M1 Geology of Sandstone in Ecuador 2017

Marine and Petroleum Geology 86 (2017) 1207 e1223

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Marine and Petroleum Geology j o u r n a l h o m e p a g e : w w w . e l s e v i e r . co co m / l o c a t e / m a r p e t g e o

Research paper

Geology of the Campanian M1 sandstone oil reservoir of eastern Ecuador: A delta system sourced from the Amazon Craton Cristian Vallejo a , Diego Tapia b, Janeth Gaibor c, Ron Steel d, Mario Mario Cardenas b, Wilfried Wilfried Winkl Winkler e, Anne Valdez b , Jose Esteban f , Mariana Figuera c, Jose Leal c, Dario Cuenca b ,

*

a

Departamento de Geología, Escuela Polit ecnica Nacional, Quito, Ecuador Petroamazonas EP, Quito, Ecuador c Halliburton Latin America, Quito, Ecuador d Jackson School of Geosciences, University of Texas at Austin, 1 University Station, C1100, C1100, Austin, TX, United States e Department of Earth Sciences, ETH-Zurich, Switzerland f Departamento de Geodinamica, Facultad de Ciencia y Tecnología, Universidad del País Vasco, Apdo. 644, 48080 Bilbao, Spain 

b

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a r t i c l e

i n f o

Article history: Received 11 January 2017 Received in revised form 20 June 2017 Accepted 23 July 2017 Available online 25 July 2017 Keywords: M1 sandstone Oriente Basin Delta Napo Formation Tena Formation Ecuador Provenance analysis

a b s t r a c t

The Campanian M1 sandstone Member of the Napo Formation is one of the main reservoirs of the Capiron-Tiputini oil play system of the eastern part of the Oriente Basin of Ecuador. This study presents a geological model for this reservoir at the Apaika and Nenke oil � elds included within the Block 31, and the Eden Yuturi oil �eld, operated by PETROAMAZONAS EP. The geological characterization of the M1 sandstone is based on an integrated study, which includes core description, electric log analyses and seismic data interpretation. The sedimentological descriptions from available cored sections show that M1 sandstone records a variety of � uvial to shallow marine processes, including mass � ow and delta-front turbidity currents not previously documented. The stratal stacking pattern interpreted from electric logs indicates that the succession re�ects deltaic progradation rather than the backstepping pattern typically displayed by the underlyin underlying g Albian Albian to Santonian sediments sediments of the Napo Formation. Formation. Seismic Seismic RMS amplitude amplitude de �nes clinoform geometries with sigmoidal sand body body architecture prograding prograding from east to west. Correlation Correlation with neighboring regions in Colombia suggest that the M1 sandstone was probably part of a large riverdominated delta system sourced from the east during major Campanian progradation. The location of the source area of sediment was determined using U-Pb detrital zircon dating. Provenance ages are uniform within the M1 Sandstone cluster in a population that ranges from 1.4 to 1.6 billion years. These ages indicate that sands were derived mostly from erosion of the lithotectonic Rio Negro-Jurena province, located to the northeast of the studied area. Detrital ages from the overlying Maastrichtian Tena Formation include ages between 80 and 200 Ma pointing to an important change in the source areas, located in the Eastern Cordillera of Ecuador. The change in sediment supply from east to west recorded in the Tena Formation coincide with the initial episodes of the Andean orogeny, and may be driven by the collision of fragments of the Caribbean large igneous province (CLIP) against the continental margin of northwestern South America. © 2017 Elsevier Ltd. All rights reserved.

1. Introduction

With nearly 30 billion barrels of oil in place (Rivadeneira ( Rivadeneira and Baby, 2004), 2004), the Oriente Basin of Ecuador is one of the most proli�c Sub-Andean oil basins of northwestern South America (Fi ( Fig. g. 1). *

Corresponding author. E-mail address: [email protected] (C. Vallejo).

http://dx.doi.org/10.1016/j.marpetgeo.2017.07.022 0264-8172/© 2017 Elsevier Ltd. All rights reserved.

The oil is hosted in Cretaceous (Aptian-Maastrichtian) reservoirs. Contemporaneous, organic-rich sediments (e.g., Basal and upper shale shale members members of the Napo Napo Format Formation) ion) were were deposit deposited ed over over a large large area area and are considered considered the source source of almost almost all hydro hydrocar carbon bonss (Dashwood and Abbotts, 1990; 1990; Mello et al., 1995). 1995). In the western and central part of the basin, oil exploration and production is most mostly ly focuse focused d on the the Albia Albiann-Ce Ceno noma mani nian an �uvio-estuarine

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C. Vallejo et al. / Marine and Petroleum Geology 86 (2017) 1207 e1223

Fig. 1. Tectonic sketch map of Ecuador showing the geological setting of the Andean Amazon Basin in Ecuador and adjacent regions. Compiled from Spikings et al. (2001) and Gombojav and Winkler (2008). Location of the area of study is represented by a black square in the map.

sandstones of the Hollín and Napo formations (T and U sandstone members). The Campanian M1 sandstone member of the Napo Formation is a reservoir restricted to the eastern part of the Oriente Basin of Ecuador and its equivalent, the Vivian Formation in the Maranon Basin of Peru. The M1 sandstone is unconformably overlain by continental deposits of the Maastrichtian Tena Formation, which marks the end of the marine sedimentation within the Oriente Basin and neighbouring areas. The deposition of the Tena Formation coincided in time with the initiation of the Andean Orogeny (Dashwood and Abbotts, 1990). However, its continuity and trap potential has not been de �ned due to the lack of data integration and interpretation, including the origin of the M1 sandstone. Previous models based on lithostratigraphic correlations (e.g., Barragan et al., 2004) proposed that the M1 sandstone was deposited within incised valleys during transgression. Among the available data, Jaillard (1997) recognized the dif �culty in determining the depositional environment of the M1 sandstone and suggested a subaqueous and continental environment for this member at the Tiputini Minas 1 well of the Tiputini oil �eld (eastern border of the Oriente Basin). This contribution is one of the �rst published detailed geological overviews of the M1 sandstone reservoir of the Napo Formation; another is a published MS thesis by Yu Ye (2014). We propose a new model for the origin of the M1 sandstone member, integrating core description, biostratigraphy, electric log analyses, and seismic data interpretation from Eden Yuturi, Apaika and Nenke oil �elds. In addition, new UePb detrital zircon ages were obtained within the M1 sandstone and the overlying Tena Formation. The objectives of this study are: (1) to interpret depositional processes from core descriptions; (2) calibrate the depositional facies with wireline logs; (3) establish sand-body geometries using stratigraphic correlations of well logs and seismic data; (4) develop a depositional model by integrating core, outcrop, log, and seismic data; and (5) discuss the tectonic setting during the deposition of ~

