Abstract: Stratigraphic and Operational Controls on Remaining Oil in Carbonate-Ramp Reservoirs

LUCIA, F. JERRY and CHARLES KERANS

A major goal in reservoir characterization is to describe the 3-D spatial distribution of remaining hydrocarbon saturation. One prevailing concept is that undrained compartments remain in depleted reservoirs, and implicit in this concept is the idea that the compartments are isolated from the depleted reservoir; that is, they are not in pressure communication. Outcrop studies of San Andres and Grayburg reservoir analogs have revealed lateral changes in permeability to be gradational on the scale of thousands of feet. Whereas no lateral impediments to flow have been observed in outcrop studies, vertical impediments are common; typically thin, dense mud layers located at the base of high-frequency cycles. Vertical flow impediments restrict cross flow of injected water, which results in bypassed oil. Flow simulation studies show the bypassed layers to be in pressure communication with the flooded reservoir. Therefore, they can not be reservoir compartments.

Examination of 11 Permian carbonate-ramp reservoirs in the Permian Basin has revealed no pressure-isolated compartments. Instead, bypassed oil resulting from heterogeneous permeability distribution and operational considerations appear to be the cause of low recovery efficiency. The distribution of remaining oil saturation in carbonate-ramp fields commonly results from poor waterflood conformance that is related to a complex geologically controlled permeability structure and an operational history controlled by past recovery activities. No conventional reservoir compartments have been found in this geological setting because lateral permeability changes are sufficiently gradational on the scale of well spacing that no lateral impediments to flow exist.

AAPG Search and Discovery Article #90942©1997 AAPG International Conference and Exhibition, Vienna, Austria