Prospecting for Stratigraphic Traps1

Abstract

Many stratigraphic traps are related directly to their respective environments of deposition. An understanding of the depositional environment is essential to successful prospecting for oil or gas in this type of reservoir. Isopach studies of shale sequences directly above, or both above and below, a lenticular reservoir sandstone are of considerable value in reconstructing depositional environments. Variations in thickness of such shale intervals, either directly above a reservoir sandstone, or embracing it, are completely independent of present-day structural configuration. Isopach maps of such genetic sequences serve as realistic indicators for locating certain lenticular sands. Depositional trends of beach sands, strike valley sands, and offshore bars are determined r adily from such studies. Structure maps, constructed on a reliable time marker within the arbitrarily selected genetic interval, serve as a means of locating oil or gas accumulation within any of these reservoir types. In all such studies electrical log data are essential since such genetic sequences seldom are named formational units. The thinner the genetic sequence the greater the necessity for accurate selection of correlation points on electric logs. Deltaic reservoirs are poorly understood and only rarely recognized by the geologist. This type of reservoir is, nevertheless, abundantly preserved in the sedimentary section. Regional isopach studies of depositional environment are a prerequisite for the construction of meaningful exploration maps of this type of reservoir. An understanding of the trends of distributary fingers and of the influence of differential compaction in producing drape structures, likewise, is essential.