Magoon, L B, and w.G. Dow, eds, 1994, The petroleum system-from source to trap: AAPG M Chapter 13 ydrocarbon Traps Kevin T Biddle Charles C. wielchowsk Exxon Exploration Commpany Houston Teras, U.S.A abstract rap identification is a first step in prospect evaluation and an important part of any exploration or assessment program. Future success in exploration will depend increasingly on an improved understanding of how traps are formed and an appreciation of the numerous varieties of trap type that exist. We define a trap as any geometric arrangement of rock that permits significant accumula tion of hydrocarbons in the subsurface. a trap must include a reservoir rock in which to store hydrocarbons, and a seal or set of seals that impede or stop migration out of the reservoir. Although it is the geometric arrangement of reservoirs and seals that determines if a trap is present, both reservoir and seal analysis should be an integral part of trap evaluation. raps can be divided into three broad categories: structural traps, stratigraphic traps, and combi- nation traps, which exhibit both structural and stratigraphic elements. We have subdivided struc- tural traps into fold traps, traps associated with faults, traps associated with piercement features, and combination traps that require elements of both faults and folds for effectiveness. Stratigraphic traps can be grouped into primary or depositional traps, traps associated with unconformities (either above or beneath the unconformity), and secondary or diagenetic stratigraphic traps. We note that although each trap has unique characteristics, early recognition of trap type will aid in mapping and evaluating a prospect INTRODUCTION as shown on the events chart( Chapter 1, Figure 1.5),is important in a petroleum system study Trap evaluation is fundamental in the analysis of a trap forms before the hydrocarbon-forming process, the prospect and an important part in any successful oil and evidence(oil and gas) that a petroleum system exists is gas exploration or resource assessment program. A trap preserved. The volume of oil and gas preserved depends can be defined as any geometric arrangement of rock, on the type and size of the trap, which is important in the regardless of origin, that permits significant accumula- evaluation of the prospect tion of oil or gas, or both, in the subsurface (modified The critical components of a trap(the reservoir, seal, from North, 1985). Although we define a trap as the and their geometric arrangement with each other) can be geometric configuration that retains hydrocarbons, combined in a variety of ways by a number of separate several critical components must be in place for a trap to processes. This variability has led to many different trap be effective, including adequate reservoir rocks and classifications(e.g, Clapp, 1929; Wilson, 1934; Hero s, and each of these must be addressed during trap 1941; Wilhelm, 1945; Levorsen, 1967; Perrodon, 1983 evaluation North, 1985: Milton and Bertram, 1992). Different The oil and gas within a trap is part of a petroleum authors have focused on various trap attributes as the system, whereas the trap itself is part of one or more key element or elements of their classification. Some edimentary basins and is evaluated as part of a prospect have emphasized trap geometry, while others have or play (see Chapter 1, Figure 1. 1, this volume). The concentrated on the mechanisms of trap formation. hydrocarbon-forming process and the trap-forming Others have considered reservoir or seal characteristics process occur as independent events and commonly at as the major parts of their classification Space limitations different times. The timing of the trap-forming process, preclude a thorough review of the various classifications
Magoon, L. B, and W. G. Dow, eds., 1994, The petroleum system—from source to trap: AAPG Memoir 60. Chapter 13 Hydrocarbon Traps Kevin T. Biddle Charles C. Wielchowsky Exxon Exploration Company Houston, Texas, U.S.A. Abstract Trap identification is a first step in prospect evaluation and an important part of any exploration or assessment program. Future success in exploration will depend increasingly on an improved understanding of how traps are formed and an appreciation of the numerous varieties of trap types that exist We define a trap as any geometric arrangement of rock that permits significant accumulation of hydrocarbons in the subsurface. A trap must include a reservoir rock in which to store hydrocarbons, and a seal or set of seals that impede or stop migration out of the reservoir. Although it is the geometric arrangement of reservoirs and seals that determines if a trap is present, both reservoir and seal analysis should be an integral part of trap evaluation. Traps can be divided into three broad categories: structural traps, stratigraphic traps, and combination traps, which exhibit both structural and stratigraphic elements. We have subdivided structural traps into fold traps, traps associated with faults, traps associated with piercement features, and combination traps that require elements of both faults and folds for effectiveness. Stratigraphic traps can be grouped into primary or depositional traps, traps associated with unconformities (either above or beneath the unconformity), and secondary