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the Campanian to Maastrichtian sedimentation, particularly the transition from the M1 sandstone member of the Napo Formation to the Tena Formation. 2. Material and methods

This study included core descriptions, electric log analyses, seismostratigraphic interpretation and provenance analysis. The sedimentological study of a 92-ft core from the APKA-02 well (Block 31, eastern part of the Oriente Basin; Fig. 1) allowed for characterizing the depositional environment and processes during deposition of the M1 sandstone member. The sedimentological and stratigraphic data are complemented with a seismostratigraphic analysis using root mean square (RMS) amplitude-seismic attributes, whichallow us to de�ne the geometries of the reservoir. This study also integrates a petrophysical analysis, including porosity and permeability measured directly from rock samples collected from a cored section. The source area of the sediment was determined from a zircon provenance analysis, which included zircon U-Pb ages from detrital grains obtained by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Zircons were extracted using conventional separation techniques (i.e., crushing, sieving, magnetic, and heavy-liquid separation) at the University of the Basque Country. Only the cleanest crack and inclusion-free zircons were mounted into epoxy and polished to reveal their internal structure and imaged using a JEOL 6400 scanning electron microscope. Appropriate zircons were ablated with a NewWave UP-213 excimer ablation system at a 30- mm diameter beam size, 10-Hz repetition rate, 60-s signal, and a beam intensity of 2.5 J/cm 2. The isotopic ratios were measured using a Thermo Scienti �c Xseries II. External reference standards used to calibrate and monitor fractionation and consistency in the measured U-Pb dates were either GJ1 (608.5 ± 4 Ma; Jackson et al., 2004) zircon or Ple sovice (337.13 ± 0.37 Ma; Slama et al., 2008) zircon. Data reduction was carried out by the 

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laboratory staff using Iolite 3 (Paton et al., 2011) and VizualAge (Petrus and Kamber, 2012) software. 3. Regional geology

The Oriente Basin is part of a retro-arc foreland basin, one of the Sub-Andean basins of northwestern South America (Gombojav and Winkler, 2008; Baby et al., 2013), located between the Andes to the east and the South American Craton to the west ( Fig. 1). Development of the Oriente Basin was closely controlled by the early dynamics of the Northern Andes starting in the Jurassic period (Spikings et al., 2015). The basin is bordered to the west by the primordial Eastern Cordillera and the Amazon Craton (Guyana Shield) to the east (e.g., Ruiz et al., 2007; Gombojav and Winkler, 2008; Spikings et al., 2015). The Cretaceous Ecuadorian Oriente Basin was developed as a part of an epicontinental sea that included the Mara non Basin in Peru, as well as the Putumayo, Magdalena, and Llanos basins in Colombia and the Barinas-Apure Basin in Venezuela (Balkwill et al., 1995; Vallejo et al., 2002). The Oriente Basin, including the Napo Uplift, has further developed as an eastward-pinching foreland basin since the Maastrichtian (Gombojav and Winkler, 2008) and has yielded the majority of Ecuador hydrocarbons (Rivadeneira and Baby, 2004). There is general agreement that the Oriente Basin initiated latest at 115e100 Ma. This occurred after a long period of extension between 145 and 120 Ma, which led to the formation of grabens and half-grabens that were �lled by evaporites, continental debris �ows, and shallow marine sediments of Middle Jurassic age (Diaz et al., 2004; Gaibor et al., 2008; Spikings et al., 2015). The extension was followed by a period of compression along the northern South American plate margin, which closed a series of fore-, inter-, and back-arc basins (Guamote, Peltetec, Alao, and Upano basins) against the continent (Litherland et al., 1994; Spikings et al., 2015). In the Oriente Basin, this compression created an unconformity above the pre-Cretaceous strata, which is clearly observed in the seismic lines of the Oriente Basin (Balkwill et al., 1995) and occurs below the Aptian to Albian Hollin Formation in this basin. Earlier authors ascribed this deformational event (Peltetec event) to the collision of allochthonous terranes between 120 and 110 Ma (e.g., Litherland et al., 1994; Spikings et al., 2015), creating a primordial Andean chain in the Eastern Cordillera located to the west of the basin (Ruiz et al., 2007; Gombojav and Winkler, 2008). Compression was followed by a period of punctuated thermal subsidence, during which the Cretaceous strata were deposited. The pre-Aptian unconformity formed paleotopographic depressions that became �lled by �uvial deposits of the Hollín Formation (Fig. 2). Shanmugam et al. (2000) proposed four stages of deposition for this formation from oldest to youngest: 1) �uvial channels of low sinuosity, which may represent braided rivers; 2) tide-dominated estuaries; 3) continued drowning of the tidedominated estuaries; and 4) a shelf environment. This transition and the component environmental changes were clearly the result of a major, long-term transgression (Boyd et al., 2006), probably driven by tectonic downwarping and relative sea-level rise across the new basin � oor. Detrital sediment supply during deposition of the Hollín Formation was routed from both the South American craton and from a primordial, Cretaceous Andean cordillera (Gombojav and Winkler, 2008). The Napo Formation, overlying the Hollín Formation (Fig. 2), is a stratigraphic series that includes at least �ve major stratigraphic sequences deposited from the Albian to the Campanian ( White et al., 1995; Vallejo et al., 2002; Barrag an et al., 2004). The �rst two stratigraphic sequences (T and U) were deposited during Albian and Cenomanian transgressions; they comprise marine ~

