in the foreland basin is characterized by two distinct styles of deposition: (1) the gradual decrease in depositional energy in the coastal plain system toward the basin centre (i.e. in the landward direction), signifies the predominance of deposition influenced by sea storms/waves and tides, giving rise to an underfilled basin with transgressive system tracts and shallow marine bay-fill deposition. This pattern is attributed to thrust loading, which deepened the basin faster than the rate of sedimentation, and (2) the broad and homogeneous spread of energy observed in the in the fluvial system, however, with a slight tendency to decrease toward the distal end (i.e. towards the sea). Such a wide lateral spread of energy can only be attained by the high energy fluvial and meandering streams that sourced their sediment loads directly from the thrust fronts in the southeast. “The change from paralic sedimentation in the Magoffin trough to sand-dominated alluvial plain deposition was rapid, probably due rejuvenation (arrival) of the source terrane. Coarse clastics were shed from the orogene faster than the subsiding foreland basin could accommodate it, resulting in an overfilled basin”, Tankard (1986). Braided and coarse-grained meandering stream deposits with subordinate amounts of paleovalley fills, bay-fill and splay deposits are mostly common in the fluvial-deltaic system of the Four Corners Formation. The dip and strike crosssections show successions consisting of amalgamated fluvial sandstones, interpreted to lie within incised valleys (units 15 and 16). Going up the stratigraphic column the depositional environment becomes more influenced by tides or marine transgressions with a considerably large proportion of shales and heterolithics deposited, including a few isolated bodies of sandstones (units 17, 18, and 19). This is again followed by a succession of vertically stacked and laterally extensive channel sandstones bodies (units 20 and 21). These changes in deposition are mainly attributed to changes in relative sea level and tectonics. The tectonic influence on the drainage pattern and distribution of sands in the fluvial and upper coastal systems (the Four-Corners and Hyden Formations) is further exhibited by the presence of a series of fold structural elements (small anticlines and synclines) in the basin, which together with the shales may influence the mechanisms of flow, sealing/trapping and accumulation of hydrocarbons in the reservoir sandstone bodies. Analysis of these series of fold structures in Broad bottom reveals tectonic movements whose intensity gradually attenuated down the stratigraphic column, from the top stratas of the Four Corners Formation through the base of the Pikeville Formation. This observation confirms the remarks by Greb et al. (2002) that the majority of movements in the Appalachian foreland basin are post-Middle Pennsylvanian, most probably during the Ouachita–Alleghenian Orogeny. The Fluvial system offers better sandstone reservoirs (laterally and vertically stacked sandstone bodies) than the coastal plain system, where laterally extensive shales, sandwiching the sandstones, may act as barriers to fluid flow.
108 The reservoirs in the fluvial system are also optimally inclined to allow free fluid flow by gravity drainage. Conglomerates are widely spread in both the upper coastal plain system and in the lower part of the fluvial system, possibly deposited as forced regressions and incised channel fills during periods of lowstand. From the petrophysical point of view these units are highly permeable and may act as thief zones during drilling and production of hydrocarbons. However, careful facies analysis may provide guidelines as to the positioning, spacing, and number of wells planned for the production or to attain enhanced oil recovery (EOR). Chesnut (1996) and Greb et al. (2002) estimated a duration 400 ka for the deposition of each coal-clastic cycle, which is analogous to the long-earth eccentricity cycle. Based on this assumption it can be concluded that the mapped interval of the Breathitt Group in Broad bottom, ranging from the base of the Pikeville Formation almost to the top of the fluvial system in the Four Corners Formation, was deposited in approximately 10 Million years. These coal-clastic cycles are therefore eustatically controlled fourth-order sequences grouped into sequence sets to form third-order sequences. The distribution of the extensive coal seams can be used as indicators of the paleoclimate in this part of the Central Appalachian Foreland Basin. Generation of such peats required an ever-wet climate to initiate doming and leaching that gave rise to the production of good quality coals mined in Eastern Kentucky. The ever-wet and warm climates dominated the early and Middle Pennsylvanian, whereas, seasonally drier climates dominated the late Middle and Upper Pennsylvanian (Eble and Grandy, 1990; Esterle, 1992; Greb et al., 2002).
