Department of Geology University of Peshawar & Pakistan Association of Petroleum Geoscientists
Principles and techniques of cross section balancing and its implication in hydrocarbon exploration Bara Gali August 30- September 01, 2008 Presented by: Dr. Sajjad Ahmad
Introduction Warm well come to this short course on cross section balancing techniques and its implication in hydrocarbon exploration. The next three days cover an intense review of the fold-thrust belt structures, balancing principles and techniques with special emphasis on its use in hydrocarbon exploration. The first two days will be spent on covering theoretical and practical aspects, supported by numerous exercises in fold-thrust belts interpretation and balancing. The last day will be spent in field to observe fold-thrust assemblages in Mesozoic-Eocene rocks of the Khan Pur valley.
DEFINITION “A balanced cross section is a deformed state cross section that is both admissible and viable” Admissibility means that the section must show realistic-looking fold and fault geometries and has to obey local rules Viability means that the amount of rock area before deformation has to be equal to the amount of rock area after the deformation and the predeformational fault geometries must be admissible or simply the section should be retrodeformable A balanced cross section does not mean that the interpretation is unique and correct. A balanced cross section is possibly one of the correct interpretations whereas a section that does not balance is probably wrong.
WHY WE BALANCE A SECTION • • • • •
For correct palinspatic restoration To determine geologic sequence of events To understand geometry of prospect To construct more accurate prospect and reservoir maps To discover significant additional reserves
WHAT IS MOST IMPORTANT? The interpreter’s knowledge of structural geology and the application of techniques that lead to geologically reasonable and sound interpretations
STRUCTURAL ELEMENTS OF FOLD-THRUST BELTS Fold-thrust belts are typical of convergent plate tectonic habitat. These are belts of deformed sedimentary rock in which the layers are folded and duplicated by thrust faults and are common at the external edges of orogens. Folds and reverse faults are the dominant structures in un-metamorphosed rocks exposed in a belt that may be tens or hundreds of kilometres wide. Internal zone The internal zone is the portion of the belt where plastic deformation dominates, penetrative strains develop and metamorphism occurs (hinterland) (Fig.1). External zone Boarders the undeformed continental interior and is characterized by less plastic deformation, nonmetamorphic conditions and nonpenetrative strain (foreland) (Fig.1).
Fig.1.Generalized geologic map of the NW Himalayas
Detachment A detachment or decollement is a subhorizontal or shallowly dipping fault along which a sheet of rocks has moved relative to the underlying substrate. In stratified rocks detachment commonly lies in the plane of bedding. Several detachments may lie in a vertical sequence. The lowest is called as basal detachment (Fig.2).
Fig.2. Schematic cross section showing the concept of detachments in thin-skinned deformed belt (external zone). In external zones the basal detachment commonly forms at or near the contact between sedimentary units and crystalline basement whereas in internal zones the basal detachment lies within crystalline basement rocks. Detachments commonly run within weak rocks but it can within hard as well.
Thrust sheet The package of rocks above a fault is called thrust sheet. Thrust sheets are named after their underlying faults (Fig.3).
Fig.3. Schematic cross section showing the concept of step like geometry of a thrust fault.
Ramps and flats Many thrust faults are non-planar. Most commonly, steeper and more gently dipping segments are present. Ramps are regions on thrust faults where stratigraphy is truncated at relatively steep angles (typically > 10°) Flats are regions where thrust faults do not cut stratigraphy, or where the strata are cut at a very low angle. Typically, ramps and flats correlate with the stratigraphic units cut. Massive, competent units usually show thrust faults that climb steeply through stratigraphy, whereas shaly, incompetent units contain thrusts that stay in one unit over large areas. We need to clarify the definitions for ramps and flats that make clear whether we are talking about the hanging wall or the footwall. These are distinguished as under; Footwall ramp Footwall flat Hanging wall ramp Hanging wall flat Frontal, lateral and oblique ramps Ramps need not be oriented parallel to the overall strike of the orogen. A frontal ramp is a ramp whose strike is orogen-parallel. An oblique ramp is oriented at an angle to the overall strike of the orogen A lateral ramp strikes perpendicular to the orogen. Lateral ramps can be quite steep, as the direction of thrusting is typically parallel to the strike, so the hanging wall block does not have to climb up the dip slope. Such steep strike-slip faults at high angles to thrust belt strike are generally called tear faults.