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successions that retrograde from � uvial to tide-dominated estuarine, and then to open marine shelf environments. Facies transitions within the next three stratigraphic units, the T, U, and M2 sequences, are similar in stratigraphic architecture and transgressive trend to the Hollín Formation. For most of the Hollín, T, U, and M2 sedimentationperiods, the basin was remarkably protected from large, open, oceanic waves and swells, a feature supporting the presence of a partial topographic barrier to the west ( Jaillard, 1997; Vallejo et al., 2002; Gombojav and Winkler, 2008) The con�guration of the Cretaceous Oriente Basin was, thus, somewhat comparable with the late Cretaceous Western Interior Seaway (WIS) of the USA, though this basin shows abundant evidence of ocean waves, as well as tides, and is not dominated by repeated transgressive intervals (Steel et al., 2012). Consequently, the Oriente Basin appears to have been narrower than the WIS, relatively protected from large waves and in�uenced mainly by river and tidal currents moving through the shallow Ecuadorian Cretaceous seaway (e.g., Jaillard, 1997). The M1 shale and M1 limestone of the Napo Formation ( Fig. 2) were deposited during the Santonian transgression ( Ordonez et al., 2006) and comprise mainly carbonates and shallow offshore and shelf marine shales. During this period, the sea transgressed across the whole basin and the M1 limestone and associated shales is considered to be the maximum � ooding zone of the Napo secondorder stratigraphic cycle. The organic-rich Upper Napo Shale (Fig.2) is the youngest part of the Napo depositional system. In the westernmost part of the Oriente Basin, this member was deposited in a shallow marine environment during the Campanian period ( Jaillard, 1997; Vallejo et al., 2002). The deposition of the various overlying M1 sandstones and shales is more complex and somewhat different from the previous four stratigraphic sequences in the basin and is the main subject of this paper. The establishment of prevailing continental depositional environments in the Oriente Basin occurred from Maastrichtian times and is represented by �uvial sandstones and red beds from the continental Tena Formation (Tschopp, 1953; Can�eld et al., 1982). There is a regional erosional unconformity separating the Napo Formation from the Tena Formation. This unconformity represents erosion of the upper part of the Napo Formation and marks a signi�cant lithological change from shallow marine sediments of the Napo Formation to overlying continental deposits of the Tena Formation. Along the western border of the basin, the erosion level below the Tena Formation reaches the Turonian-Coniacian M2 limestone of the Napo Formation; whereas, along the eastern border of the basin, this unconformity separates the �uvial to coastal sediments of the Campanian M1 sandstone from the red beds of the Tena Formation (Fig. 2). During this late Cretaceous deformation period, thick-skinned tectonics produced the inversion of pre-Cretaceous extensional, mainly northward-striking fault systems (Balkwill et al., 1995; Baby et al., 2013). The inverted faults propagated to the Cretaceous sedimentary series and gave rise to the formation of N-S elongated folds, which form the main structural oil traps of the basin. This late Cretaceous deformation event is likely related to the collision of fragments of the Caribbean Plateau against the continental margin of the Northern Andes (Luzieux et al., 2006; Vallejo et al., 2006). The Tena Formation is, in turn, overlain by the Paleocene to Eocene alluvial fan conglomerates from the Tiyuyacu formation. The Tiyuyacu Formation depicts the accelerated uplift of the Andean cordillera located to the west of the Oriente Basin (e.g., Gombojav and Winkler, 2008). ~

4. The M1 sandstone

The M1 sandstone member is one of the most proli �c reservoirs

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Fig. 2. Regional Stratigraphic framework of the Cretaceous interval at the Oriente Basin, including the oil reservoir intervals (in yellow) and the sequence stratigraphic interpretation (modi �ed from Barragan et al., 2004). (For interpretation of the references to colour in this � gure legend, the reader is referred to the web version of this article.) 

along the eastern reaches of the Oriente Basin, including the Capiron-Tiputini oil play (Rivadeneira and Baby, 2004), where the Eden Yuturi, Apaika, Nenke, and the giant Ishpingo-TambocochaTiputini (ITT) oil � elds are located. The M1 sandstone member includes several sandstone intervals (Fig. 3) that were deposited on top of thick marine shale with some limestone intercalations. This underlying �ne-grained succession, de�ned as the M1 shale member, is dated to Santonian age at the Amo-1 well in the eastern part of the Oriente Basin ( Jaillard, 1997). A regional stratigraphic marker within the M1 limestone was identi�ed as the L marker, a surface interpreted as the maximum �ooding level of the Santonian transgression (Fig. 2), and is used as a chronostratigraphic datum for well correlations (Fig. 3). In the eastern part of the Oriente Basin, the M1 sandstone is overlain by red beds and discontinuous sandy strata from the Tena Formation. There is an east-to-west thinning of the strata between the L marker and the top of the M1 sandstone. Regional thickness maps for the M1 sandstone, itself, also indicate a maximum thickness of 200 ft near the Peruvian border (Dashwood and Abbotts, 1990; Barragan et al., 2004), but the unit thins or disappears in the central part of the basin (Sacha-Shushu �ndi corridor). No outcrop or subsurface type section has been described for the M1 sandstone. In Peru, the lateral equivalent of the M1 sandstone is the Vivian Formation, which is the main reservoir of the Maranon Basin 

~

(Mathalone and Montoya, 1995). In this study, we present cross sections from the M1 shale member of the Napo Formation to the Tena Formation, depicting the stratigraphic structure of the M1 sandstone in the Eden Yuturi and Apaika � elds (Fig. 3). These stratigraphic sections are � attened to the regional chronostratigraphic L marker. The overall stratal stacking pattern of the M1 sandstone, as observed in wireline logs of the studyarea, differs fromwhat is observed in lower cycles (T, U) of the Napo Formation stratigraphy. Fig. 3A shows a NE-SW stratigraphic correlation for the M1 sandstone member at the Eden Yuturi oil � eld located 30 km north of Block 31. The M1 sandstone here consists of at least three sand bodies that thin and almost disappear towards the southwest, where the sandstones are replaced by shale. An eastward thickening of the sands towards the proximal region is clearly observed in the well correlation. The gamma ray logs through the M1 sandstone succession shows clearly that it consists of an alternation of shale and sandstone. Both the uppermost and lowermost sandstone horizons in Fig. 3A are shown to be progradational because of the upwardcoarsening shale-to-sandstone log patterns. However, the middle of the three sandstones is sharp based. Fig. 3B is a N-S stratigraphic correlation within the Apaika oil �eld, depicting a lateral thickness change of the M1 sandstone. The


Fig. 3. Stratigraphic sections across the M1 Sandstone. (A) West to East stratigraphic section of the M1 sandstone at the Eden Yuturi oil Sandstone at the Apaika oil � eld. Note the wedge-shaped geometry of the sands.

thickest part of the sandstone is located within the APKA-06 well area; whereas, a progressive thinning of the sandstone occurs towards the APKA-02 well, becoming more dramatic at the APKA-14 well, where the sandstone disappears. The stratigraphic correlation of the M1 sandstone in Apaika (Fig. 3B), is somewhat similar to Eden Yuturi; however, the lowermost sandstone of the Eden Yuturi wells (Fig. 3A) is missing, as well

� eld.