109
7.0. RECOMMENDATIONS This thesis tried to establish a facies model including the sequence stratigraphic framework and facies architecture of the upper part of the Breathitt Group in Eastern Kentucky (which is part of the Central Appalachian Foreland Basin), primarily by correlating and interpreting laterally extensive coal seams and shale lithofacies obtained chiefly from cored borehole data. The study should be able to serve as a basis for further research on analysis of facies architecture and sequence stratigraphy in other coal-bearing fluvial and marginal marine analogs and subsurface reservoir deposits in foreland basins. More detailed work is required on the distinction of tectonics versus eustatic controls and on regional variations of the paleoclimate, based on field studies. The use of coal seams as genetic stratigraphic sequence boundaries also requires further research to be confirmed correct. Nevertheless, thick laterally extensive coal seams are considered generally to be a good sensitive indicator of a high accommodation potential, which occurs during marine transgression at the time of maximum flooding (Aitken, 1995). From a hydrocarbon exploration and production point of view, very deep wells penetrating the entire thickness of the Breathitt Group should only be drilled for exploration purpose during the appraisal phase and not for production. The deeper parts of the Breathitt Group, particularly the lower coastal plain (e.g. units 0 to 6), has been observed predominantly to be marine with lithofacies deposited under the influence of tides, waves and storm actions with little fluvial interaction. However, since this system is at a great depth and rich in organic matter it should therefore serve a perfect source region for the generation of hydrocarbons. The upper coastal plain, however, consists of thick clean laterally connected sandstone bodies encased in thin continuous shale layers. There is no vertical communication between the sandbodies due to the sealing action of the shales. Drilling vertical wells for production purpose from this region can be quite costly, as production casings have to be perforated in several reservoir sections. The solution to this problem could be to drill a few vertical wells from which several side tracking horizontal wells can be drilled through each reservoir unit for several kilometers in the dip direction. There are several conglomerates or very coarse grained sediment bodies observed in the upper coastal plain. These are local sources of highly permeable zones to fluid flow and may be considered thief zones when directly drilled through and produced from them. This may lead to a rapid depletion of the reservoir causing an early water breakthrough and abandonment. A good analysis of the facies architecture and distribution should serve as a guide for the reservoir engineers to avoid making perforations in the production casing at the locations of such bodies. To maintain pressure in the reservoirs by water flooding, these thief zones should be shut off for the injection to reach the target reservoirs. When left unsealed, the thief zones will take most of the injected water, causing it to be rapidly swept out of the reservoir, leading to an early water
110 breakthrough at the production well. This will reduce the driving force needed to push most of the oil out of the reservoirs. Another disastrous effect of such thief zones is that they may cause huge losses of drilling mud if left unsealed during drilling. It is therefore essential that such zones are detected during the exploration phase and sealed off with a liner or plugged with special cements or fibrous clogging agents before drilling can resume. It has been observed that the fluvial-deltaic system consists of two parts: (1) the reservoir sandstone zones in the lower part of the system (units 15 and 16), next to the Magoffin transgression which are characteristically similar to those in the upper coastal plain, and have several very coarse grained sediment bodies (conglomerates). Here the conglomerates are even more laterally continuous than they are in the upper coastal plain. They too may be considered thief zones (excessively permeable) and direct production from their localities should be avoided, and (2) the upper part of the fluvial system, separated from its lower counterpart by a marine transgression zone (unit 17), is however characteristically different. In this part of the system the reservoir facies are both laterally and vertically connected, forming very thick and laterally extensive reservoirs which can be effectively produced by vertical or inclined wells.
111
8.0. REFERENCES Aitken, J.F. (1995) Utility
of Coal Seams as Genetic Stratigraphic Sequence Boundaries in Nonmarine Basins: An Example from the Gunnedah Basin, Australia: Discussion. AAPG
Bulletin, vol. 79, No. 8, 1179—1181. Aitken, J.F. and Flint, S.S. (1996) Variable expressions of interfluvial
sequence boundaries in the Breathitt Group (Pennsylvanian), eastern Kentucky, USA. From Howell, J.A. and Aitken, J.F. (eds), High resolution sequence stratigraphy: innovations and applications, Geological Society Special Publication 104, 193–206. Aitken, J.F., Howell, J.A. (1996) High
resolution sequence stratigraphy: innovations, applications and future prospects.
From Howell, J.A. and Aitken, J.F. (eds), High resolution sequence stratigraphy: innovations and applications, Geological Society Special Publications 104, 1–9. Andrews, W.M. Jr., Hower, J.C., Hiett, J.K. (1994) Lithologic and
geochemical investigations of the Fire Clay coal bed, southeastern Kentucky, in the vicinity of sandstone washouts. International Journal of Coal Geology 26, 95–115. Brennington, J.B. (1996) Stratigraphic and biofacies patterns in the Middle Pennsylvanian Magoffin Marine Unit in the Appalachian Basin, U.S.A. International Journal of Coal Geology 31, 169–193. Buter, S.J., de Leeuw, C.S., The, R.D.L., and Verstralen, I.R.M.J. (1991)
Architecture of delta plain deposits in a sequence stratigraphic framework: the Upper Carboniferous of Eastern Kentucky. Field studies. Chesnut, D.R (1996) Geologic framework for the coal-bearing rocks of the Central Appalachian Basin. International Journal of Coal Geology 31, 55-66. Chesnut, D.R. (1980) The Mississippian-Pennsylvanian Eastern Kentucky. Kentucky Geological Survey.
contact in
112 Chesnut, D.R. (1979) Source
of the volcanic ash deposit (Flint Clay) in the Fire Clay Coal of the Appalachian Basin.
Kentucky Geological Survey, 145—154. DeCelles, P.G., and Giles, K.A. (1996) Foreland Basin Systems. Basin Research 8, 105—123. Dorsch, J., Bambach, R.K., and Driese, S.G. (1994) Basin-rebound
origin for the “Tuscarora unconformity” in Southwestern Virginia and its bearing on the nature of the Taconic Orogeny. American Journal of Science, vol. 294, 237—255. Eble, C.F., and Hower, J.C. (1995) Palynologic, petrographic and geochemical characteristics of the Manchester coal bed in Eastern Kentucky. International Journal of Coal Geology 27, 249 – 278. Eble, C.F., and Grady, W.C. (1990) Paleoecological
interpretation of a Middle Pennsylvanian coal bed in the central Appalachian basin, U.S.A. International Journal of Coal Geology 16, 255 –286. Eble, C.F., Hower, J.C., and Andrews Jr., W.M. (1999) Compositional variations in the Fire Clay Coal Bed of Eastern Kentucky: Geochemistry, Petrography, Palynology, and Paleoecology. Kentucky Geological Survey, series 11, Report of Investigations 14. Einsele, G. (1992) Subsidence, chapter 8, 313—320. In:
Sedimentary Basins, Evolution, Facies, and Sediment budget. 2nd edition, Springer Verlag 2000, 792 pp. Ferm, J.C., Weisenfluh, G.A. (1989) Evolution
of some depositional models in Late Carboniferous rocks of the Appalachian coal fields. International Journal of Coal Geology 12, 259 –292. Filer, J.K. (2003) Stratigraphic evidence for a Late Devonian possible back-bulge basin in the Appalachian basin, United States. Basin Research 15, 417—429. Greb, S.F., Eble, C.F and Chesnut Jr., D.R. (2002) Comparison of the Eastern and Western Kentucky coal fields (Pennsylvanian), USA—why are coal distribution patterns and sulfur contents so different in these coal fields? International Journal of Coal Geology 50, 89–118.