or diagenetic stratigraphic traps. We note that although each trap has unique characteristics, early recognition of trap type will aid in mapping and evaluating a prospect. INTRODUCTION Trap evaluation is fundamental in the analysis of a prospect and an important part in any successful oil and gas exploration or resource assessment program. A trap can be defined as any geometric arrangement of rock, regardless of origin, that permits significant accumulation of oil or gas, or both, in the subsurface (modified from North, 1985). Although we define a trap as the geometric configuration that retains hydrocarbons, several critical components must be in place for a trap to be effective, including adequate reservoir rocks and seals, and each of these must be addressed during trap evaluation. The oil and gas within a trap is part of a petroleum system, whereas the trap itself is part of one or more sedimentary basins and is evaluated as part of a prospect or play (see Chapter 1, Figure 1.1, this volume). The hydrocarbon-forming process and the trap-forming process occur as independent events and commonly at different times. The timing of the trap-forming process, as shown on the events chart (Chapter 1, Figure 1.5), is important in a petroleum system study because if the trap forms before the hydrocarbon-forming process, the evidence (oil and gas) that a petroleum system exists is preserved. The volume of oil and gas preserved depends on the type and size of the trap, which is important in the evaluation of the prospect. The critical components of a trap (the reservoir, seal, and their geometric arrangement with each other) can be combined in a variety of ways by a number of separate processes. This variability has led to many different trap classifications (e.g., Clapp, 1929; Wilson, 1934; Heroy, 1941; Wilhelm, 1945; Levorsen, 1967; Perrodon, 1983; North, 1985; Milton and Bertram, 1992). Different authors have focused on various trap attributes as the key element or elements of their classification. Some have emphasized trap geometry, while others have concentrated on the mechanisms of trap formation. Others have considered reservoir or seal characteristics as the major parts of their classification. Space limitations preclude a thorough review of the various classifications 219
220 Biddle and Wiechowski potential base seal fault seal Hydrocarbon accumulation Migration pathway Figure 13.1. Key elements for(A) structural and B)stratigraphic hydrocarbon traps here, but we note a general consensus on three broad Reservoir rock ategories of traps( Levorsen, 1967): those created by structural deformation, those formed by stratigraphic The reservoir within a trap provides the storage space nomena, and those that combine aspects of both In for the hydrocarbons. This requires adequate porosity ddition, dynamic fluid conditions in the subsurface can within the reservoir interval. The porosity can be modify the capacity of some structural and stratigraphic primary(depositional), secondary (diagenetic),or traps, or perhaps lead to hydrocarbon accumulations in fractures, but it must supply enough volume to accom- xpected locations. This chapter covers what we modate a significant amount of fluids consider to be two critical components of a trap. It also The reservoir must also be capable of transmitting and describes the major structural and stratigraphic types of exchanging fluids. This requires sufficient effective traps and provides suggestions for trap evaluation permeability within the reservoir interval and also along the migration conduit that connects the reservoir with a pod of active source rock. Because most traps are initially TWO CRITICAL COMPONENTS water filled, the reservoir rock must be capable of OF A TRAP exchan fluids as the original formation water r Is displaced by hydrocarbons. As North(1985, P. 254) To be a viable trap a subsurface feature must be noted, Traps are not passive receivers of fluid into capable of receiving hydrocarbons and storing them for otherwise empty space; they are focal points of active some significant length of time. This requires two funda- fluid exchange mental components: a reservoir rock in which to store the a trap that contains only one homogeneous reservoir hydrocarbons, and a seal (or set of seals)to keep the rock is rare. Individual reservoirs commonly include h hydrocarbons from migrating out of the trap(Figure lateral and /or vertical variations in porosity and perme- 13.1). Both seal and reservoir are discussed in more detail ability. Such variations can be caused either by primary elsewhere in this volume(see Morse, Chapter 6; Jordan depositional prod and Wilson, Chapter 7: Downey, Chapter 8), but these deformational effects and can lead to hydrocarbon- are such basic parts of a trap that some of their aspects saturated but nonproductive waste zones within a trap must also be covered here (Figure 13. 2A). Variations in porosity and, more impor We do not consider the presence of hydrocarbons to tantly, permeability can also create transitions that occur e a critical component of a trap, although this is over some distance between the reservoirs and the major certainly a requirement for economic success. The seals of a trap (Figure 13. 2C and D). These intervals may absence of hydrocarbons may be the result of failure of contain a significant amount of hydrocarbons that are other play or prospect parameters, such as the lack of a difficult to produce effectively. Such intervals should be pod of active source rock or migration conduits, and it viewed as uneconomic parts of the reservoir and not part may have nothing to do with the ability of an individual of the seal. Otherwise, trap spill points may be mis-iden- feature to act as a trap. After all, "a trap is a trap, whether tified. Many traps contain several discrete reservoir rocks or not it has a mouse in it"(attributed to w c Finch, in with interbedded impermeable units that form internal Rittenhouse, 1972, p 16) eals and segment hydrocarbon accumulations into parate compartments with separate gas-oil-water