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(B) Stratigraphic section of the M1

as the shale between the sandstones. Careful viewing of the sandstones in Apaika M1 wells in Fig. 3B shows that the M1 sandstone consists of an upper sand that is upward coarsening and is amalgamated to a lower, sharp-based sand, just as in Eden Yuturi. The intervening shale is absent because the Apaika wells lie in a more proximal paleogeographic position. The upward-coarsening, upper sandstone level in both � elds (and the lowermost sandstone in the

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Eden Yuturi Field) represent deltas that clinoformed as they prograded westwards, and an abrupt pinch-out of sand can be observed towards the southwest. However, the sharp-based, blocky sandstone (the middle sand in Eden Yuturi wells, and the lower sand in Apaika) is likely to be transgressive and of estuarine origin. It is very common in stratigraphy that transgressive and regressive sands alternate, during the construction of shelf platforms, by repeated back-and-forth transits of the sediment delivery system (Steel et al., 2008). Discontinuous sand packages also occur towards the base of the M1 sandstone member, most likely representing the distal parts of older clinoforms. Fig. 4 represents a net-sand isopach map of the combined M1 sandstone reservoir for the Apaika and Nenke oil � elds of Block 31. The map depicts lobate geometries for the M1 sandstone member, with an overall west-northwestward progradation direction. Distally on the Apaika wedge, both the transgressive basal sand and the overlying regressive clinoform sand become thinner. At the Apaika area, the lateral discontinuity of the sand distribution within the M1 sandstone member was con�rmed when drilling the APKA-14 well (Fig. 4), which failed to �nd sandstones. This well crossed a mud-rich interlobate area that separates the Apaika from the Apaika Sur sandy lobes.

the APKA-02 well (Fig. 5). The samples yielded abundant palynomorphs, but were barren in foraminifera and calcareous nannofossils. Palynomorphs included the cysts of the dino�agellate, Dinogymnium sp, Dinogymnium aff rigaudae of Campanian to Maastrichtian age (Boltenhagen, 1977), Dinogymnium cf. digitatus of Coniacian-Maastrichtian age, and Dinogymnium acuminatum of Campanian to Maastrichtian age (Evitt et al., 1967). Palynomorphs also include the pollen, Tricolporites sp, and Camarozonosporites sp trilete spores. Therefore, the palynomorphic association suggests a Campanian to Maastrichtian age, with sediments re�ecting a shallow marine to coastal environment. These data are consistent with previous biostratigraphic studies in the M1 sandstone member, which indicated a Campanian age for this reservoir at the Eden Yuturi and ITT oil �elds (Raynaud et al., 1993; Miles, 2003; Ordonez et al., 2006). As indicated earlier, the Upper Napo Shale member of the Sub-Andean zone is also dated as Campanian; therefore, the M1 sandstone of the eastern part of the Oriente Basin was coeval with the Upper Napo Shale of the westernmost part of the Oriente Basin. According to Jaillard (1997) and Vallejo et al. (2002), the palynomorph assemblage of the Upper Napo Shale along the western parts of the basin is dominated by a high diversity of dino�agellates, suggesting a normal marine environment for this member.

4.1. Age of the M1 sandstone in the Apaika oil � eld

5. Lithofacies of the M1 sandstone in the APKA-02 well core

Three samples were collected in this study for biostratigraphic analyses within the M1 sandstone member from a cored section of

There have been few published sedimentological studies on the M1 sandstone member, mostly due to the lack of a continuous cored section of this reservoir. For this study, we had access to a cored interval of the M1 sandstone from the APKA-02 well located in the Apaika area (Fig. 5). Cores were examined for (1) bedding contacts, (2) bed-thickness variations, (3) grain size, (4) lithologic variations, (5) primary physical-sedimentary structures, (6) biological sedimentary structures, (7) syndepositional and postdepositional sedimentary structures, and (8) oil staining. The core description and the environmental interpretations have subsequently been calibrated against the local and regional seismic data. Despite the fact that the analyzed cored section is not complete, the cores help us to interpret the depositional processes involved during the sedimentation of the M1 sandstone member at the Apaika and neighbouring areas. The APKA-02 well includes a cored section of 92 ft, and the lithofacies characteristics are summarized in Table 1.

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5.1. Mudstones (F1) 5.1.1. Description Lithofacies F1 is located at the base of the M1 cored section from the Apaika and consists of massive, black to grey-colored claystone (Fig. 6A). Bioturbation is absent in this lithofacies. 5.1.2. Interpretation The muddy nature of this lithofacies and the absence of bioturbation suggest stressed conditions and probably highsedimentation rates consistent with a shallow marine settings. 5.2. Breccia (F2)

Fig. 4. Net sand isopach map from the base to the top of the M1 Sandstone Member at the Apaika and Nenke oil � elds. Note the lobate nature of the sands.

5.2.1. Description Lithofacies F2 include centimetric blocks of intra-basinal reworked material, supported by a matrix of poorly sorted medium-to coarse-grained sand. Some of the clasts, as observed in the cored section and image logs (Fig. 5), are estimated at approximately 30-cm long. Clasts are subangular and internally deformed (Fig. 6B). A petrographic analysis (Fig. 7A) shows that the


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Fig. 5. Logs showing the lithology, stratigraphy, sedimentological features, lithofacies associations, image log and paleocurrent data of the M1 sandstone from a cored section in APKA-02 well at the Apaika oil � eld.