113 Greb, S.F., Eble, C.F., J.C. Hower, J.C. (1999) Depositional history
of
the Fire Clay Coal bed (Late Duckmantian), Eastern Kentucky, USA. International Journal of Coal Geology 40, 225–280. Greb, S.F., Eble, C.F., J.C. Hower, J.C., W.M. Andrews, W.M. (2002)
Multiple-bench architecture and interpretations of original mire phases—Examples from the Middle Pennsylvanian of the Central Appalachian Basin, USA. International Journal of Coal Geology 49, 147– 175. Greb, S.F., Weisenfluh, G.A. (1996) Paleoslumps
in coal-bearing strata of the Breathitt Group (Pennsylvanian), Eastern Kentucky Coal Field, U.S.A. International Journal of Coal Geology
31, 115 – 134. Greb, S.F., Hiett, J.K., Weisenfluh, G.A., Andrews, R.E., and Sergeant, R.E. (1999) Geology of the Fire Clay Coal in parts the Eastern Kentucky Coal Field. Kentucky Geological Survey, Series12. Report of Investigations2. Greb, S.F., Chesnut Jr., D.R., and Eble, C.F. (2004) Temporal changes
in coal-bearing depositional sequences (Lower to Middle Pennsylvanian) of the central Appalachian Basin, U.S.A., in J.C. Pashin and R.A. Gastaldo, eds., Sequence stratigraphy, paleoclimate, and tectonics of coal-bearing strata: AAPG Studies in Geology 51, 89—120. Greb, S.F., and Chesnut Jr., D.R. (1992) Transgressive
channel filling in the Breathitt Formation (Upper Carboniferous), Eastern Kentucky Coal Field, USA. Sedimentary Geology, 75,
209—221. Greb, S.F., and Popp, J.T. (1999) Mining
geology of the Pond Creek seam, Pikeville Formation, Middle Pennsylvanian, in part of the Eastern Kentucky Coal Field, USA. International Journal of
Coal Geology 41, 25–50.
114 Haworth, R.T., Hipkin, R., Jacobi, R.D., Kane, M., Lefort, J.P., Max, M.D., Miller, H.G., and Wolf, F. (1988) Geophysical framework
and the Appalachian-Caledonide connection. In: A.L. Harris, and D.J. Fettes (eds), The Caledonian-Appalachian Orogen. Geological Society, London, Special Publications, vol. 38, 3—20. Heckel, P.H., Gibling, M.R., King, N.R. (1998) Stratigraphic model
for Glacial-Eustatic Pennsylvanian Cyclothems in Highstand nearshore detrital regimes. Journal of Geology, vol. 106, 373—383. Hower, J.C., and Pollock, J.D. (1989) Petrology of River Gem coal bed, Whitley County, Kentucky. International Journal of Coal Geology 11, 227 –245. Hower, J.C., Hiett, J.K., Wild, G.D., and Eble, C.F. (1994) Coal
Resources, Production, and Quality in the Eastern Kentucky Coal Field: Perspectives on the Future of Steam Coal Production. University of Kentucky, Centre for Applied Energy Research. Howell, P.D., and van der Pluijm, B.A. (1990) Early
history of the Michigan Basin: Subsidence and Appalachian tectonics.
Geology, vol. 18, 1195—1198. Martino, R.L., and Sandrson, D.D. (1991) Fourier
and autocorrelation analysis of estuarine tidal rhythmites, Lower Breathitt Formation (Pennsylvanian), Eastern Kentucky, USA. Journal of Sedimentary Petrology, vol. 63, No. 1, 105—119. Miall, A.D. (1991) Stratigraphic Sequences and their Chronostratigraphic correlation. Journal of Sedimentary Petrology vol. 61, No. 4, 497–505. Reed, J.S., Eriksson, K.A., and Kowalewski, M. (2005) Climatic,
depositional and burial controls on diagenesis of Appalachian Carboniferous sandstones: qualitative and quantitative methods. Sedimentary Geology, 176, 225—246. Shanley, K.W., and McCabe, P.J. (1994) Perspectives on the Sequence Stratigraphy of Continental Strata. AAPG Bulletin, vol. 78, No. 4, 544 – 568.
115 Tankard, A.J. (1986) Depositional
response to Foreland deformation in the Carboniferous of Eastern Kentucky. The
American Association of Petroleum Geologists Bulletin, vol. 70, No. 7, 853–868. Ver Straeten, C.A., and Brett, C.E. (2000) Bulge Migration and
Pinnacle Reef Development, Devonian Appalachian Foreland Basin. Journal of Geology, vol. 108, 339—352.
116
9. Other relevant literatures Amorosi, A., Centineo, M.C., Colalongo, M.L., Pasini, G., Sarti, G., and Vaiani, S.C. (2003) Facies architecture and Latest Pleistocene-
Holocene Depositional History of the Po Delta (Comacchio Area), Italy. Journal of Geology, vol. 111, 39—56. Anderson, W.H. (2008) Foundation Problems and Pyrite Oxidation in the Chattanooga Shale, Estill County, Kentucky. Kentucky Geological Survey, series 12. Report of Investigations 18. Bartov, Y., Enzel, Y., Porat, N. and Stein, M. (2007) Evolution of the Late Pleistocene–Holocene Dead Sea Basin from Sequence Stratigraphy of Fan Deltas and Lake-Level reconstruction. Journal of Sedimentary Research, vol. 77, 680–692. Carey, D.I., Hounshell, T.D. (2008) Kentucky Terrain: Kentucky Geological Survey, series 12. Map and Chart Series 187. Carey, D.I., Hounshell, T.D., and Kiefer, J.D. (2008) Geologic Hazards in Kentucky: Kentucky Geological Survey, series 12. Map and Chart Series 185. Carter, R.M., Fulthorpe, C.S., Naish, T.R. (1998) Sequence concepts
at seismic and outcrop scale: the distinction between physical and conceptual stratigraphic surfaces. Sedimentary Geology 122, 165–179. Coueffe, R., Tessier, B., Gigot, P., and Beaudoin, B. (2004) Tidal
rhythmites as possible indicators of very rapid subsidence in a Foreland Basin: An example from the Miocene marine molasse formation of the Digne Foreland Basin, SE France. Journal of Sedimentary Research, vol. 74, No. 6, 746—759. Darmadi, Y., Willis, B.J., and Dorobeki, S.L. (2007) Three-
Dimensional seismic architecture of fluvial sequences on the low-gradient Sunda shelf, offshore Indonesia. Journal of Sedimentary Research, vol. 77, 225—238. Davies, S.J., and Gibling, M.R. (2003) Architecture
of coastal and alluvial deposits in an extensional basin: the Carboniferous Joggins Formation of eastern Canada. Sedimentology 50, 515—
439.
