Assessing Oil and Gas Plays in Facies-Cycle Wedges: ABSTRACT

Abstract

Oil and gas potentials of formations in frontier areas can be assessed by reference to 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 (shale or micrite), to coarse, and back to nonmarine. Different types of potential coarse reservoir formations (plays) are identified by their distinctive vertical facies successions within this cycle: wedge base, fine over coarse (potential reservoir) over nonmarine; wedge middle, fine over coarse over fine; wedge top, nonmarine over coarse over fine; wedge edge, nonmarime over coarse over nonmarine; and a special category, subunconformity, which includes any truncated part of a wedge unconformably underlying another wedge. These play types have distinctively different spatial relations between their coarse reservoir facies and their fine oil-source and seal facies. Different wedge positions thus typically have different hydrocarbon potentials; within each position, however, there are also large ranges of potentials related to variations in source richness, reservoir quality, or trap capacity. As a result, the assessment procedures for new plays have three critical steps: (1) selecting look-alike productive plays of the same wedge position; (2) scaling the potential hydrocarbon yield to compensate for bvious differences in thickness, areal extent, etc; and (3) risking the results for other factors that might render the new plays nonproductive. End_of_Article - Last_Page 552------------