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sandstone is composed of >80% quartz grains, 5% silt and <15% autigenic clay (mainly kaolinite). Calcareous cement is locally present. The quartz grains are subangular to subrounded. 5.2.2. Interpretation The chaotic nature of these deposits suggest a gravitational collapse by subaqueous gravity-induced mass movements. The sediments were likely supplied from unstable slopes. These collapse features are similar to the slope failures identi�ed on some modern submarine slopes downdrift of large river mouths ( Nemec et al., 1988). Another possible interpretation is that the gravityinduced collapse was on the margin of an estuarine channel. Gravel or breccia can easily occur by bank collapse into the thalweg of a large channel, and the F2 lithofacies can occur above the sharp base of the lowermost part of the basal blocky M1 sandstone. 5.3. Medium-to coarse-grained heterogeneous sands (F3) 5.3.1. Description Lithofacies F3 includes poorly sorted medium-to coarse-grained sandstone. Elongated mudstone clasts and coal fragments occur at the top of individual beds and, in some cases, parallel to the strati�cation (Fig. 6C). Contorted mud layers also occur intercalated within this lithofacies. The individual beds have sharp upper contacts (Fig. 6C). A petrographic analysis indicates that the sandstone include >80% subangular to subrounded quartz grains, 5% silt, 5% detritical clays and < 10% autigenic clays (Fig. 7B). 5.3.2. Interpretation The poorly sorted nature of this lithofacies suggests mass � ow processes. The presence of �oating mudstone clasts with planar fabric near the top of the sandstone beds suggest laminar �ow conditions typical of debris �ows with plastic rheology (Shanmugam et al., 1994). Sharp upper contacts at the top of the sand beds are interpreted as evidence of deposition by en masse freezing, which is typical of debris �ow processes (Shanmugam, 2012). The coal fragments were derived from terrestrial areas and transported by rivers. 5.4. Rhythmic layers of ripple-laminated sand and mud (F4) 5.4.1. Description Lithofacies F4 includes rhythmic layers of ripple-laminated sandstone and mudstone. Rhythmic occurrence of mud and sand layers present normal grading and sharp-based beds (Fig. 6D). Small-scale load structures are also characteristic of these intercalations. The ripples are symmetrical, resembling wave ripples, and sometimes truncated at the top. Hummocky crossstrati�cation can also occur within this lithofacies. Cm-scale beds of F3 lithofacies are frequently intercalated with this lithofacies, which is composed of 30e60% quartz, 20 e40% silt, 5% detritical clays, and <15% autigenic clays (Fig. 7C). The quartz grains are subangular to subrounded. 5.4.2. Interpretation The sharp-based beds capped by ripples suggest decelerating waning � ows from river � oods (Bhattacharya and Walker, 1991) or dilute turbiditic currents as the � ow emerged from a channel and changed from con�ned to uncon�ned (Ambrose et al., 2009). The loading indicates that the muds were less dense than the overlying denser sandstones. The presence of wave ripples and mud drapes, together with occasional hummocky cross-strati �cation, suggest possible wave-reworking processes (Shanmugam, 2012). Therefore, this lithofacies, when present in the upward-coarsening, part of the sandy core, may represent � ne-grained turbiditic � ows, possibly of

hyperpycnite character, which would be common on the front of a river-dominated delta. There was occasional reworking of these thin turbidites by waves. Where F4 occurs in the lower part of the core, the rhythmic beds could be of tidal current origin, probably associated to estuarine tidal channels. 5.5. Inverse-graded beds (F5) 5.5.1. Description Lithofacies F5 is dominated by inverse-graded sandstone beds approximately 1-ft thick (Fig. 6E). Grain size varies from �nemedium sands to granule-size quartz grains. The graded beds are capped by thin layers of rippled sandstones. Wood, leaves, and other plant fragments occur occasionally. A petrographic analysis shows that this lithofacies is composed of >70% quartz, 20% silt, and <10% authigenic clays (Fig. 7D). The quartz grains are subangular to subrounded. The thin layers of rippled sandstones present organicrich laminae. Paleocurrent data obtained from this lithofacies and the associated F3 lithofacies indicates a west-northwest transport direction (Fig. 5). 5.5.2. Interpretation The inverse grading of the sandy matrix can be explained by dispersive pressure in grain � ows (Bagnold, 1954) or in other types of sediment gravity- �ow deposits (Naylor, 1980). This is because larger particles in high-concentration granular �ows tend to be pushed upward toward the free, upper surface of the � ow due to internal grain collision (Shanmugam, 2012). Inverse grading is also diagnostic of the acceleration of sediment-laden hyperpycnal �ows that have reached maximum capacity during a waxing river � ood (Mulder et al., 2003; Bhattacharya, 2010). The association of the inverse-graded beds with small, sandy debris �ows is consistent with hyperpycnal delta-front � ows from the river mouth in association with other delta-front collapse episodes. 5.6. Heterolithic lithofacies (F6) 5.6.1. Description Lithofacies F6 occurs at the top of the analyzed section and consists of intercalations of silt-to �ne-grained sands with mud. The individual beds are greater than 1-cm thick, with sharp bases and normal grading. The parallel-laminated sand presents smallscale load structures of �ne to very �ne grain size. Some of the sandstone beds are overlain by ripples (Fig.6F). Manyof the thinner siltstone or very-�ne-grained sandstone laminae are lenticular and appear to have formed starved ripples (Fig. 6F). This lithofacies also displays low intensities of bioturbation and phytodetrital material. 5.6.2. Interpretation Sedimentary structures include sharp-based beds that show normal grading and are capped by ripples indicative of sustained �ows that waned. The associated thin-bedded mudstones and �negrained sandstones may suggest a dilute hyperpycnal �ows, as they contain organic material with sharp but not erosive bases ( Petter and Steel, 2006). This lithofacies is similar to modern hyperpycnite sediments produced by direct � uvial discharges generally related to river �oods (e.g., Mulder and Syvitski,1995), deposited in a distal delta front to prodeltaic setting. 6. Seismic data interpretation

Three-dimensional (3-D) post-stack seismic data from Block 31 was available for this study. The seismic data were migrated in time with a bin size of 25  25 m. After a frequency analysis, the seismic resolution was estimated to be 80 ft on average. Stratigraphic


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Table 1

Lithofacies description and palaeoenvironmental interpretation. Lithofacies

Lithology

Processes and depositional environment

F1: Mudstones

Massive claystone of black to grey color.

F2: Breccia

Centimetric intrabasinal blocks in a matrix formed by medium to coarse grained sands.

F3: Medium to coarse grained heterogeneous sands

Poorly sorted medium to coarse grained sandstone. Elongated mudstone clasts and coal fragments occur at the top. Locally contorted mud layers. Rythmic layers of sand and mud intercalated with wavy rippled sandstones.

Low energy shallow marine to prodeltaic environment. Gravitational collapse by recurrent subaqueous gravity induced mass movements in submarine slopes, or gravity-induced collapse on the margin of an estuarine channel. Debris � ows deposited by en masse freezing in a delta front.