220 Biddle and Wielchowsky B Hydrocarbon accumulation • Migration pathway Figure 13.1. Key elements for (A) structural and (B) stratigraphic hydrocarbon traps. here, but we note a general consensus on three broad categories of traps (Levorsen, 1967): those created by structural deformation, those formed by stratigraphic phenomena, and those that combine aspects of both. In addition, dynamic fluid conditions in the subsurface can modify the capacity of some structural and stratigraphic traps, or perhaps lead to hydrocarbon accumulations in unexpected locations. This chapter covers what we consider to be two critical components of a trap. It also describes the major structural and stratigraphic types of traps and provides suggestions for trap evaluation. TWO CRITICAL COMPONENTS OF A TRAP To be a viable trap, a subsurface feature must be capable of receiving hydrocarbons and storing them for some significant length of time. This requires two fundamental components: a reservoir rock in which to store the hydrocarbons, and a seal (or set of seals) to keep the hydrocarbons from migrating out of the trap (Figure 13.1). Both seal and reservoir are discussed in more detail elsewhere in this volume (see Morse, Chapter 6; Jordan and Wilson, Chapter 7; Downey, Chapter 8), but these are such basic parts of a trap that some of their aspects must also be covered here. We do not consider the presence of hydrocarbons to be a critical component of a trap, although this is certainly a requirement for economic success. The absence of hydrocarbons may be the result of failure of other play or prospect parameters, such as the lack of a pod of active source rock or migration conduits, and it may have nothing to do with the ability of an individual feature to act as a trap. After all, "a trap is a trap, whether or not it has a mouse in it" (attributed to W. C. Finch, in Rittenhouse, 1972, p. 16). Reservoir Rock The reservoir within a trap provides the storage space for the hydrocarbons. This requires adequate porosity within the reservoir interval. The porosity can be primary (depositional), secondary (diagenetic), or fractures, but it must supply enough volume to accommodate a significant amount of fluids. The reservoir must also be capable of transmitting and exchanging fluids. This requires sufficient effective permeability within the reservoir interval and also along the migration conduit that connects the reservoir with a pod of active source rock. Because most traps are initially water filled, the reservoir rock must be capable of exchanging fluids as the original formation water is displaced by hydrocarbons. As North (1985, p. 254) noted, "Traps are not passive receivers of fluid into otherwise empty space; they are focal points of active fluid exchange." A trap that contains only one homogeneous reservoir rock is rare. Individual reservoirs commonly include lateral and/or vertical variations in porosity and permeability. Such variations can be caused either by primary depositional processes or by secondary diagenetic or deformational effects and can lead to hydrocarbonsaturated but nonproductive waste zones within a trap (Figure 13.2A). Variations in porosity and, more importantly, permeability can also create transitions that occur over some distance between the reservoirs and the major seals of a trap (Figure 13.2C and D). These intervals may contain a significant amount of hydrocarbons that are difficult to produce effectively. Such intervals should be viewed as uneconomic parts of the reservoir and not part of the seal. Otherwise, trap spill points may be mis-identified. Many traps contain several discrete reservoir rocks with interbedded impermeable units that form internal seals and segment hydrocarbon accumulations into separate compartments with separate gas-oil-water
13. Hydrocarbon Traps 21 TRIASSIC RED BEDS waste zone within reservoi nd skeletal boundary zones from North, 1985) Transition zone Depositional or diagenetic E Hydrocarbon accumulation Migration pathway Figure 13. 2. Common trap limitations. (A)Waste or nonproductive zones in trap. (B)Multiple impermeable layers in creating several individual oil-water contacts. (C)Non- to poorly productive transition zone(from reservoir to seal above productive reservoir. (D)Lateral transition from reservoir to seal.(E Lateral, stratigraphically controlled leak (F)Lateral leak point or thief bed
