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Oil and gas potentials of formations in frontier areas can be assessed by comparison with formations in corresponding parts of facies-cycle wedges documented in producing areas. The transgressive-regressive facies-cycle wedge is a body of rock bounded above and below by regional unconformities or the tops of major nonmarine tongues. The ideal wedge includes, from base to top, facies successions from nonmarine, to coarse (sandstone or grain carbonate), to fine (marine shale or micrite), to coarse, and back to nonmarine. Five types of potential coarse reservoir plays, representing the wedge base, middle, top, edge, and subconformity positions, are identified by their distinctive vertical facies successions within this cycle.
Different play types have different risks that can affect assessments. Least risky of sandstone plays are the transgressive wedge base, which is capped by marine shale that commonly is both a good source and seal, and the wedge middle, which is both underlain and overlain by thick shale. Most risky are the regressive wedge-top sandstones with no thick shale seal above and a typically poor-source shale beneath, and the wedge edge, which has no thick marine shales whatever. Subunconformity plays, which include any of the other wedge parts truncated beneath another wedge, have intermediate risks.
Carbonate plays, with the exception of the wedge top, in general are riskier than their corresponding sandstone plays, probably because of more poorly developed porosity and permeability in the carbonate rocks. The exceptional wedge-top carbonate rocks have excellent leached porosity and anhydrite caps. The wedge classification contrasts different types of reefs for exploration purposes. Transgressive wedge-base reefs are encased in the overlying shale or micrite and are apt to be localized over unconformity topography. Wedge-middle reefs are both capped and underlain by shale, and many are localized by depositional topography. Typical regressive wedge-top reefs, which also may form at the edge of depositional slopes, overlie shale or micrite and commonly are capped by anhydrite and r d beds.
Comparison based on wedge position considers the basic differences in source-reservoir-seal relations. Wedge position alone, however, cannot reflect all the other critical controls of oil and gas occurrence, such as source richness and maturation, reservoir quality, or trap capacity. Large variations in productivity thus can occur within any single play type. Accordingly, assessment procedures for new plays have three key steps: (1) selecting look-alike productive plays of the same wedge position; (2) scaling the potential hydrocarbon yields to compensate for obvious differences in thickness or areal extent; and (3) risking the results for the other factors that might render the new plays nonproductive.
This study is based on stratigraphic cross sections from 80 producing basins of the free world. Selected examples are from the Eastern Venezuela, Alberta, Gulf Coast, Permian, and Paradox basins.
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