TYLER, NOEL, Bureau of Economic Geology, University of Texas at Austin, Austin, TX

Abstract: Architectural Controls on Recovery Response in Deltaic Reservoirs in Foreland Basin Settings

Deltaic reservoirs in foreland basins are prolific producers. As a broad class, deltaic reservoirs show a wide, but predictable, range in conventional recovery efficiencies. Recovery efficiency is a function of reservoir architectural style, which is, in turn, a process response to the interplay between sediment supply, base-level dynamics, and the processes active in the receiving basin. With an understanding of depositional history, recovery efficiency of deltaic reservoirs and the potential for incremental recovery become predictable.

Outcrop characterization of deltaic sandstones in a thrust-fault–bounded foreland basin in conjunction with subsurface characterization of analogous reservoirs shows that the sedimentary succession consists of a syntectonic progradational phase followed by aggradational and retrogradational pulses as the upthrusted sediment provenance is denuded. Resulting deltaic sediments of this tectono-depositional cycle undergo a progressive change in sedimentation process and resultant architecture. With the onset of thrusting, delta progradation is vigorous and sediment is largely captured in the delta-mouth-bar system. With incipient base-level rise, sediment is increasingly stored in the distributary system of superjacent deltas, and bifurcation increases, as do the complexity and discontinuity of the reservoir system. Continued base-level rise results in increasing marine-process effectiveness and compression of the systems tract. Delta shape changes from elongate to lobate. Internally complex and discontinuous distributary sand bodies become the dominant sediment repository. The delta-front profile continues to steepen, and delta architecture depends on the morphology of the coast. If the coast is straight, wave-dominated deltas result; however, if the coast is irregular, tide-dominated deltas can be formed. Overall, reservoir continuity, and hence, recovery efficiencies, increase with increasing marine influence, with the exception of tide-dominated deltas where recoveries of typically less than a quarter of the in-place oil attest to sand-body dimensions less than well spacing.

AAPG Search and Discovery Article #90922©1998-1999 AAPG International Distinguished Lectures