24 Biddle and wielchowusk ment of geologic (usually bedding)surfaces(after In addition, the mechanism of fold generation in part Dennis, 1967). Therefore, folds include not only tector controls secondary faulting, which can play a major role cally induced phenomena but also primary depositional in trap segmentation and disruption even though the features, gravity-induced slumping, compaction effects, secondary faults are not integral to fold gene and so on. It is convenient to divide prospect-scale fold Fold traps tend to change significantly in their into two categories-those that are directly fault related geometry with depth. For example, detachments in fold nd those that are largely fault free and thrust belts, angular unconformities, primary strati- Most fault-related folds result from bending above a graphic convergence of reservoir units, and the tendency planar fault surface(Figures 13 4A and B). Crys- of parallel folds to die upward in synclines and talline basement may or may not be involved, and stratal downward in anticlines cause major vertical changes in shortening, extension, or transcurrent movements may trap capacity. In addition, regional tilting affects trap have occurred. Common examples are fault bend folds capacity because structural relief(the height that a (Figure 134A)( Suppe, 1983) and fault propagation folds reservoir unit rises above the regional slope)can become (Figure 13 4B)(Suppe and Medwedeff, 1984)in detached ineffective as a fold's crest in profile drops below the fold and thrust belts fault bend folds are also common horizontal (Levorsen, 1967) in extensional terranes, Other fault-related folds include drag folds, or folds formed by frictional forces acting Fault-Dominated traps across a fault(Figure 13 4C)(Suppe, 1985), and drap folds, those formed by flexure above a buried fault along As already pointed out, faults can be extremely which there has been renewed movement Figure 134D) important to the viability of a trap by providing either by slip over a nonplanar fault surface. Also, drape folds lateral, or base seals by juxtaposing relatively imperme- do not involve significant stratal shortening or extension able rock units against more permeable reservoir units at the reservoir-seal level (Figure 13.5), or by acting as sealing surfaces due to the Fault-free, decollement, or lift-off folds(Figure 13 4E) impermeable nature of the material along the fault. In Sf . Namson, 1981)result from buckling caused by addition, they may act as leak points by juxtaposition of #hiz al shortening above a decollement, usually within a permeable units or by creation of a fracture network.The thick or very efficient(ie, weak and ductile) sequence of term fault is descriptive in that it refers to a surface across evaporites or shale Kink bands and chevron folds are which there has been displacement without reference to pecial types of fault-free folds(Figure 13 4F). Other the cause of that displacement(i.e, whether it is tectoni- types of fault-free folds may form by bending above cally, gravitationally, diagenetically, or otherwise without significant stratal shortening or extension at the the reservoir-seal level (the fault itself makes the trap by reservoir-seal interval(Figure 134G). This would sealing the reservoir without an ancillary fold) can be Isually include folding related to flow and diapirism of divided into three categories based on the type of separa salt and shale, although some prospect-scale folds ar tion, or slip if it is known, that geologic surfaces exhibit related to intrusive igneous activity. Drape folding can be across the fault( Dennis, 1967). These are normal, reverse, caused not only by faulting, as previously mentioned, and strike separation or slip fault traps but also by differential compaction above buried topog ormal fault traps are the most common faul raphy, reefs, or other relatively immobile subsurface dominated structural traps. They are of two fundamen- masses(Figure 13 4H). Initial depositional dips may also tally different geometries and are most common in two produce a drape fold geometry, but we would classify different tectonostratigraphic settings. Normal faults such features as a type of stratigraphic trap. Broad involving the basement occur in areas of significant olding or warping of unknown genesis above basement crustal extension, such as the Gulf of Suez and North Sea, arches and domes would fall into this latter category as and are characterized by tilted fault blocks that exhibit a The distinction between fault-related and fault-free Probably the most important trap geometry is the trap folds is somewhat artificial because the dominant fold door closure at fault intersections(Figure 136A). Syn generation mechanism may vary with time. For example, and postdepositional normal faults that are detached a fold may nucleate above a thick detachment horizon as from the basement occur in areas of rapid subsidence a fault-free fold that is subsequently modified by fault and sedimentation, commonly on passive continental propagation out of the detachment zone. Also, fold margins, such as the U.S. Gulf Coast or Niger Delta geometry may result from the action of more than one of (Weber et al., 1978), and are characterized by a listric the preceding mechanisms, such as extensional fault profile and a cuspate map pattern that is usually concave d folding above a rising salt dapi anthro yn side of distin huish carbon ee Iorahtionis its af be forportan tor maithe displa ement hicrmul fauits na ted s getinge smalelr a variety of reasons. These include predicting trap Keystone normal fault-dominated traps above deep- geometry where the subsurface is incompletely imaged seated salt intrusions are also common (North, 1985) y seismic data and untested by the drill bit, mapping Reverse fault traps may be associated with detached or migration pathways, and analyzing fracture distribution. basement-involved thrust (low angle)or high-angle
