TYLER, NOEL, MICHAEL H. GARDNER, and MARK D. BARTON, Bureau of Economic Geology, The University of Texas at Austin, Austin, TX

ABSTRACT: Architecture and Recovery Response of Deltaic Reservoirs to Base-Level Dynamics

Deltaic reservoirs are among the most prolific producers of the domestic resource base and account for almost three quarters of the historic oil production from sandstones in Texas. Recovery efficiencies in deltaic reservoirs vary widely but predictably. Recovery is a function of heterogeneity style and is, in turn, a process response to the interplay between sediment supply and base-level dynamics. Deltaic systems deposited in the progradational cycle following base-level fall experience a progressive change in depositional process and resultant architecture. On base-level fall, existing deposits are incised and reworked. Subsequent delta progradation is vigorous, and sediment is largely captured in the mouth bar system. Distributaries are primarily zones of sediment bypass, are char cterized by low sinuosity, and are poorly developed but internally well interconnected. With incipient base-level rise, sediment begs to be stored in the distributary system of subsequent deltas, and bifurcation increases as does the complexity of the reservoir system. With continued base level rise, the systems tract is compressed and marine processes exert increasing geomorphologic impact. Delta shape becomes lobate, reflecting increasing storm and wave processes. Distributaries become the dominant sediment repository but are laterally stacked and internally highly complex. At the apex of marine onlap, the delta-front profile becomes increasingly steep and delta architecture depends on the morphology of the shore. If straight, wave-dominated deltas result; however, if the shorezone is rregular, tide-dominated deltas can be formed. Overall, 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 #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.