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,^ <^Ns/.Top seal :.; .<-?VS> 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 strati￾graphic trap for features "in which a variation in stratig￾raphy 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 reservoir￾seal(s) combination were formed by any variation in the stratigraphy that is independent of structural deforma￾tion, 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 classifi￾cation and supply abundant examples of types of strati￾graphic traps (e.g, Levorsen, 1936; Dott and Reynolds, 1969; King, 1972; Busch, 1974; Halbouty, 1982; Foster and Beaumont, 1988,1991). Here, we generally follow Ritten￾house (1972) and divide stratigraphic traps into primary or depositional stratigraphic traps, stratigraphic traps associated with unconformities, and secondary strati￾graphic 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 rela￾tively short lateral distances in either siliciclastic or carbonate settings. The lateral transition from reservoir to seal is generally gradational, leading to possible noneco￾nomic 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 hydrocar￾bons. 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-