117 Davis, Jr. R.A., Klay, J., and Jewell, P. (1993) Sedimentology
and Stratigraphy of tidal sand ridges southwest of Florida inner shelf. Journal of Sedimentary Petrology, vol. 63, No. 1, 91-104.
Di Celma, C., Ragaini, L., Cantalamessa, G. and Landini, W. (2005)
Basin physiography and tectonic influence on sequence architecture and stacking pattern: Pleistocene succession of the Canoa Basin (central Ecuador). Geological Society of America Bulletin 117, 1226– 1241. Dickinson, W.R., and Gehrels, G.E. (2008) Sediment
delivery to the Cordilleran Foreland Basin: Insights from U-Pb ages of detrital zircons in Upper Jurassic and Cretaceous strata of the Colorado plateau. American Journal of Science, vol. 308, 1041—1082. DiMichele, W.A., Eble, C.F., Chaney, D.S. (1996) A drowned lycopsid forest above the Mahoning coal (Conemaugh Group, Upper Pennsylvanian) in eastern Ohio, U.S.A. International Journal of
Coal Geology 31, 249 – 276. DiMichele, W.A., Pfefferkorn, H.W., Gastaldo, R.A. (2001) Response
of Late Carboniferous and Early Permian plant communities to climate change. Annual Review of Earth Planetary Sciences 29, 461–487. DiMichele, W.A., and Phillips, T.L. (1994) Paleobotanical and
paleoecological constraints on models of peat formation in the Late Carboniferous of Euramerica. Paleogeography, Paleoclimatology, Palaeoecology 106, 39—90. Donnelly, L.J. (2006) A review of coal mining induced fault reactivation in Great Britain. Quarterly Journal of Engineering Geology and Hydrogeology, vol. 39, 5—50. Dott, Jr. R.H. (1988) Perspectives: Something old, something new,
something borrowed, something blue—a hindsight and foresight of sedimentary geology. Journal of Sedimentary Petrology, vol. 58, No. 2, 358—364.
118 Duke, W.L. (1990) Geostrophic
circulation or shallow marine turbidity currents? The dilemma of paleoflow patterns in storm-influenced prograding shoreline systems. Journal of
Sedimentary Petrology, vol. 60, No. 6, 870—883. Esterle, J.S., Ferm, J.C., and Ylu-Liong, T. (1989) A
test for the analogy of tropical domed deposits to “dulling up” sequences in coal beds—Preliminary results. Organic Geochemistry, vol. 14,
No. 3, 333—342. Frederiksen, K.S., Clemmensen, L.B., and Lawaetzi, H.S. (1998)
Sequential architecture and cyclicity in Permian desert deposits, Brodick Beds, Arran, Scotland. Journal of the Geological Society, London, vol. 155, 677—683. Garcia-Castellanos, D., Fernandez, M. and Torne, M. (1997) Numerical
modeling of Foreland Basin formation: a program relating thrusting, flexure, sediment geometry and lithosphere rheology. Computers and Geosciences vol. 23, No. 9, 993—1003. Gardner, T.W. (1983) Paleohydrology and Paleomorphology of a Carboniferous, meandering, fluvial sandstone. Journal of Sedimentary Petrology, vol. 53, No. 3, 991–1005. Gibling, M.R. (2006) Width and thickness of fluvial
channel bodies and valley fills in the geological record: a literature compilation and classification. Journal of Sedimentary Research,
vol. 76, 731—770. Greb, S.F., and Weisenfluh, G.A. (2000) Mining
Geology of the Lower Elkhorn Coal: Kentucky Geological Survey, series 12.
Map and Chart Series 4. Habib, D., and Groth, P.K.H. (1967) Paleoecology of migrating carboniferous peat environments. Paleogeography, Paleoclimatology, Palaeoecology 3, 185—195. Helland-Hansen, W. and Martinsen, O.J. (1996) Shoreline trajectories
and sequences: Description of variable depositional-dip scenarios. Journal of Sedimentary Research, vol. 66, No. 4, 670—688.
119 Hower, J.C., Greb, S.F., Cobb, J.C., Williams, D.A. (2000) Discussion
on origin of vanadium in coals: parts of the Western Kentucky (USA) No. 9 coal rich in vanadium. Journal of the Geological Society, London, vol. 157, 1257—1259. Jogensen, P.J., and Fielding, C.R. (1999) Debris-flow
deposits in an alluvial-plain succession: the Upper Triassic Callide Coal measures of Queensland, Australia. Journal of Sedimentary
Research, vol. 69, No. 5, 1027—1040. Keep, M. (2003) Physical modeling of deformation in Orogenic zone. Tectonophysics 375, 37—47. Kim, W., Paola, C., Voller, V.R., and Swenson, J.B. (2006)
the Tasman
Experimental measurement of the relative importance of controls on shoreline migration. Journal of Sedimentary Research, vol. 76, 270 – 283. Kovac, M., Barath, I., Harzhauer, M., Hlavaty, I., and Hudackova, N. (2004) Miocene depositional systems and sequence stratigraphy of the Vienna Basin. Course Forschungs Institut Senckenberg 246, 187—212. Kusznir, N., and Karner, G. (1985) Dependence of the flexural
rigidity of the continental lithosphere on the rheology and temperature. Letters to Nature, vol. 316, 138—142. Laurin, J., Meyers, S.R., Sageman, B.B., and Waltham, D. (2005) Phaselagged amplitude modulation of hemipelagic cycles: A potential tool for recognition and analysis of sea-level change. Geology 33, No. 7, 569—572. Leckie, D.A. and Krystinik, L.F. (1989) Is there evidence for geotrophic currents preserved in the sedimentary record of inner to middle–shelf deposits? Journal of sedimentary Petrology, vol. 59, No. 5, 862—870. Lee, H.J., and Yoon, S.H. (1997) Development
of stratigraphy and sediment distribution in the Northeastern Yellow sea during Holocene and sea-level rise. Journal of Sedimentary Research, vol.