13. Hydrocarbon Traps 221 B TRIASSIC RED BEDS ^* f **& ^ (from North, 1985) zone of anhydrite impregnation multi-level oil/water boundary zones V^V^KSP^^D : "How permeability chalky limestone seat-seals Hi algal and skeletal reservoirs <^^/&&^g&i0iqg^&jjieHt*^l^i**i*&*if*&i Depositions^ or diagenetic transition between reservoir and seal Hydrocarbon accumulation Migration pathway Figure 13.2. Common trap limitations. (A) Waste or nonproductive zones in trap. (B) Multiple impermeable layers in trap creating several individual oil-water contacts. (C) Non- to poorly productive transition zone (from reservoir to seal) rock above productive reservoir. (D) Lateral transition from reservoir to seal. (E) Lateral, stratigraphically controlled leak point. (F) Lateral leak point or thief bed
222 Biddle and wielchowsky contacts and different pressure distributions (Figure STRUCTURAL TRaPs 3.2B). As illustrated, these are complications of a single trap and are not multiple traps Structural traps are created by the syn- to postdeposi tional deformation of strata into a geometry(a structure) that permits the accumulation of hydrocarbons in the subsurface. The resulting structures involving the The seal is an equally critical component of a trap reservoir, and usually the seal intervals, are dominated volume). Without effective seals, hydrocarbons will of the foregoing(Figures 133A-D). Traps formed by migrate out of the reservoir rock with time and the trap gently dipping strata beneath an erosional unconformit will lack viability. Most effective seals for hydrocarbon are commonly excluded from the structural category accumulations are formed by relatively thick, laterally (North, 1985)(Figure 133E) subang continuous, ductile rocks with high capillary entry mity deformation increases, this distinction becomes pressures(Downey, 1984 and Chapter 8, this volume), ambiguous(Figure 13 3F). Superposed multiple defor but other types of seals may be important parts of indi- mation may also blur the foregoing distinctions(e. g (e. g, fault zone material, volcani asphalt, and permafrost Subdivisions of structural traps have been proposed All traps require some form of top seal(Figure 13. 1 ). by many authors based on a variety of schemes.For When the base of the top seal is convex upward in three example, in his general trap classification, Clapp(1929) dimensions, the contours drawn to represent this surface distinguished between anticlinal, synclinal, homoclinal, (called the sealing surface by Downey, 1984)close in map quaquaversal, and fault-dominated traps. Harding and view). If this is the case, no other seal is necessary to form owell (1979)based their classification of structural trap an adequate trap. In fact, some authors (e.g, Wilhelm, on the concept of structural styles, which emphasizes 1945: North, 1985)have used the basic convex or basement involvement or noninvolvement, inferred nonconvex geometry of the sealing surface as a way of deformational force, and mode of tectonic transport Levorsen(1967) divided structural traps into those Many traps are more complicated and require that, in caused by folding, faulting, fracturing, intrusion, and addition to a top seal, other effective seals must be combinations of these processes. North(1985), under the resent(Figure 13. 1). These are the poly-seal traps of category of convex traps, distinguished between buckle- Milton and Bertram(1992). Lateral seals impede hydro- or thrust-fold, bending fold and immobile convexity and are a common element of successful stratigraphic convex traps are caused by faults ( e, the folding is a traps. Facies changes from porous and permeable rocks response to the faulting rather than the other way to rocks with higher capillary entry pressures(Figures around). However, the reverse is true under certain 13. 1B and 13. 2 D)can form lateral seals, as can lateral conditions in which prospect-scale faulting results from diagenetic changes from reservoir to tight rocks. Other the folding process, such as in the development of lateral seals are created by the juxtaposition of dissimilar chevron folds(Ramsey, 1974)or in keystone normal rock types across erosional or depositional boundaries. faulting above a rising salt diapir(Harding and Lowell, Traps in incised valley complexes commonly rely type of lateral seal(Figure 13 2F). Stratigraphic variability The following sections discuss in more detail the two in lateral seals poses a risk of leakage and trap limitation. most important structural trap types: fold dominated Even thinly interbedded intervals of porous and versus fault dominated. In our experience, fol permeable rock(thief beds)(Figures 132E and F)in a dominated traps are by far the most important structural potential lateral seal can destroy an otherwise viable trap. traps, We agree with North(1985)that purely fault Base seals(Figure 13. 1)are present in many traps and dominated traps(those on which the fault itself creates are most commonly stratigraphic in nature. The presence the trap without the presence of a fold) are relatively or absence of an adequate base seal is not a general trap uncommon. Traps dominated by piercement (in which requirement, but it can play an important role in the reservoir is sealed by intrusion of salt or shale) and deciding how a field will be developed those resulting from combinations of faulting, folding, Faults can be important in providing seals for a trap, and piercement are treated by Harding and lowell nd fault leak is a common trap limitation(Smith 1966 (1979), Lowell ( 1985), and North( 1985) 1980: Dot 1984; Allan, 1989). Faults can create or modify seals by juxtaposing dissimilar rock types across Fold-Dominated Traps the fault (Figure 13. 