F4: Rhythmic layers of sand and mud intercalated with ripples F5: Inverse graded beds F6. Heterolithic lithofacies

Inverse graded � ne-medium sandstones capped by thin layers of rippled sandstones. Intercalations of silt to � ne grained sands with muds.

surfaces identi�ed from core and/or well logs were tied to seismic horizons and mapped for the entire area. Root mean square (RMS) amplitude extractions from the 3-D seismic cube were performed to de�ne the geometry of the sands within the M1 sandstone member in the Apaika and Nenke areas. This techniquewas applied to better de �ne the sandstone limits. Fig. 8 represents a plan view of a time window within the M1 sandstone. In the map, the cooler colours (lower amplitudes) represent muddy sediments, while the areas with higher amplitude (yellow to red) are interpreted to be sand rich. The geometries de �ned after this amplitude extraction depict lobe geometries for the M1 sandstone in the Apaika and Nenke areas and the neighbouring Apaika Sur. Fig. 9A and B show a southwest-to-northeast-oriented seismic fence and another seismic line depicting the internal structure of the M1 sandstone lobe in the Apaika area. The wedge-shaped geometry of the bed succession is evident and is similar to what was observed from the wireline log correlations ( Fig. 3), demonstrating that the prograding clinoforms that form the upper level of M1 sandstone oil reservoirs can be seismically mapped. The clinoform slope geometry of the individual sand bodies is also evident, which (in the context of a river-dominated delta front) can also explain the gravitational sedimentary processes observed in the analyzed cored section of the APKA-02 well. 7. Depositional environment of the M1 sandstone

According to the log patterns and cored lithofacies as described above, the M1 sandstone in the Apaika area of the Oriente Basin of Ecuador is likely to have been deposited in two different environments: the sharp-based basal beds with blocky log pattern, rhythmic tidal beds and breccia-collapse bank margins are likely to have originated in estuarine channels developed thinly during the transgression of the area; the thicker and upward-coarsening succession dominated by mass-�ow deposits, particularly sandy mass-transport deposits, are likely to represent regressive deltas that prograded westwards across the Apaika area. The presence of �oating mudstone clasts, inverse grading, and sharp upper contacts of individual beds, which are typical of hyperpycnal �ows and sandy debris �ows, support this interpretation. There was also some in�uence of wave-reworking of sediments. The absence of bioturbation suggests high sedimentation rates (e.g., MacEachern et al., 2005). The presence of mass-�ow deposits and hyperpycnites suggests river � ooding and gravitational instability on the river-dominated delta front, which, according to the seismic interpretation, can be related to low-angle, subaqueous, prograding clinoforms.

Fine-grained turbiditic � ows, possibly of hyperpycnite character, reworked by waves in a river-dominated delta front. Hyperpycnal delta-front � ows. Low-density hyperpycnal �ow in a distal delta front to prodeltaic environment.

The geometries depicted by the stratigraphic correlations, sedimentological processes identi�ed in the core descriptions, and the seismic data indicate that the M1 sandstone reservoir in the Apaika, Nenke and Eden Yuturi areas was deposited as a thin lower tier of estuarine channels and a thicker upper tier of prograding delta lobes. This would explain the lateral discontinuity of the sandstone observed in the Apaika and Nenke areas, a feature typical of debris �ow lobes (Shanmugam and Moiola, 1991; Shanmugam et al., 1994). In the case of the deltas, the delta-front slope is known to be inherently unstable and prone to debris- �ow processes (Nemec, 1990). The palynomorph association, which is interpreted in terms of coastal environments, supports this interpretation. The high abundance of coal fragments and the abundance of phytodetrital material derived from continental areas identi�ed in the cored intervals suggest direct river input into the basin ( PlinkBjorklund and Steel, 2006). Down-dip, clinoform cross-sectional architecture identi�ed in the seismic data is also consistent with westward-dipping, prograding delta lobes. In our interpretation, debris �ows were likely sourced by a delta-front collapse, a process known to occur elsewhere in the geological record (e.g., Nemec et al., 1988). Cross-strati�cation is rare, which suggests that sedimentation was rapid, impeding the development of these traction structures. However, farther east for the Vivian Formation of northern Peru, Radomski et al. (2010) reported the presence of abundant trough cross-bedded sandstones with interbedded coaly organics, gutter scours, rhizoliths, calcrete horizons, and a lack of observable trace fossils. Similar facies with abundant cross-bedded sandstones were observed in cores of the M1 sandstone at the ITT oil � elds (Vallejo et al., 2015), east of the Apaika and Nenke areas (Fig. 1). These observations suggest the presence of channelized facies to the east of the study area, in a proximal part of the system. In the studied deltaic part of the succession, we recognized delta-front and pro-delta lithofacies (Fig. 5), with no representation of proximal delta-front facies or evidence of subaerial exposures. However, in the present study, we did not have a continuous cored section of the transition from the described deltaic sediments to shallower facies. Overlying the Campanian M1 sandstone, there is an important change in sedimentation. Electric log data and cuttings description indicate a regional change from the subaqueous sediments of the M1 sandstone to red beds of the Tena Formation, which represent a different sedimentary cycle when compared with the M1 sandstone (see provenance analysis). €

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Fig. 6. Lithofacies of the M1 sandstone at the APKA-02 well cored section. (A) Claystone sediments underlying the base of the M1 Sandstone Member (F1 lithofacies); (B) Brecciated sediments presenting deformed clats of intrabasinal origin (F2 lithofacies); (C) Poorly sorted medium to coarse grained sandstone. It includes elongated mudstone clasts at the top of the bed, parallel to the strati �cation, and sharp upper bed contact (F3 lithofacies); (D) Rythmic layers of ripple-laminated sandstone and mudstone, including wave ripples and hummocky cross strati�cation (F4 lithofacies); (E) Inverse-graded sandstone beds capped by thin layers of rippled sandstones (F5 lithofacies); (F) Sharp-based sandstone intercalated with mudstones in cm-scale beds. Sandstone show grading, load structures and organic matter laminae (F6 lithofacies).