67, No. 2, 341–349.
120 Lee, K., McMechan, G.A., Gani, R., Bhattacharya, J.P., Zeng, X., and Howell, Jr.C.D. (2007) 3-D architecture and sequence
stratigraphic evolution of a forced regressive toptruncated mixed-influenced delta, Cretaceous wall creek sandstone, Wyoming, U.S.A. Journal of Sedimentary Research, vol. 77, 303 – 323. Lericolais, G., Berne, S., and Fenies, H. (2001) Seaward
pinching out and internal stratigraphy of the Gironde incised valley on the shelf (Bay of Biscay). Marine Geology 175, 183—197. Libertin, M. and Bek, J. (2004) Huttonia spicata (Sternberg) emend. and its spores, the Radnice Basin (Bolsovian), Carboniferous continental basins of the Czech Republic. Review of Palaeobotany and Palynology 128, 247—261. Martinsen, O.J., Ryseth, A. Helland-Hansen, W., Flesche, H., Torkildsen, G. and Idil, S. (1999) Stratigraphic base level
and Fluvial architecture: Ericson Sandstone (Campanian), Rock Springs Uplift, SW Wyoming, U.S.A. International Association of
Sedimentologists, Sedimentology, 46, 235–259. Miller, D.J. (1998) Depositional Environments
and Sequence Stratigraphy of Upper Mississippian Strata in the Central Appalachian Basin. PhD Dissertation in Geological Sciences, at
Faculty of the Virginia Polytechnic Institute and State University. Miller, D.J., and Eriksson, K.A. (1997) Late Mississippian Prodeltaic
rhythmites in the Appalachian Basin: A hierarchical record of tidal and climatic periodicities. Journal of Sedimentary Research, vol. 67, No. 4, 653—660. Noad, J. (2004) The use of field analogues
in the correlation and static reservoir methodology used in the Tern Field, Northern North Sea, UK. Marine and Petroleum Geology 21 485–497.
Olsen, T., Steel, R., Hogseth, K., Skar, T., and Roe, S-L. (1995)
Sequential architecture in a fluvial succession: Sequence stratigraphy in the Upper Cretaceous Mesaverde group, Price Canyon, Utah. Journal of Sedimentary Research, vol. B65, No. 2, 265—280.
121 Petter, A.L., and Muto, T. (2008) Sustained
alluvial aggradation and autogenic detachment of the alluvial river from the shoreline in response to steady fall of relative sea-level. Journal of
Sedimentary Research, vol. 78, 98—111. Phillips, T.L., (1979) Reproduction of heterosporous
arborescent lycopods in the Mississippian—Pennsylvanian of Euramerica. Review of Palaeobotany and Palynology, 27, 239—289. Pierce, B.S., Stanton, R.W., and Eble, C.F. (1991) Facies development in the Lower Freeport coal bed, west-central Pennsylvania, U.S.A. International Journal of Coal Geology 18, 17 –43. Posamentier, H.W., and Weimer, P. (1993) Siliciclastic Sequence Stratigraphy and Petroleum Geology—Where to From Here? American Association of Petroleum Geologists Bulletin, vol. 77, No. 5, 731—742. Proust, J.N., Chuvashov, B.I., Vennin, E., and Boisseau, T. (1998)
Carbonate platform drowning in a Foreland setting: The MidCarboniferous platform in Western Urals (Russia). Journal of Sedimentary Research, vol. 68, No. 6, 1175—1188. Raitala, J. (1987) Implications of lithospheric bending and faulting in lunar mare-terra tectonics. Earth, Moon, and Planets 38, 137—147. Read, W.A. (1997) Syntactic simulation of 1-D Sedimentary
sequences in a coal-bearing succession using a stochastic Context-free grammar—discussion. Journal of Sedimentary Research, vol. 67, No. 6, 1097–1099. Rodriguez, A.B., Fassell, M.L., and Anderson, J.B. (2001) Variations
in shoreface progradation and ravinement along the Texas coast, Gulf of Mexico. Sedimentology 48, 837—853. Rodriguez, A.B., and Meyer, C.T. (2006) Sea-level variation during the Holocene deduced from the morphologic and stratigraphic evolution of Morgan Peninsula, Alabama, U.S.A. Journal of Sedimentary Research, vol. 76, 257—269.