1A), by sm permeable material into the fault zone, by forming a less Structural traps that are dominated by folds at the eable gouge because of differential sorting and/or reservoir-seal level exhibit a wide variety of geometries cataclasis, or by preferential diagenesis along the fault. and are formed or modified by a number of significantly Fault-induced leakage may result from juxtaposition of different syn and postally considered to result from depositional deformation mecha fault ( fi 13.1A)or by formation of a fracture network along the tectonically induced deformation, the term fold is purely fault itself descriptive and refers to a curved or nonplanar arrange-
222 Biddle and Wielchowsky contacts and different pressure distributions (Figure 13.2B). As illustrated, these are complications of a single trap and are not multiple traps. Seal The seal is an equally critical component of a trap (Milton and Bertram, 1992; Downey, Chapter 8, this volume). Without effective seals, hydrocarbons will migrate out of the reservoir rock with time and the trap will lack viability. Most effective seals for hydrocarbon accumulations are formed by relatively thick, laterally continuous, ductile rocks with high capillary entry pressures (Downey, 1984 and Chapter 8, this volume), but other types of seals may be important parts of individual traps (e.g., fault zone material, volcanic rock, asphalt, and permafrost). All traps require some form of top seal (Figure 13.1). When the base of the top seal is convex upward in three dimensions, the contours drawn to represent this surface (called the sealing surface by Downey, 1984) close in map view). If this is the case, no other seal is necessary to form an adequate trap. In fact, some authors (e.g., Wilhelm, 1945; North, 1985) have used the basic convex or nonconvex geometry of the sealing surface as a way of classifying traps. Many traps are more complicated and require that, in addition to a top seal, other effective seals must be present (Figure 13.1). These are the poly-seal traps of Milton and Bertram (1992). Lateral seals impede hydrocarbon movement from the sides of a trap (Figure 13.1B) and are a common element of successful stratigraphic traps. Fades changes from porous and permeable rocks to rocks with higher capillary entry pressures (Figures 13.IB and 13.2D) can form lateral seals, as can lateral diagenetic changes from reservoir to tight rocks. Other lateral seals are created by the juxtaposition of dissimilar rock types across erosional or depositional boundaries. Traps in incised valley complexes commonly rely on this type of lateral seal (Figure 13.2F). Stratigraphic variability in lateral seals poses a risk of leakage and trap limitation. Even thinly interbedded intervals of porous and permeable rock (thief beds) (Figures 13.2E and F) in a potential lateral seal can destroy an otherwise viable trap. Base seals (Figure 13.1) are present in many traps and are most commonly stratigraphic in nature. The presence or absence of an adequate base seal is not a general trap requirement, but it can play an important role in deciding how a field will be developed. Faults can be important in providing seals for a trap, and fault leak is a common trap limitation (Smith, 1966, 1980; Downey, 1984; Allan, 1989). Faults can create or modify seals by juxtaposing dissimilar rock types across the fault (Figure 13.1A), by smearing or dragging less permeable material into the fault zone, by forming a less permeable gouge because of differential sorting and/or cataclasis, or by preferential diagenesis along the fault. Fault-induced leakage may result from juxtaposition of porous and permeable rocks across the fault (Figure 13.1 A) or by formation of a fracture network along the fault itself. STRUCTURAL TRAPS Structural traps are created by the syn- to postdepositional deformation of strata into a geometry (a structure) that permits the accumulation of hydrocarbons in the subsurface. The resulting structures involving the reservoir, and usually the seal intervals, are dominated by either folds, faults, piercements, or any combination of the foregoing (Figures 13.3A-D). Traps formed by gently dipping strata beneath an erosional unconformity are commonly excluded from the structural category (North, 1985) (Figure 13.3E), although as subunconformity deformation increases, this distinction becomes ambiguous (Figure 13.3F). Superposed multiple deformation may also blur the foregoing distinctions (e.g., Lowell, 1985). Subdivisions of structural traps have been proposed by many authors based on a variety of schemes. For example, in his general trap classification, Clapp (1929) distinguished between anticlinal, synclinal, homoclinal, quaquaversal, and fault-dominated traps. Harding and Lowell (1979) based their classification of structural traps on the concept of structural styles, which emphasizes basement involvement or noninvolvement, inferred deformational force, and mode of tectonic transport. Levorsen (1967) divided structural traps