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ABSTRACT: Characterization and Distribution of Clearfork Reservoirs in the Wasson Area of the Permian Basin

John W. Hjerpe

A productive trend of Permian Leonard Series carbonates occurs along the northwest shelf margin of the Permian basin in west Texas. The trend is approximately 40 mi long, ranges from 1 to 5 mi in width, and contains 12 active waterflood units. Six of the units, concentrated in the Wasson area, are operated by Shell Western E&P Inc. They include three productive formations, the Glorieta, Clearfork, and Wichita-Albany, which occur at depths ranging from 5600 to 9000 ft. The Clearfork, with a gross thickness of about 1800 ft, is the most significant producing interval.

Deposition of the Leonard series sediments occurred on a shallow-marine shelf between a shoreline to the northwest and a marine basin to the southeast. Depositional environments range from at or near shelf margin in the lower Clearfork-Wichita interval to near shore subtidal, intertidal, and supratidal in the upper Clearfork-Glorieta interval. The entire section represents the regressive leg of a second-order (10-m.y.) eustatic sea level change which resulted in progradation of the shelf margin toward the Midland basin to the southeast. Superimposed on this overall regressive sequence, five third-order (1 m.y.) sea level fluctuations are recognized within the Leonard Series. The third-order low sea level stands are characterized by clastic influx into what is predominantly a carbonate mud environment. The clastic intervals act as the major seals that control the vertical distribution of the accumulations.

Reservoir quality is controlled primarily by depositional texture with pore-filling anhydrite playing an important secondary role. Grainstones are the best reservoir rocks, but are relatively rare. Packstones are the most common reservoir rock type and can range considerably in quality depending on the amount of mud present. Mudstones and wackestones are the most dominant rock type and act as barriers to fluid migration during the life of a field, but may be imperfect seals over geologic time due to microfractures. The grainstones and packstones occur in thin discontinuous lenses that grade vertically and laterally to mudstones and wackestones. Because of the discontinuous nature of the reservoirs, most Clearfork floods in this area have benefited significantly from both infill drilli g and injection pattern modification.

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