122 Root, S., and Onasch, C.M. (1999) Structure
and tectonic evolution of the transitional region between the central Appalachian foreland and interior cratonic basins. Tectonophysics 305, 205—
223. Runkel, A.C., Miller, J.F., McKay, R.M., Palmer, A.R., and Taylor, J.F. (2007) High-resolution sequence stratigraphy of Lower
Paleozoic sheet sandstones in central North America: The role of special conditions of cratonic interiors in development of stratal architecture. Geological Society of America Bulletin 119, 860—881. Sakurai, R., Ito, M., Ueno, Y., Kitajima, K., and Maruyama, S. (2005)
Facies architecture and sequence-stratigraphic features of the Turban Formation in the Pilbara Craton, northwestern Australia; Implications for depositional environments of oxygenic stromatolites during the Late Archean. Precambrian Research 138, 255—273. Snedden, J.W., and Swift, D.J.P. (1991) Is there
evidence for geotrophic currents preserved in the sedimentary record of inner- to middle-shelf deposits?—Discussion. Journal of
Sedimentary Petrology, vol. 61, No. 1, 148—151. Stevens, C.H., and Stone, P. (2005) Structure and
regional significance of the Late Permian(?) Sierra Nevada—Death Valley thrust system, east-central California. Earth-Science
Reviews 73, 103—113. Storms, J.E.A., and Hampson, G.J. (2005) Mechanisms
for forming
discontinuity surfaces within shoreface—shelf parasequences: Sea-level, sediment supply, or wave regime? Journal of Sedimentary Research, vol. 75, No. 1, 67—81. Storms, J.E.A., and Swift, D.J.P. (2003) Shallow-marine
sequences as the building blocks of stratigraphy: insights from numerical modeling. Basin Research 15, 287—303.
123 Thomas, R.G., Smith, D.G., Wood, J.M., Visser, J., Calverley-Range, E.A. and Koster, E.H. (1987) Inclined heterolithic stratification—
Terminology, description, interpretation, and significance. Sedimentary Geology 53, 123-179. Trincardi, F., Coerggiari, A. and Roveri, M. (1994) Late quaternary
transgressive erosion and deposition in a modern epicontinental shelf: the Adriatic semi enclosed Basin. GeoMarine Letters 14, 41—51. Trincardi, F., and Field, M.E. (1991) Geometry,
lateral variation, and preservation of downlapping regressive shelf deposits: Eastern Tyrrhenian Sea margin, Italy. Journal of Sedimentary
Petrology, vol. 61, No. 5, 775—790. Walker, R.G. (1990) Perspective Facies modeling and sequence stratigraphy. Journal of Sedimentary Petrology, vol. 60, No. 5, 777— 786. Weber, N., Chaumillon, E., Tesson, M., and Garlan, T. (2004)
Architecture and morphology of the outer segment of a mixed tide and wave-dominated incised valley, revealed by HR seismic reflection profiling: the paleo-Charente River, France. Marine Geology 207, 17—38. Willett, S.D., and Beaumont, C. (1994) Subduction of Asian lithospheric mantle beneath Tibet inferred from models of continental collision. Letters to Nature, vol. 369, 642—645. Wilson, R.C.L. (1998) Sequence stratigraphy: a revolution without a cause? In: Blundell, D.J. and Scott, A.C. (eds) Lyell: the past is the key to the present. Geological Society, London, Special Publications, 143, 303—314. Zecchin, M., Mellere, D., and Roda, C. (2006) Sequence
stratigraphy and architectural variability in growth fault-bounded basin fills: a review of Plio-Pleistocene stratal units of the Crotone Basin, southern Italy. Journal of Geological Society, vol. 163, 471—
486.
124
10. APPENDICES
125
Fig. 10.1. (a) A composite of topographic map of surface BLR B and boreholes that penetrate this surface.
126
Fig. 10.2. (a) A composite of topographic map of surface CLN B and boreholes that penetrate this surface.
127
Fig. 10.3. (a) A composite of topographic map of surface LEK B and boreholes that penetrate this surface.
128
Fig. 10.4. (a) A composite of topographic map of surface UE1 B and boreholes that penetrate this surface.
129
Fig. 10.5. (a) A composite of topographic map of surface UE2 A and boreholes that penetrate this surface.
130
Fig. 10.6. (a) A composite of topographic map of surface UE3 A and boreholes that penetrate this surface.
131
Fig. 10.7. (a) A composite of topographic map of surface UE3 B and boreholes that penetrate this surface.
132
Fig. 10.8. (a) A composite of topographic map of surface AMB A and boreholes that penetrate this surface.
133
Fig. 10.9. (a) A composite of topographic map of surface AMB B and boreholes that penetrate this surface.
134
Fig. 10.10. (a) A composite of topographic map of surface WHI A and boreholes that penetrate this surface.
135
Fig. 10.11. (a) A composite of topographic map of surface WHI B and boreholes that penetrate this surface.
136
Fig. 10.12. (a) A composite of topographic map of surface FCL B and boreholes that penetrate this surface.
137
Fig. 10.13. (a) A composite of topographic map of surface FCR A and boreholes that penetrate this surface.
138
Fig. 10.14. (a) A composite of topographic map of surface HML A and boreholes that penetrate this surface.
139
Fig. 10.15. (a) A composite of topographic map of surface HML B and boreholes that penetrate this surface.
140
Fig. 10.16. (a) A composite of topographic map of surface HDX A and boreholes that penetrate this surface.
141
Fig. 10.17. (a) A composite of topographic map of surface HZD A and boreholes that penetrate this surface.
142
Fig. 10.18. (a) A composite of topographic map of surface HZ7 B and boreholes that penetrate this surface.
143
Fig. 10.19. (a) A composite of topographic map of surface PCH A and boreholes that penetrate this surface.
144
Fig. 10.20. (a) A composite of topographic map of surface PCH I and boreholes that penetrate this surface.
145
Fig. 10.21. (a) A composite of topographic map of surface PCH B and boreholes that penetrate this surface.
146
Fig. 10.22. (a) A composite of topographic map of surface BRS A and boreholes that penetrate this surface.
147
Fig. 10.23. (a) A composite of topographic map of surface BRS B and boreholes that penetrate this surface.