into those caused by folding, faulting, fracturing, intrusion, and combinations of these processes. North (1985), under the category of convex traps, distinguished between buckleor thrust-fold, bending fold, and immobile convexity traps. North (1985) appropriately pointed out that many convex traps are caused by faults (i.e., the folding is a response to the faulting rather than the other way around). However, the reverse is true under certain conditions in which prospect-scale faulting results from the folding process, such as in the development of chevron folds (Ramsey, 1974) or in keystone normal faulting above a rising salt diapir (Harding and Lowell, 1979). The following sections discuss in more detail the two most important structural trap types: fold dominated versus fault dominated. In our experience, folddominated traps are by far the most important structural traps. We agree with North (1985) that purely faultdominated traps (those on which the fault itself creates the trap without the presence of a fold) are relatively uncommon. Traps dominated by piercements (in which the reservoir is sealed by intrusion of salt or shale) and those resulting from combinations of faulting, folding, and piercement are treated by Harding and Lowell (1979), Lowell (1985), and North (1985). Fold-Dominated Traps Structural traps that are dominated by folds at the reservoir-seal level exhibit a wide variety of geometries and are formed or modified by a number of significantly different syn- and postdepositional deformation mechanisms. Although usually considered to result from tectonically induced deformation, the term fold is purely descriptive and refers to a curved or nonplanar arrange-
13. Hydrocarbon tra A Fold 8 Fault c Piercement D. Combination fold /fault E Subunconformi unconformIty Hydrocarbon accumulation of structural traps: (A)fold, (B)fault, (C)pierce subunconformities. The situation in(E)is commonly excluded from the structural category
13. Hydrocarbon Traps 223 A Fold B Fault . . °-'.*V'. ' •%- 0 - - o fc ?', * j*^'-* o .••v.o-.» . , r ' o-* , _^>- . c " ' 0 - • *. * . • * * "- ^ " * ' . • ' * u " " O °.. ' : •.•>•••; • 11 \*K - • '"•»" ° - * 1 . ' » ' • *" i * • » . ° • • ; * . * • *•'»•, • •.°- *\7 o* i^i-^li- ^ ^ ^ ^ ^ ^ ^%^ - f^§ \>>^\^>^\ ^ C Piercement D Combination fold/fault E Subunconformity l^^^rf^W^^^^^g^^ ^ F Subunconformity Hydrocarbon accumulation Figure 13.3. Major categories of structural traps: (A) fold, (B) fault, (C) piercement, (D) combination fold-fault, (E) and (F) subunconformities. The situation in (E) is commonly excluded from the structural category
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
Biddle and wielchousk A Lateral seal Base seal 一一 Hydrocarbon accumulation Figure 13. 5. Combination fold and fault traps in which both are critical to trap viability ( A)Complex fault-bend fold showing associated sealing fault. B)A duplex structure with a thrust fault forming an element of the base seal. selected fault sealing properties are also illustrated. fault-dominated traps because of attendant folding, graphic traps(e.g, Levorsen, 1936; Dott and Reynolds, However, Figure 136C shows how regional dip plus 1969: King, 1972; Busch, 1974; Halbouty, 1982, Foster and hrusting can produce a viable reverse fault-dominated Beaumont, 1988, 1991). Here, we generally follow Ritten- ithout folding at the relevant reser house(1972)and divide stratigraphic traps into primar interval and how minor footwall drag can provide a or depositional stratigraphic traps, stratigraphic traps viable trap sealed by an overlying thrust fault. associated with unconformities, and secondary strati Figure 13. 6D is an example of a strike-slip fault trap in graphic traps the Los angeles basin of the United States(harding, 1974). Folding and a tar seal also play a significant role in Primary or Depositional Stratigraphic Traps this trap are created by changes in contemporaneous deposition STRATIGRAPHIC TRAPS (see MacKenzie, 1972). As described here, such traps are not associated with significant unconformities. Two In 1936(p. 524), Levorsen proposed the term strati- general classes of primary stratigraphic traps can be graphic trap for features"in which a variation in stratig. recognized: those formed by lateral depositional raphy is the chief confining element in the reservoir changes, such as facies changes and depositional which traps the oil. "The existence of such nonstructural pinchout( Figure 137A), and those created by buried traps has been recognized since at least the late 1800s depositional relief(Figure 137B) Carll, 1880). Today, we would define a stratigraphic trap Facies changes(Figure 137A) may juxtapose potential as one in which the requisite geometry and reservoir- reservoir rocks and impermeable seal rocks over rela seal(s)combination were formed by any variation in the tively short lateral distances in either siliciclastic or tratigraphy that is independent of structural deforma- carbonate settings. The lateral transition from reservoir to tion, except for regional tilting (modified from North, seal is generally gradational, leading to possible nonego- nomic segments within the reservoir. Particular care Many attempts have been made to classify types of must be taken to identify strike closure in this type of stratigraphic