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 °.. ' <=,' - -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