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ABSTRACT: Geologic Controls on the Production Characteristics of Cyclical, Mixed Carbonate-Clastic Gas Reservoirs, Lower Permian Chase Group. Guymon-Hugoton Field, Oklahoma

W. Terry Siemers, Wayne M. Ahr

The Lower Permian Chase Group, Guymon-Hugoton field, Oklahoma, is composed of interlayered carbonates, siliciclastics, and evaporites deposited in cyclical, shallowing-upward sequences on a gently dipping, low-relief shelf. Each cycle forms a reservoir layer comprised of laterally continuous, successively shallower water marine carbonates and siliciclastics capped and separated by shaly red beds and paleosols. Reservoir studies indicate that individual reservoir layers have contrasting permeabilities and volumes, are not in pressure communication, and exhibit different depletion characteristics. No virgin pressures have been revealed by recent drilling, indicating that no new gas has been discovered and that the current well-spacing pattern has been efficiently draining t e field.

Siltstones and sandstones consist of framework quartz, feldspar, mica, and rock fragments and interstitial clay, carbonate, and anhydrite; porosity is primarily intergranular. Within cycles, limy or dolomitic skeletal, pelletal, and oolitic subtidal grainstones to wackestones grade upward into intertidal to supratidal wackestones and mudstones. Intergranular and crystal- or grain-moldic pore types and a well-developed, laterally continuous, intercrystalline pore system, produced by dolomitization and independent of depositional texture and other pore types, provide fair to excellent porosity and permeability within individual reservoir layers. Lateral continuity of the intercrystalline pore network and individual reservoir layers promotes good field drainage.

Shaly layers are composed of shaly mudstones and argillaceous carbonates that exhibit low permeabilities and high threshold entry pressures. Primary components include illitic, chloritic, and smectitic clays, authigenic carbonate and anhydrite, and detrital quartz, feldspar, and micaceous silt. Lateral continuity, low permeability, and high threshold entry pressures make shaly layers effective regional barriers to vertical fluid flow and prevent pressure communication between reservoir layers.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990