148
Table 10.1. Raw data from the KGS, from which input data for creating well headers in Petrel (Table 10.2) was extracted. S.E = surface elevation, D = total depth of the borehole and Y is its year of drilling. Hole_ID BRDBTTM004 BRDBTTM005 BRDBTTM010 BRDBTTM011 BRDBTTM012 BRDBTTM013 BRDBTTM014 BRDBTTM015 BRDBTTM020 BRDBTTM023 BRDBTTM024 BRDBTTM028 BRDBTTM029 BRDBTTM072 BRDBTTM073 BRDBTTM076
utm17_83_N utm17_83_E 4159258.44 4159575.37 4157700.4 4156435.33 4157915.4 4157830.32 4159310.42 4161045.42 4158635.44 4161356.38 4160113.37 4156470.37 4156290.37 4163797.47 4162952.42 4161635.42
Source
360789.68 USGS 359626.59 DIAMOND COAL 356869.6 BETH ELKHORN 357239.64 BETH ELKHORN 357474.6 BETH ELKHORN 357394.58 BETH ELKHORN 357034.57 BETH ELKHORN 358214.58 BETH ELKHORN 358404.6 BETH ELKHORN 357885.6 BETH ELKHORN 358936.61 BETH ELKHORN 357959.63 BETH ELKHORN 357020.63 BETH ELKHORN 359634.6 UNKNOWN 361050.64 UNKNOWN 362110.64 UNKNOWN
County Quadrangle
DDH_NUM
Pike Pike Floyd Pike Pike Pike Floyd Pike Pike Floyd Pike Pike Floyd Pike Pike Pike
DDH-8
Broad Bottom Broad Bottom Broad Bottom Broad Bottom Broad Bottom Broad Bottom Broad Bottom Broad Bottom Broad Bottom Broad Bottom Broad Bottom Broad Bottom Broad Bottom Broad Bottom Broad Bottom Broad Bottom
102 BE-73-16 BE-73-17 BE-73-18 BE-73-18A BE-73-19 BE-73-20 BE-78-3 BE-83-7 BE-83-6 BE-83-2 BE-83-1 DH-D-2 DH-D-5 E-85-2
S.E (ft)
D (ft) Y drilled Upper_Bed
1625 1109.2 1380 678 669.76 370 1175.73 701 1190.05 471 1060.05 300 700 315 1634.26 900 754.95 501.65 1530 407 1520 426 1535 422 1220 627 1125 750 1220 645 1350 730.33
1962 Broas 1979 Hazard 1973 Lower Elkhorn 1973 Hamlin 1973 Hamlin 1973 Whitesburg 1973 Upper Elkhorn No.1 1973 Broas 1978 Upper Elkhorn No.3 1983 Broas 1983 Broas 1983 Peach Orchard 1983 Hamlin 0 Firecreek 0 Firecreek 1985 Hazard
Lower_Bed Lower Elkhorn Upper Elkhorn No.2 Eagle Lower Elkhorn Upper Elkhorn No.3 Kendrick Shale Clintwood Kendrick Shale Clintwood Copland Magoffin Shale Fire Clay Rider Lower Elkhorn Lower Elkhorn Upper Elkhorn No.2 Upper Elkhorn No.2
149 Table 10.2. Well header data for all wells in Broad bottom used for input into Petrel. KB = Kelly bushing, here used as surface elevation and TD = Total Depth of the borehole. Well
y
x
KB
TD
BRDBTTM004
4159258.44
360789.68
1625.00
1109.2
BRDBTTM005
4159575.37
359626.59
1380.00
678
BRDBTTM010
4157700.40
356869.60
669.76
370
BRDBTTM011
4156435.33
357239.64
1175.73
701
BRDBTTM012
4157915.40
357474.60
1190.05
471
BRDBTTM013
4157830.32
357394.58
1060.05
300
BRDBTTM014
4159310.42
357034.57
700.00
315
BRDBTTM015
4161045.42
358214.58
1634.26
900
BRDBTTM020
4158635.44
358404.60
754.95
501.65
BRDBTTM023
4161356.38
357885.60
1530.00
407
BRDBTTM024
4160113.37
358936.61
1520.00
426
BRDBTTM028
4156470.37
357959.63
1535.00
422
BRDBTTM029
4156290.37
357020.63
1220.00
627
BRDBTTM072
4163797.47
359634.60
1125.00
750
BRDBTTM073
4162952.42
361050.64
1220.00
645
BRDBTTM076
4161635.42
362110.64
1350.00
730.33
150 Table 10.3. Litho-log data for well BRDBTTM004 obtained from KGS used as input data into Petrel for the creation of FermFacies logs. FaciesCurves and Facies logs are derived from the FermFacies logs. Well BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004
LithCode
Top depth
543.5 134 21 107 544 324.3 27 123 323 543.5 124 21 127 21 125 541 123 27 127 546 332 543.6 125 29 124 27 123 125 748 28 137 540 21 307 21 749 324 748 326 323 129 329 137 323 540 324.3 748 748 743 323 543 123 21 137 123 137 21 125 540 322.5 21 127
0.00 13.10 45.90 47.40 48.40 108.50 110.90 114.30 115.90 118.40 140.30 146.30 147.00 149.90 153.30 153.60 163.10 163.80 165.50 169.00 169.50 171.70 181.20 190.10 192.40 228.40 234.70 236.00 254.20 257.90 258.60 260.90 318.80 319.95 322.50 322.90 346.80 348.30 357.70 371.70 400.20 401.80 432.60 435.70 460.10 489.40 490.00 490.70 491.10 495.20 501.70 504.90 511.00 511.15 515.30 520.10 522.90 523.05 523.20 586.30 588.80 591.00
Well BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004
LithCode
Top depth