traps. Early efforts, while not specifically trap. Depositional pinchout (Figure 137A)may lead to using the term stratigraphic, led to broad categories of reservoir and seal combinations that can trap hydrocar- that considerable variability exists among such traps also a risk for pinchout trap nge aoi traps that were"closed"because of varying porosit bons. The transition from reservoir to lateral seal within rock (e. g, Wilson, 1934). Later work re abrupt, in contrast to facies ch raps. Strike closure is (e. g, Levorsen, 1967), and subdivisions became more Both lateral facies change and depositional pinchout numerous. A number of treatments of stratigraphic traps traps generally require a component of regional dip to be provide information on different approaches to classifi- effective. Both types are common elements of combina-
226 Biddle and Wielchowsky A , ' r 11 i ~ J_A "TLTL'Z,^ s , \ \ ^ Lateral seal T-A---S±*&^^ :- B y / / *' ' '' y y '''' " ' ' ' y y y y y y ' s y s- y y y^ Base seal-C'/y •^y /y'/yZt —-* y y.^&fi'*? ~~ ^ y •^SdE&a •• "•;•••:•' : •'• irinn.; ::•'.•:: .y-~'~r ~-I~-IZZ~— y _ y ~— _ - -- _ ~ - -s \ \ \ \ Hydrocarbon accumulation Figure 13.5. Combination fold and fault traps in which both are critical to trap viability. (A) Complex fault-bend fold showing associated sealing fault. (B) A duplex structure with a thrust fault forming an element of the base seal. Selected fault sealing properties are also illustrated. reverse faults. These structures tend not to produce pure fault-dominated traps because of attendant folding. However, Figure 13.6C shows how regional dip plus thrusting can produce a viable reverse fault-dominated trap without folding at the relevant reservoir-seal interval and how minor footwall drag can provide a viable trap sealed by an overlying thrust fault. Figure 13.6D is an example of a strike-slip fault trap in the Los Angeles basin of the United States (Harding, 1974). Folding and a tar seal also play a significant role in this trap. STRATIGRAPHIC TRAPS In 1936 (p. 524), Levorsen proposed the term stratigraphic trap for features "in which a variation in stratigraphy is the chief confining element in the reservoir which traps the oil." The existence of such nonstructural traps has been recognized since at least the late 1800s (Carll, 1880). Today, we would define a stratigraphic trap as one in which the requisite geometry and reservoirseal(s) combination were formed by any variation in the stratigraphy that is independent of structural deformation, except for regional tilting (modified from North, 1985). Many attempts have been made to classify types of stratigraphic traps. Early efforts, while not specifically using the term stratigraphic, led to broad categories of traps that were "closed" because of varying porosity within rock (e.g., Wilson, 1934). Later work recognized that considerable variability exists among such traps (e.g., Levorsen, 1967), and subdivisions became more numerous. A number of treatments of stratigraphic traps provide information on different approaches to classification and supply abundant examples of types of stratigraphic traps (e.g, Levorsen, 1936; Dott and Reynolds, 1969; King, 1972; Busch, 1974; Halbouty, 1982; Foster and Beaumont, 1988,1991). Here, we generally follow Rittenhouse (1972) and divide stratigraphic traps into primary or depositional stratigraphic traps, stratigraphic traps associated with unconformities, and secondary stratigraphic traps. Primary or Depositional Stratigraphic Traps Primary or depositional stratigraphic traps (Figure 13.7) are created by changes in contemporaneous deposition (see MacKenzie, 1972). As described here, such traps are not associated with significant unconformities. Two general classes of primary stratigraphic traps can be recognized: those formed by lateral depositional changes, such as facies changes and depositional pinchouts (Figure 13.7A), and those created by buried depositional relief (Figure 13.7B). Facies changes (Figure 13.7A) may juxtapose potential reservoir rocks and impermeable seal rocks over relatively short lateral distances in either siliciclastic or carbonate settings. The lateral transition from reservoir to seal is generally gradational, leading to possible noneconomic segments within the reservoir. Particular care must be taken to identify strike closure in this type of trap. Depositional pinchouts (Figure 13.7A) may lead to reservoir and seal combinations that can trap hydrocarbons. The transition from reservoir to lateral seal may be abrupt, in contrast to facies change traps. Strike closure is also a risk for pinchout traps. Both lateral facies change and depositional pinchout traps generally require a component of regional dip to be effective. Both types are common elements of combina-
A Basement-involved normal fault, trap door Cross section Map view B Synthetic detached listric normal fault Cross section A. A c Reverse fault trap Associated with a fault-bend fold Associated with ductile deformation Cross section Cross section D Strike-slip fault Cross section M ap view Hydrocarbon accumulation 13.6. Types of traps in which faulting dominates the reservoir-seal interval (A)Basement-involved normal fault trap ap door.( B)Synthetic detached listric normal fault traps. (C)Two types of reverse fault traps. D)Strike-slip fault traps