8. Provenance analysis

Three samples from the M1 sandstone member at the APKA-02 and one sample from the ITT oil �elds (ISHP-01M) were collected for provenance analysis. These samples were taken at different depths within the Campanian M1 sandstone member ( Fig. 5). In addition, two samples of the Maastrichtian Tena Formation were

also selected for detrital zircon dating. One of the samples (ISHP01T) came from the ISHP-01 well in the neighbouring ITT oil � elds (Fig. 1), and a second sample (BT-001) was collected in the Subandean zone of the Oriente Basin (Puyo area). The ISHP-01T sample of the Tena Formation was taken above the late-Cretaceous unconformity, which separates the M1 sandstone member from the Tena Formation at the ITT. The BT-001 sample was also taken above


Fig. 7. Representative thin sections of the M1 Sandstone oil reservoir from the APKA-02 well at the Apaika oil

the late Cretaceous unconformity that separates the M2 limestone from the Tena Formation in the Sub-Andean zone. Histograms represent the age distribution of the four analyzed samples of the M1 sandstone (Fig. 10). Concordant U/Pb of individual zircon grains for the analyzed samples were used for the age histogram. The age histograms for the M1 sandstone in the Apaika and ITT areas show a relatively wide range of ages, ranging from 1 to 3 billion years. However, the population of detrital zircons in the range of 1.5e1.6 billion years is the most important. The ages of the source regions overlap with the ages reported for the Rio Jurena lithotectonic province (Tassinari and Macambira, 1999). This province is located in the western part of the Amazon Craton, along a NW-SE trend approximately 2000-km long and 600-km wide, and underlies parts of Brazil, Venezuela, and Colombia (Fig. 11). The basement of the Rio Negro-Jurena province is primarily composed of gneiss and granitoids, with an age range of 1.8 to 1.55 Ga. Its northern part is predominantly composed of biotite monzogranites, while in the southern part, the basement rocks are composed of granitic gneisses and migmatites with tonalitic compositions. In general, the rocks are metamorphosed to amphibolite facies, although some granulites are also present (Dall'Agnol and Macambira, 1992). The location of the Rio Negro-Jurena province is consistent with the expected direction of rivers that fed the delta de �ned for the upper M1 sandstone; therefore, we can con �rm an east-to-west direction of deltaic progradation. This direction is also consistent with the direction of clinoform progradation, as interpreted from seismic data in the Apaika and Nenke areas. Detrital zircon ages of the Tena Formation (Fig.10A and B) reveal at least �ve different age populations. The youngest zircons yielded ages between 80 and 100 Ma. Source regions with these ages are not known from the neighbouring areas. However, Barragan et al. (2005) dated a series of volcanic cones restricted to the western part of the Oriente Basin. Vasquez (2007) also reported the presence of Cretaceous volcanism at both � anks of the Eastern Cordillera of Colombia. Both authors indicated that the Cretaceous 

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� eld.

volcanism had an intraplate origin and was restricted to the western part of the Cretaceous Putumayo-Oriente-Maranon basins. Therefore, wesuggest that the 80 to 100 Ma zircons within the Tena Formation were derived from volcanic rocks of these ages exposed in the Eastern Cordillera of Ecuador during the Maastrichtian. These rocks have been eroded in the Eastern Cordillera due to the uplift of the Andes. An important age population recorded in the Tena sandstone ranges from 240 to 220 Ma. This imprint on the Tena Basin was possibly derived from the erosion of the Tres Lagunas S-type granitoid, which is exposed in the Eastern Cordillera and is dated between 240 and 220 Ma (Litherland et al., 1994; Spikings et al., 2015). The zircons dated from 400 to 600 Ma overlap ages reported in the Palaeozoic rocks of the Eastern Cordillera (Spikings et al., 2015). Few zircon ages are dated between 1000 and 1300 Ma, which may correlate with zircons eroded from rocks belonging to the Sunsas province (Litherland et al.,1985). A notable equivalent of such source rocks lies in southern Colombia, where the Garz on Granulitic Belt is interpreted to represent an extension of the Sunsas Belt. In general, the detrital zircon ages of the Tena Formation suggest that this formation was derived from the erosion of the Eastern Cordillera of Ecuador, as well as from the reworking of material from the South American Craton. ~



9. Geological setting of eastern Ecuador during the Campanian to Maastrichtian period

The tectonic evolution of the Northern Andes during Campanian to Maastrichtian time was strongly in�uenced by collision of the Caribbean Large Igneous Province (CLIP) at 75 Ma (Luzieux et al., 2006; Vallejo et al., 2006). In Ecuador, this event caused a change from shallow marine deposition of the Napo Formation to nonmarine deposits of the Tena Formation (Gombojav and Winkler, 2008). The analysed stratigraphic interval, which includes the M1 sandstone of the Napo Formation and the lower part of the Tena z

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Fig. 8. RMS amplitude time slice map from a window spanning the M1 sandstone in the Block 31 area. Brighter colors (yellow to red) correspond to layers that are interpreted to be sand rich. Note the lobate shape of the anomalies representing the Nenke, Apaika, Apaika Sur oil �elds. (For interpretation of the references to colour in this �gure legend, the reader is referred to the web version of this article.)

Formation, coincidewith the time of the early collision between the CLIP with the northwestern margin of South America. Deltaic progradation can be caused by either (1) an anomalously high siliciclastic sediment � ux with a relatively stable sea level or (2) a smaller sediment � ux during deltaic forced regression with a relative sea-level fall (Edwards, 1981; Galloway, 1989). Local uplift of the South American craton caused an increase in clastic input to the Oriente Basin shelf margin during the Campanian, forcing progradation of clastic wedges. This resulted in delta-driven

clinoforms downlapping onto the former, shallow-dipping transgressive deposits of the pre-Campanian section of the Napo Formation. Progradation of the Campanian delta occurred in the eastern part of the basin (Fig. 11), whereas marine conditions prevailed in the central and western parts. Maastrichtian continuation of the regional deformation event in Ecuador and the early phases of the Andean Orogeny caused a dramatic change in sedimentation and the abandonment of the east-to-west drainage system that dominated during the


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Fig. 9. Seismic interpretation of the M1 sandstone at the Apaika area. (A). Fence diagram showing three-dimensional RMS amplitude variation from the Apaika oil � eld. Sandstones in the M1 interval are likely indicated by high amplitude orange re �ectors, which represents the prograding clinoforms; (B) Southwest to northeast seismic section showing the Apaike deltaic clinoforms. Clinoforms wedge towards the southwest, which is also consistent with the stratigraphic correlations based on wireline logs. (For interpretation of the references to colour in this � gure legend, the reader is referred to the web version of this article.)

deposition of the Napo Formation. According to provenance analyses, the sediments of the Maastrichtian Tena Formation were derived from a different source than the underlying M1 sandstone. The source of the Tena Formation was likely located in the Eastern Cordillera, as suggested by the detrital zircon ages presented in this study and previous provenance analyses (Ruiz et al., 2007; Gombojav and Winkler, 2008); Therefore, during this period, the drainage system switched from west to east, similar to the present drainage patterns in the Amazon Basin. 10. Comparison with neighbouring areas

The Campanian deltaic system in the eastern part of the Oriente Basin was probably not limited to Ecuador. The Campanian Guadalupe Group of the Llanos Basin and Eastern Cordillera of