540 322 543.5 323 27 133 21 124 21 127 21 330 540 129 3 540 3 540 129 543 129 21 123 541 21 137 748 21 124 324.3 543 123 27 124 21 124 124 540 124 21 124 546 124 540 124 124 540 21 123 323 124 21 124 540 324.3 124 21 134 540 543 322 540 543
592.20 598.60 604.40 623.10 625.10 627.40 648.70 649.00 649.60 650.80 652.60 653.30 654.30 704.70 717.00 717.00 750.00 750.00 795.00 834.00 844.40 876.50 877.00 878.30 882.40 886.60 888.00 894.70 895.90 897.00 914.70 927.50 928.40 931.80 932.70 932.90 937.00 943.50 946.20 947.20 948.40 952.90 954.30 955.30 961.60 1024.40 1033.90 1039.90 1041.20 1042.50 1055.30 1056.40 1057.65 1058.80 1063.60 1065.60 1071.50 1071.65 1073.20 1079.60 1082.60 1090.10 1095.20
151
Table 10.4. Codes used for labeling the various lithologies encountered in the subsurface while drilling in the quadrangle Broad bottom. LithCode
Lithology
1 3 4 5 12 18 20 21 23 24 27 28 29 32 33 34 38 70 103 104 107 114 119 123 124 125 127 129 133 134 137 302 307 312 320 322 322.5 323 324 324.3 325 326 327 328 329 330 332 503 510 540 541 543 543.5 543.6 544 546 547 548 549 550 640 741 742 743 748 749
SURFACE ZERO TAG LOST CORE OPEN MINE GRY SH W/COAL STKS SDY SH MUDFLOW COAL COMMON BANDED COAL BONEY COAL DULL COAL COAL W/SH LAYERS COAL W/SH STKS COAL W/PYRITE STKS COAL & BONE BONE W/COAL STKS BONE COAL BONE & SH PYRITE SH W /COAL STKS SHALE CLAYSTONE BLK SH BLK SH W/FOS SHLS GRY SH W/COAL STKS DARK GRAY SHALE DRK GRY CHRND SH GRAY FIRECLAY SH & BONE LT GRY GRN SH W/COAL STKS LT GRY GRN SH LT GRY GRN FIRECLAY SH & SS SDY FIRECLAY BLK SH+INTERBED SS GRY SDY SH DRK GRY SH+INTERBED SS DRK GRY SH+INTERBED RIP SS GRY SH W/SS STKS SANDY SHALE DRK GRY SDY SH W/COAL STKS DRK GRY MASS CHRND SDY SH DRK GRY CHRND SDY SH DARK GRAY SANDY FIRECLAY DRK GRY BURROWED SDY SH DRK GRY SDY SH W/FOS SHLS LT GRY GRN SDY SH LT GRY GRN SH+INTERBED SS SS W/SH STKS GRY SS W/LS SANDSTONE GRY CROSSBED SS GRY SS W/SH STRKS GRY RIP SS W/SH STKS GRY SS W/FLAT SH STKS GRAY SANDSTONE GRY CHRND SS GRY ROOTED SS GRY BURROWED SS GRY SS W/FOS SHLS XTLIZED SS GRY CO3 CMNTD SS GRY SH+/OR IRNSTN PEB CGL GRY SH PEB CGL GRY IRNSTN PEB CGL GRAY SANDSTONE WITH COAL BANDS GRAY SANDSTONE WITH COAL SPARS
152 Table 10.5. Coal-logs data for wells in Broad bottom obtained from KGS used as input data into Petrel for the creation of comment logs. Well
Top MD
Base MD
Seam
BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM004 BRDBTTM005 BRDBTTM005 BRDBTTM005 BRDBTTM005 BRDBTTM005 BRDBTTM010 BRDBTTM011 BRDBTTM011 BRDBTTM011 BRDBTTM011 BRDBTTM011 BRDBTTM011 BRDBTTM011 BRDBTTM012 BRDBTTM012 BRDBTTM012 BRDBTTM014 BRDBTTM015 BRDBTTM015 BRDBTTM015 BRDBTTM015 BRDBTTM015 BRDBTTM020 BRDBTTM020 BRDBTTM020 BRDBTTM020 BRDBTTM023 BRDBTTM023 BRDBTTM023 BRDBTTM024 BRDBTTM024 BRDBTTM024 BRDBTTM024 BRDBTTM028 BRDBTTM029 BRDBTTM029 BRDBTTM029 BRDBTTM029 BRDBTTM029 BRDBTTM029 BRDBTTM029 BRDBTTM072 BRDBTTM072 BRDBTTM072 BRDBTTM073 BRDBTTM073 BRDBTTM074 BRDBTTM076 BRDBTTM076 BRDBTTM076 BRDBTTM076
45.90 110.90 149.90 228.40 588.80 717.00 750.00 882.40 928.40 947.20 1039.90 328.50 452.90 507.90 619.70 664.15 101.00 94.59 252.44 301.52 397.86 466.82 459.42 538.90 150.59 300.67 349.50 111.58 97.33 147.08 259.94 328.63 656.93 64.20 75.42 167.62 424.62 40.70 164.01 217.15 31.00 79.12 196.02 273.02 87.76 145.63 299.98 331.98 454.10 526.90 519.59 596.48 187.40 444.54 619.20 252.00 512.60 73.33 346.90 487.00 543.60 635.00
47.40 114.30 153.30 234.70 591.00 717.00 750.00 886.60 931.80 948.40 1041.20 330.90 452.90 507.90 623.20 667.55 102.25 98.44 253.44 301.52 403.34 466.82 466.82 539.98 152.67 301.00 349.50 112.58 98.50 150.35 262.90 335.13 659.51 67.57 76.62 168.97 425.52 43.91 167.93 223.73 32.42 82.27 201.23 280.46 93.09 152.73 299.98 331.98 460.25 526.90 526.90 597.65 189.50 446.09 620.80 254.40 515.05 77.29 349.00 487.00 544.70 637.30
BRS B BRS A PCH B PCH A FCL B AMB B AMB A UE3 A UE2 A UE1 B LEK B FCL B AMB B AMB A UE3 A UE2 A LEK B FCL B AMB B AMB A UE3 A UE1 B UE2 A LEK B FCL B AMB B AMB A LEK B BRS B BRS A PCH B PCH A FCL B UE2 A UE1 B LEK B CLN B BRS A PCH B PCH A BRS B BRS A PCH B PCH A PCH A FCL B AMB B AMB A UE3 A UE1 B UE2 A LEK B FCL B UE3 A LEK B FCL B UE3 A UE3 A FCL B AMB B AMB A UE3 A
