Petroleum Traps in Deepwater Settings
A trap is any geometric arrangement of rock—regardless of its origin—that permits a significant subsurface accumulation of oil or gas or both (Biddle and Wielchowsky, 1994). Deepwater reservoirs produce petroleum from a wide variety of traps. As we noted in Chapter 2, about 25% of the giant deepwater fields produce from structural traps that have four-way closure, approximately 9% produce from purely stratigraphic traps, and most deepwater fields (66%) produce from combined structural-stratigraphic traps (Figure 9-1).
Many deepwater settings are characterized by syndepositional tectonics, such that the creation of structural traps is linked inextricably with the evolution of the other elements of petroleum systems. The lapout or truncation of the reservoir elements against the flank of contemporaneously active structures helps create many of the combined structural-strati-graphic traps (Figure 9-2). In addition, the continued deformation in many settings constantly changes other local elements of the petroleum systems, including the local and regional pressure systems and migration pathways.
The subject of traps in deepwater settings is extremely broad, because most deep-water sedimentary basins have multiple trapping styles. In this chapter, we begin by looking at four types of deepwater settings: (1) basins with mobile substrates (salt or shale), (2) basins with nonmobile substrates (i.e., basement blocks, wrench tectonics), (3) unconfined basins, and (4) shallow to continental reservoirs that now rest in deep water (Figure 9-3; see also Chapter 2 of this book) (Worrall et al., 2001). Within each