13. Hydrocarbon Traps 227 A Basement-involved normal fault, trap door Cross section A::. Map view B Synthetic detached listric normal fault Cross section C Reverse fault trap Associated with a fault-bend fold Cross section Associated with ductile deformation Cross section D Strike-slip fault Cross section A ^m\ AMap view 300 m ^ -V.+ [ j Hydrocarbon accumulation Figure 13.6. Types of traps in which faulting dominates the reservoir-seal interval. (A) Basement-involved normal fault trap and trap door. (B) Synthetic detached listric normal fault traps. (C) Two types of reverse fault traps. (D) Strike-slip fault traps
Biddle and wielchousk Lateral depositional Buried depositional changes toe-of-slope marls and sh carbonates porous reef and Facies change Eolian dune Depositional pinchout Submarine fan lobe Hydrocarbon accumulation itional stratigraphic traps. (A) Traps created by lateral changes in sedimentary rock type during p: juxtaposition of reservoir and seal caused by lateral facies changes. Bottom: reservoir termination due to al pinchout of porous and permeable rock units. B)Traps formed by buried depositional relief. In each entary processes form a potential trapping geometry, but require burial by younger impermeable section to sIred top sea tion structural-stratigraphic traps, particularly if the between the forereef rocks and adjacent basinal deposits structure was growing during deposition of the reservoir (potential source rocks) can create excellent migration and seal rocks pathways. Formation of a top seal requires that reef The second general class of primary stratigraphic growth is terminated and that the reef is buried beneath traps is associated with burea p ejeogeomorphic type of trap is accurate prediction of porosity and perme- a cap of low-permeability material. a key risk for this traps are equivalent to the constructive pa such traps, a few of which are illustrated in Figure 137B. of the Western Canada sedimentary basin are excellent Each of these has distinct characteristics and attendant examples of this type of trap (Hemphill et al. 1970; Barss Carbonate reefs provide a classic example of potential Another type of buried depositional relief is associ- raps associated with buried depositional relief. Reef ated with some submarine fan deposits (Figure 13 7B).In growth with time enhances depositional relief, and the such depositional settings, sand-rich depositional lobes transition from tight lagoonal rocks to porous and may be encased in shale. The Balder oil field in the permeable backreef-reef-forereef rocks may provide a Norwegian section of the North Sea is an example of this good reservoir-lateral seal combination. The relationship type of trap(Sarg and Skjold, 1982)
228 Biddle and Wielchowsky A Lateral depositional changes Facies change Depositional pinchout B Buried depositional relief *9&J2* 1 1 " 1 " f ^*V l ' i i i tight ^v i . i i carbonates ">*U . 1 ' y^*^i^_\ porous reef and forereef carbonates toe-of-slope and basin carbonates, ^. marls and shales I A*s. i ^V^ \ ^ i M X-T"1 ^^™™ Reef Eolian dune Submarine fan lobe ||f| Hydrocarbon accumulation Figure 13.7. Primary or depositional stratigraphic traps. (A) Traps created by lateral changes in sedimentary rock type during deposition. Top: juxtaposition of reservoir and seal caused by lateral facies changes. Bottom: reservoir termination due to the depositional pinchout of porous and permeable rock units. (B) Traps formed by buried depositional relief. In each example, sedimentary processes form a potential trapping geometry, but require burial by younger impermeable section to create the required top seal. tion structural-stratigraphic traps, particularly if the structure was growing during deposition of the reservoir and seal rocks. The second general class of primary stratigraphic traps is associated with buried depositional relief. These traps are equivalent to the constructive paleogeomorphic traps of Martin (1966). There are many different types of such traps, a few of which are illustrated in Figure 13.7B. Each of these has distinct characteristics and attendant trap risks. Carbonate reefs provide a classic example of potential traps associated with buried depositional relief. Reef growth with time enhances depositional relief, and the transition from tight lagoonal rocks to porous and permeable backreef-reef-forereef rocks may provide a good reservoir-lateral seal combination. The relationship between the forereef rocks and adjacent basinal deposits (potential source rocks) can create excellent migration pathways. Formation of a top seal requires that reef growth is terminated and that the reef is buried beneath a cap of low-permeability material. A key risk for this type of trap is accurate prediction of porosity and permeability within the reef complex. The Devonian reef fields of the Western Canada sedimentary basin are excellent examples of this type of trap (Hemphill et al., 1970; Barss etal,1970). Another type of buried depositional relief is associated with some submarine fan deposits (Figure 13.7B). In such depositional settings, sand-rich depositional lobes may be encased in shale. The Balder oil field in the Norwegian section of the North Sea is an example of this type of trap (Sarg and Skjold, 1982)