Lower and middle Guadalupian Shelf Carbonates, Eastern Margin of Central Basin Platform, Permian Basin, West Texas

R. F. Ward, R. G. Chalcraft

Lower and middle Guadalupian shelf carbonates serve as the reservoir for a nearly continuous band of oil fields extending 100 mi along the eastern margin of the Central Basin platform of west Texas. Approximately 5 billion bbl of oil have been produced from stratigraphic-structural traps within the Upper Permian (Guadalupian Series) dolomites of the San Andres and Grayburg Formations in Upton, Crane, Ector, Pecos, and Andrews Counties, Texas.

The San Andres and Grayburg Formations are cyclical shallowing-upward carbonate sequences of open shelf through sabkha facies whose depositional strike parallels the eastern margin of the Central Basin platform. Porosity and permeability of reservoir rock are governed by diagenetic processes such as dolomitization, anhydrite porosity occlusion, leaching, silicification, and authigenic clay formation. Shelf sediments are primarily burrowed wackestones and packstones that locally contain pelletal, skeletal, and ooid grainstones. Typical subtidal shelf sediments are capped by algal-laminated dolomite, nodular anhydritic dolomite, and bedded anhydrite. The fauna is normally sparse and dominated by foraminifera and algae. Less common faunal components include pelecypods, crinoids, sponges, Bryozoa, brachiopods, gastropods, and coral that are associated with the development of small scattered patch reefs.

San Andres fields (Yates-Harris) have a combined cumulative production exceeding 3 billion bbl of oil. Grayburg fields (McCamey-Fuhrman-Mascho) account for 1.8 billion bbl of oil production.

Lowering sea level during the early Guadalupian initiated basinward progradation of San Andres carbonate facies with hydrocarbon reservoirs best developed in shallow shelf fusulinid wackestones to packstone and oolitic grainstone. Reservoir dolomites of the Grayburg formation are present east of San Andres fields with optimal reservoir properties occurring near the San Andres outer shelf margin.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.