ABSTRACT: Carbonate Reservoirs Deposited During Sea Level Lowstands, Permian Basin: Occurrence, Geometry, Facies, and Origin of Porosity of In-Situ Buildups

S. J. Mazzullo, A. M. Reid, S. T. Reid

Carbonate reservoirs composed of in-situ reefs and associated facies, mostly deposited basinward of shallow platforms during sea level lowstands, are common in Pennsylvanian rocks in the Permian basin. Specific examples include some Atokan and Strawn fields in the Delaware basin, and Canyon-Cisco fields in the Midland basin. Such reservoirs are conspicuous by their absence in Permian rocks, where lowstand facies are instead siliciclastics and detrital carbonates. In-situ lowstand carbonate reservoirs are represented by phylloid algal reefs (Atokan, Canyon-Cisco), crinoid banks locally with some Chaetetes and phylloid algal reefs (Strawn), and bryozoan-algal boundstone reefs (Canyon-Cisco). These facies are associated with bioclastic and, locally, oolitic sandstones. Lowst nd reservoirs are both underlain and overlain by deep-water facies, and field geometries range from equidimensional to slightly elongate, generally parallel to platform margins. Porosity in many fields is due to extensive leaching and karsting accompanying subaerial or shallow subsurface meteoric exposure. Accordingly, porosity predictions are best based on knowledge of reconstructed sea level curves. However, many reservoir pore systems have resulted from deep-burial dissolution accompanying chemical compaction and the migration of fluids out of the basin. Porosity predictions in such cases must rely on knowledge of diagenesis and aspects of basin hydrodynamics.

AAPG Search and Discovery Article #90096©1990 AAPG Southwest Section Meeting, Wichita Falls, Texas, March 11-13, 1990