224 Biddle and Wielchoivsky merit of geologic (usually bedding) surfaces (after Dennis, 1967). Therefore, folds include not only tectonically induced phenomena but also primary depositional features, gravity-induced slumping, compaction effects, and so on. It is convenient to divide prospect-scale folds into two categories—those that are directly fault related and those that are largely fault free. Most fault-related folds result from bending above a nonplanar fault surface (Figures 13.4A and B). Crystalline basement may or may not be involved, and stratal shortening, extension, or transcurrent movements may have occurred. Common examples are fault bend folds (Figure 13.4A) (Suppe, 1983) and fault propagation folds (Figure 13.4B) (Suppe and Medwedeff, 1984) in detached fold and thrust belts. Fault bend folds are also common in extensional terranes. Other fault-related folds include drag folds, or folds formed by frictional forces acting across a fault (Figure 13.4C) (Suppe, 1985), and drape folds, those formed by flexure above a buried fault along which there has been renewed movement Figure 13.4D) (Suppe, 1985). These latter folds, however, are not caused by slip over a nonplanar fault surface. Also, drape folds do not involve significant stratal shortening or extension at the reservoir-seal level. Fault-free, decollement, or lift-off folds (Figure 13.4E) (e.g., Namson, 1981) result from buckling caused by stratal shortening above a decollement, usually within a thick or very efficient (i.e., weak and ductile) sequence of evaporites or shale. Kink bands and chevron folds are special types of fault-free folds (Figure 13.4F). Other types of fault-free folds may form by bending above material that moves vertically or horizontally by flow without significant stratal shortening or extension at the reservoir-seal interval (Figure 13.4G). This would usually include folding related to flow and diapirism of salt and shale, although some prospect-scale folds are related to intrusive igneous activity. Drape folding can be caused not only by faulting, as previously mentioned, but also by differential compaction above buried topography, reefs, or other relatively immobile subsurface masses (Figure 13.4H). Initial depositional dips may also produce a drape fold geometry, but we would classify such features as a type of stratigraphic trap. Broad folding or warping of unknown genesis above basement arches and domes would fall into this latter category as well. The distinction between fault-related and fault-free folds is somewhat artificial because the dominant fold generation mechanism may vary with time. For example, a fold may nucleate above a thick detachment horizon as a fault-free fold that is subsequently modified by fault propagation out of the detachment zone. Also, fold geometry may result from the action of more than one of the preceding mechanisms, such as extensional fault bend folding above a rising salt diapir. In hydrocarbon exploration, it can be important to distinguish among the mechanisms of fold formation for a variety of reasons. These include predicting trap geometry where the subsurface is incompletely imaged by seismic data and untested by the drill bit, mapping migration pathways, and analyzing fracture distribution. In addition, the mechanism of fold generation in part controls secondary faulting, which can play a major role in trap segmentation and disruption even though the secondary faults are not integral to fold genesis. Fold traps tend to change significantly in their geometry with depth. For example, detachments in fold and thrust belts, angular unconformities, primary stratigraphic convergence of reservoir units, and the tendency of parallel folds to die upward in synclines and downward in anticlines cause major vertical changes in trap capacity. In addition, regional tilting affects trap capacity because structural relief (the height that a reservoir unit rises above the regional slope) can become ineffective as a fold's crest in profile drops below the horizontal (Levorsen, 1967). Fault-Dominated Traps As already pointed out, faults can be extremely important to the viability of a trap by providing either seals or leak points. They are capable of acting as top, lateral, or base seals by juxtaposing relatively impermeable rock units against more permeable reservoir units (Figure 13.5), or by acting as sealing surfaces due to the impermeable nature of the material along the fault. In addition, they may act as leak points by juxtaposition of permeable units or by creation of a fracture network. The term fault is descriptive in that it refers to a surface across which there has been displacement without reference to the cause of that displacement (i.e., whether it is tectonically, gravitationally, diagenetically, or otherwise induced). Structural traps that are dominated by faults at the reservoir-seal level (the fault itself makes the trap by sealing the reservoir without an ancillary fold) can be divided into three categories based on the type of separation, or slip if it is known, that geologic surfaces exhibit across the fault (Dennis, 1967). These are normal, reverse, and strike separation or slip fault traps. Normal fault traps are the most common faultdominated structural traps. They are of two fundamentally different geometries and are most common in two different tectonostratigraphic settings. Normal faults involving the basement occur in areas of significant crustal extension, such as the Gulf of Suez and North Sea, and are characterized by tilted fault blocks that exhibit a zig-zag map pattern (Harding and Lowell, 1979). Probably the most important trap geometry is the trap door closure at fault intersections (Figure 13.6A). Synand postdepositional normal faults that are detached from the basement occur in areas of rapid subsidence and sedimentation, commonly on passive continental margins, such as the U.S. Gulf Coast or Niger Delta (Weber et al., 1978), and are characterized by a listric profile and a cuspate map pattern that is usually concave basinward (Figure 13.6B). On the downthrown side of major displacement normal faults in this setting, smaller synthetic and antithetic fault-dominated traps are typical. Keystone normal fault-dominated traps above deepseated salt intrusions are also common (North, 1985). Reverse fault traps may be associated with detached or basement-involved thrust (low angle) or high-angle
FAULT REL ATED A Fault bend B Fault propagation c Fault drag D Fault drape FAULT FREE E Lift off F Chevron/kink band G Diapir H Dif 二 Hydrocarbon accumulation ure 13. 4. Types of traps in which folding dominates the reservoir-seal interval. Fault-related types include(A) fault fault propogation, (C)fault drag, and (D)fault drape. FauMt-free types include(E)lift off, (F)chevronkink band, (G) dia nd( h)differential co
FAULT RELATED A Fault bend ^_"V"_-^-_j-_j^l ; 8 K • J: -'';^m2Ss>i;'.; . ; , V.'-V-.v.' C Fault drag ---^-'V^ v V ^ - FAULT FREE E Lift off G Diapir 13. Hydrocarbon Traps 225 B Fault propagation D Fault drape F Chevron/kink band - < v. :-^^^s S ^WZ^^Tf :..'Xi .-^-!.'?:V-'-?vrj :\x?.-'.- .-'•...'.- d^V/alg g H Differential compaction Hydrocarbon accumulation Figure 13.4. Types of traps in which folding dominates the reservoir-seal interval. Fault-related types include (A) fault bend, (B) fault propogation, (C) fault drag, and (D) fault drape. Fault-free types include (E) lift off, (F) chevron/kink band, (G) diapir, and (H) differential compaction