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-water220 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 funda￾mental 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 accom￾modate 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 perme￾ability. Such variations can be caused either by primary depositional processes or by secondary diagenetic or deformational effects and can lead to hydrocarbon￾saturated but nonproductive waste zones within a trap (Figure 13.2A). Variations in porosity and, more impor￾tantly, 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-iden￾tified. 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