Fig.4. Diagram showing different types of ramps.
Cutoff points, lines and angle Where thrust faults cut stratigraphic boundaries there are cutoff points in cross-section (point E in figure 3). Typically, for a given stratigraphic horizon, we can identify a footwall cutoff point and a hanging wall cutoff point. The separation between them is evidence for the amount of displacement - though we need to know the slip direction to be precise about this. In 3-D each cutoff point corresponds to a cutoff line: the line of intersection between a bedding plane and a fault.
Fault tips and tip lines Sometimes thrusts lose displacement in the direction of transport. Typically this occurs when the hanging wall is folded or otherwise internally deformed but the footwall is not. Eventually, the offset may fall to zero. A fault tip point in the subsurface is a point beyond which displacement is zero the fault cannot be traced. In 3-dimensions this corresponds to a tip line. Such a thrust fault is described as a blind thrust.
Branch Lines
Tip Line
Fig.4. Diagrams showing tip lines and branch lines in 2 & 3D(from Twiss and Moores, 1992).
Arrays of thrust faults Thrusts do not occur alone. There are a many of characteristic geometries of multiple thrust faults. Because they are much studied in oil exploration, there is a great variety of terms surrounding thrust fault arrays. Imbricate thrusts In general, any thrust faults that have an en echelon arrangement when viewed in crosssection are described as imbricate. A series of imbricate thrust faults that branch out of a single, deeper 'floor' thrust is known as an imbricate fan. The points on a cross-section where the traces of two thrusts meet is called a branch point (fig.5).
Fig.5. Imbricate thrust array and branch point.
Fig.6 Imbricate fan formed from an array of fault propagation folds (After Mitra, 1990).
In 3-D each branch point corresponds to a branch line (fig.4). The branch points at the floor of an imbricate fan, where two thrusts branch when traced toward the foreland, are called trailing branch points.
Sometimes we can find other branch points where two thrusts join into one as they are traced toward the foreland. These are called leading branch points. In 3-D we recognize leading and trailing branch lines.
Duplexes Where a series of thrusts connects both with a floor thrust below and a roof thrust above, the configuration is known as a duplex. The individual thrust-bounded slices in a duplex are called horses. There are multiple trailing branch points at the floor of a duplex, at the hinterland end of each horse.
Roof thrust
Trailing edge Branch
Leading edge Branch
**
Propagation
* Sole thrust
Horses
Fig.7. Duplex strucrure and associated elements.
At the roof, thrusts join as they are traced toward the foreland; the foreland limit of each horse is marked by a leading branch point. Some duplexes exhibit very regular geometry, but in others the amount of offset between horses vary greatly. In some cases the horses are piled on top of each other to form an antiformal stack. If the displacement is higher for higher horses, they may have moved right over the antiformal stack to produce a foreland dipping duplex.
Horse
Duplex
Fig.8. Types and structural elements of duplex structure (after Davis & Reynolds, 1996).
Triangle zones and tectonic wedges There are areas in thrust belts where thrusts of opposite sense - conjugate thrusts - are developed. We can recongnize both foreland-vergent thrusts (ie those in which the hanging wall has been displaced towards the foreland) and hinterland vergent thrusts (the opposite). A body of rock that has moved between a pair of oppositely vergent thrusts is called a tectonic wedge.
Fig. simple triangle zone with passive roof back thrust. A combination of two thrusts with same basal detachment and with opposing vergence forms a triangle zone.
Fig.9. Diagram showing the geometry of triangle zone and pop up.
Thrust nappe A nappe is a recumbent, often isoclinal fold with definite asymmetry (vergence). Nappes are commonly observed with sheared out lower limbs, or thrust faults. Both the direction of shear or thrust faulting on the lower limb of the fold and the asymmetry of the fold have consistent vergence or direction of tectonic transport. Such a structure is a thrust nappe.
Folds in thrust belts Folds are almost always associated with thrusts and we can recognize a number of different fold styles that are characteristic of thrust belts. First, we will deal with some methods that can be used to construct cross-sections through folds in profile view. 1: Layer types and cross-section construction a: Layer geometry Just as thrusts faults are clearly influenced by the mechanical properties of different rocktypes, so are folds. It's common in fold and thrust belts to find that sandstone and limestone layers maintain their thickness when traced through folds, whereas shales and evaporites are strongly thickened in fold hinges. Many thrust belts show significant sedimentary packages that show Class 1 geometry, sometimes quite close to the ideal class 1B "parallel fold". Hence some constructions for cross-sections have been developed to assist with the drawing of parallel folds.