Colombia (Guerrero and Sarmiento, 1996; Villamil, 1999 ) includes sandstone beds interpreted as part of a large progradational delta system sourced from the east (Villamil, 1999; Egbue and Kellogg, 2012). In the Llanos foothills, the Guadalupe Group rests on Santonian shallow marine sediments of the Gacheta Formation (Egbue and Kellogg, 2012), which are correlatable with shallow marine sediments of the Napo Formation of Ecuador ( Jacques, 2004). Therefore, the delta system described for the M1 sandstone of the Oriente Basin may have occupied large areas (Fig.11). It is likely that collision of the CLIP � exed the South American plate, producing a distal bulge along the outboard eastern margin of the Sub-Andean basins, driving further sediment input from the Amazon Craton immediately after an earlier transgressive phase. Detrital zircon ages of the Maatrichtian Tena Formation indicate derivation from the Eastern Cordillera of Ecuador. This potential

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Fig. 10. A) Age histogram U-Pb LA-ICPMS zircon dating of the Campanian M1 sandstone and the Maastrichtian Tena Formation of the Oriente Basin.

source area continued into the Central Cordillera of Colombia (Spikings et al., 2015). These sources include the Jurassic magmatic arc (170-150 Ma), Triassic granitoids (230-220 Ma), and rocks younger than 150 Ma (Silva et al., 2013; Bayona et al., 2013; Horton et al., 2015). For the Colombian segment, however, the arrival of detritus from the Central Cordillera differs from Ecuador. In large segments of the Magdalena Basin, Eastern Cordillera, and Llanos basin, detritus from the Central Cordillera appeared in Maastrichtian to late Paleocene (Nie et al., 2012; Silva et al., 2013). During the Cretaceous, the Middle Magdalena Valley, Eastern Cordillera, and Llanos basin constituted a major integrated sedimentary basin (Cooper et al., 1995; Horton et al., 2010, 2015). The axis of the basin depocenter was located east of the Central Cordillera during Campanian-early Maastrichtian time (Mora et al., 2010a; Silva et al., 2013) and then migrated eastward with progressive uplift of the Central Cordillera (Villamil, 1999), leading to selective recycling of Maastrichtian strata (Reyes-Harker et al., 2015). In Colombia, the Maastrichtian Umir and Seca formations recorded detrital zircons with U-Pb ages younger than 150 Ma

(Silva et al., 2013), comparable to the Tena Formation. Although the Umir Formation is restricted to proximal parts of the basin, detrital input from the Central Cordillera is preserved in sediments accumulated in proximal to medial parts of the basin (including the Magdalena Valley and Eastern Cordillera) but not the distal Llanos segment of the basin (Horton et al., 2010, 2015; Saylor et al., 2011; Silva et al., 2013). In Ecuador, however, the clear and widespread input of Eastern Cordillera detritus from the western to easternmost part of the basin during the Maastrichtian may re�ect contrasts in the distribution of subsidence and/or the regional continuity of the basin relative to Colombia. Maastrichtian and Palaeocene units derived from the Eastern Cordillera were deposited continuously along the Oriente Basin of Ecuador (Dashwood and Abbotts, 1990), similar to the Late Cretaceous evolution of the neighbouring Putumayo Basin of Colombia, which shares a stratigraphic and structural framework similar to the Oriente Basin ( Higley, 2001). In the Putumayo Basin, the Rumiyacu Formation is the lithological and time equivalent to the Tena Formation and it was deposited continuously along the basin (Gonçalves et al., 2002; Mora et al., 2010b; Wolaver et al.,


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Fig. 11. Paleogeography of the Oriente Basin and neighbouring regions during the Campanian. Lithotectonic provinces map modi �ed from Cordani et al. (2000). Areal extension of the Campanian progradation in Colombia modi �ed from Villamil (1999).

2015), although no provenance data areavailable for this formation. 11. Conclusions

The architecture of the Campanian M1 sandstone in the Eden Yuturi, Apaika and Nenke areas of the eastern part of the Oriente Basin, as indicated by wireline logs, well core data, and seismic interpretation, strongly suggests an initial erosively based transgression followed by the development of a prograding delta system. The progradation of the M1 sandstone occurred above a basal, sandy estuarine unit and the underlying shale-prone Napo sedimentation cycle. The basal estuarine sand was caused by an immediately prior transgression eastwards across the region. The thickness maps of the M1 sandstone member present lobate shapes that thin rapidly westward, a pattern that is consistent with an east-to-west delta progradation. Provenance analyses indicate that the main source for the M1 sandstone were rocks within an age range of 1.4 e1.6 billion years and suggest that the sediments were mostly derived from the direct erosion of the Rio Negro Jurena lithotectonic province, located to the east of the studied area, within the South American Craton. Moreover, the westward progradation of clastic weges is consistent with the overall paleogeography, paleocurrent data and provenance analyses that point to a source region located to the east.

Correlation with neighboring regions in Colombia suggest that the documented M1 sandstone delta lobes of the study area were probably part of a large delta system sourced from the east during major Campanian progradation. Deltaic progradation was possible linked to a regional tectonic event associated with the collision of the CLIP against the Northern Andes, producing an increase in sediment supply from the South American Craton. A provenance analysis from the Maastrichtian Tena Formation indicate a continental-scale drainage reorganization during this period. Detrital U-Pb zircon ages record an input from the emerging Eastern Cordillera of Ecuador, and the Central Cordillera of Colombia. The importance of de �ning a delta system for the M1 Sandstone reservoir is that this type of reservoir is among the world's largest in areal extent (Woodroffe et al., 2006). However, because of the heterogeneities associated with the bed geometry and facies variation along the deltaic clinoforms, re�ned geological models should be considered when the reservoirs are being modelled. The potential for stratigraphic traps within the deltaic system is very high due to the compartmentalization of the individual sandstone clinoforms. Finally, the integration of core data, wireline logs, and seismic data is a powerful tool for describing the geometry and lateral extension of this type of reservoir.

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Acknowledgements

Wethank A. Carrera, M. Rivadeneira, and B. K. Horton forfruitful discussions about the geology of the Oriente Basin and the Northern Andes. We are grateful to an anonymous reviewer, and the Associate Editor, Istvan Csato, for their constructive comments and suggestions. Special thanks to F. Paz, O. Morales, R. Almeida for encouraging the realization of this project. We acknowledge funding by PETROAMAZONAS EP, GEOSTRAT SA, and EPN PIS-16-09 project. Permission for publication of this research is gratefully acknowledged to PETROAMAZONAS EP Management.

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