Fig.10. Class 1B fold ( after Ramsay, )
b: Busk or arc construction In the Busk construction we assume that folds are perfectly parallel (concentric) folds made from arcs of circles. The locations of the centre points for circular arcs are defined by drawing normals to the dip and finding their intersection points. The Busk construction produces very smoothly curved cross-sections. The construction breaks down at depth beneath anticlines and (less obviously) at high, eroded locations above synclines. This breakdown may be used to determine the depth to a detachment in areas of detachment folding. Note that although the cross-sections produced by this construction may "look" very realistic, in detail the assumption of circular arcs and perfectly parallel folds is unlikely to be true.
Fig.10. Busk method approximation. The strata are projected to depth along segments of circular arcs ( after Marshak and Mitra, 1988).
c: Kink construction More recently, another construction has become popular. Instead of being composed of perfectly circular arcs (as in the Busk construction), folds are assumed to be composed of perfectly flat limb panels separated by sharply angular hinges. In addition, layers are assumed to have equal thickness on either side of the hinge region. Geometrically, this means that fold axial surfaces must perfectly bisect the limbs. It's very easy to measure layer lengths in kink constructions, which makes section balancing (see later) easy. However, kink construction cross-sections have a tendency to look 'boxy' and angular when compared with photographs of outcrops of real structures. Using increasing numbers of axial surfaces it's possible to make 'smoother' kink constructions and approximate many real fold shapes. Remember, though, that both the kink and Busk constructions are only convenient approximations to reality!
Fig.11. Kink method approximation. The beds are projected to depth along planar limb surfaces. This method applies to majority of folds withplanar limbs ( after Tearpock & Bischke,2006)
Kink method application in hydrocarbon exploration An immediate application of the kink method construction arises when drilling the crest of the asymmetric monoclinal or asymmetric folds that are common in fold thrust belt.
The improper positioning of wells on crest can result in drilling wells offstructure or wells into synclines. In order to avoid costly mistakes the compressional regime requires a good understanding of structural styles and geometry. Figure shows two different interpretations of an asymmetric fold based on same bed dip and seismic data. The steeply dipping forelimb of the fold was not imaged. The proposed wells are spudded at different locations based on the anticipated high at the reservoir level. Which well is more likely to be successful? The frontal limb in figure a is interpreted to be thinner which is the style of high temperature mobile belts and donot contain hydrocarbons. Figure b shows a constant thickness fold where beds donot change thickness from back to fore limb. This type of folding is common in low tempraturepetroleum regime ( fold-thrust belts) On time profiles the stratigraphic intervals of constant thickness maintain about the same vertical time thickness. The depth profile showen in figure a is similar to a time profile on which the interpreter attempted to maintain the same vertical time thickness of the intervals. The result is a thin fold limb. On the other the geoscientist who constructed figure b made his interpretation an a time profile and then probably depth corrected it to generate the depth profile seen in figure b. The stratigraphic thickness was maintained and the result is a parallel fold.
Geoscientists working in fold-thrust belt regimes should know that majority of folds within hydrocarbon bearing regions approximate parallel fold rather than thin-limb folds.
If the fold is a constant thickness fold then the likely results after drilling the two wells is shown in figure . The well on the right is positione on the steeply dipping forelimb and will never test the reservoir. Perhaps the geoscientist who generated the right one profile believed that a seismic section is a geologic profile. Profile on the left demonstrates kink law that if the beds do not change thickness, then the axial surface bisects the limbs of the fold( the angles between the two fold limbs and axial surface are about the same.
Fig. Two different interpretation for same dip and seismic data.
2: Symmetry relationships of folds and thrusts In many thrust belts the folds are asymmetric (S-folds or Z folds), and also show vergence toward the foreland. In this case we recognize long backlimbs: fold limbs that dip toward the hinterland short forelimbs: fold limbs that dip toward the foreland, sometimes quite steeply. Early descriptions of fold and thrust belts adopted a terminology in which thrust were described in relation to folds.
backlimb thusts forelimb thrusts out-of-the-syncline thrusts This is a purely geometric classification. It suffers from the disadvantage that the classification of a given thrust depends on the erosion level. A backlimb thust might pass into an out-of-the-syncline or forelimb position as it is traced up or down-dip. Relationship of folds to thrust ramps, flats, and tips More recently another type of classification of folds in thrust belts has been developed, in which folds are related to thrusts (rather than the other way round). This classification is not purely geometrical; it has a relationship to thrust kinematics which we will explore more fully in the next section. a: Fault-bend folds When movement occurs on a curved fault, rotation and/or distortion of the wall rocks must occur to preserve strain compatibility. Typically, for a large-scale fault with ramps and flats, the hangingwall is folded to accommodate to the shape of the footwall ramps. Because ramps typically dip at < 40°, fault-bend folds are typically open in style. An exception might be a fault-bend fold over an antiformal stack duplex.
b:
Detachment folds Other folds in thrust belts are associated with flat thrusts, and mark locations where the displacement changes. Such folds are known as detachment folds. Some examples are particularly conspicuous in cross-sections of the Jura Mountains, in the thrust belt of the Swiss Alps, where the underlying décollement is located in a layer of Triassic evaporites.
Fig. Cross section of detachment folds in the foreland of Jura Mountains (Switzerland). c: Fault propagation folds Related geometries occur at fault tips. The best understood geometry occurs where a fault tip lies in a ramp. Strata cut by the base of the ramp are shortened by thrusting. Strata above the tip of the fault are shortened entirely by folding. Intermediate strata are shortened by a combination of folding and faulting. Kinematically, fault tips must propagate through rocks as faults develop. Hence the associated fold is called a fault propagation fold. It is possible for the fault tip to propagate beyond the area of folding, leaving the fold truncated in the hangingwall. Such folds resemble detachment folds. There may be a continuum of geometries between 'classic' detachment and 'classic' fault-propagation folds.
Fold thrust belts in map view Faults are not surfaces
e
Folding definition in time section
Folds and bedding thickness Folds are classified on whether or not the thickness of stratigraphic layers changes in dip domains across axial surfaces. Parallel folds preserve constant thickness, and are common in strata that deformed predominantly by flexural slip. Axial surfaces bisect interlimb angles in parallel folds.
Deformed state section
RECOGNIZING THRUST AND REVERSE FAULTS
Restored section
Deformed and restored state cross section the Rocky Mountain foothills of southern Alberta
Triangle zones. After McClay, K. R., 1992, Glossary of thrust tectonics terms, in McClay, K. R., ed., Thrust Tectonics: London, Chapman and Hall, p. 419-433.
Back thrust
In many thin-skinned fold and thrust belts, most of the fold and thrust structures have a definite, consistent vergence to them. That is, the sense of overturn on the folds and the dip and transport direction on the faults suggest consistent transport of material towards the foreland. A backthrust is a thrust fault that dips in a direction opposite to that of most of the structures in the belts. Foreland Thin skinned fold and thrust belts are often found on the flanks of mountain belts. The area outboard of the mountain belt consisting of undeformed sediments is known as the foreland. Since thinskinned fold and thrust belts typically detach on previously .at lying sediments andpropagate deformation towards the foreland, they are often referred to as foreland fold and thrust belts. Hinterland The core of a mountain belt, often characterized by rocks of high metamorphic grade and ductile deformation histories is the hinterland of the range. Thin-skinned fold and thrust belts are generally found between the undeformed foreland and the strongly deformed core, or hinterland of the range. Tectonic transport in foreland thrust belts is generally directed from the hinterland to the foreland. Duplex Low angle faults are often characterized by alternating ramps and .ats. Ramps are often associated with a series of imbricate (parallel or “shingled” faults joined by faults above and below them. In a thrust environment, the structural association is: two .at segments (called the .oor and roof thrusts), connected by several parallel ramp segments. The rock masses bounded by these faults are called horses, and the entire structural association is a duplex. Duplex structure Roof thrust Floor thrust Horse Horse Out of sequence thrust In a fold and thrust belt, deformation commonly propagates towards the foreland.That is, thrusts become progressively younger as you go towards the foreland (in the direction of transport). This is in sequence thrusting. A fault that is younger, but is located more hinterland ward than some other fault is, in contrast, out of sequence. Note that this has nothing to do with the transport direction or dip of the out of sequence thrust: it need not be a backthrust. Blind thrust A blind thrust is a thrust fault that does not break the surface. Instead, the tip of the fault is buried in a fold.
