McElroy Field, Permian Basin, West Texas: Cyclic Sequence Dolomite Reservoir of Central Basin Platform

R. G. Chalcraft, R. F. Ward

The McElroy field, located in Crane and Upton Counties of west Texas, is one of several carbonate reservoirs developed along the eastern margin of the Central Basin platform. The reservoir rock at McElroy consists of Permian System (Guadalupian Series) dolomite of the Grayburg Formation. The Grayburg dolomite and the underlying San Andres Formation were formed as shallowing-upward carbonate sequences. The carbonate facies are overlain by the Queen Formation, an evaporite-dominated unit that serves as the reservoir seal.

The Grayburg Formation carbonates consist of dolomitized lime mudstones, wackestones, pelletal and skeletal packstones, and ooid grainstones deposited in subtidal to supratidal environments. Facies prograde eastward toward the Midland basin, suggesting relative sea level lowering or progressive shallowing. The westward limit of the McElroy field is determined by pinch out of reservoir carbonates that interfinger with supratidal nodular and bedded anhydrite.

The reservoir petrophysical characteristics are defined by diagenetic overprinting on primary carbonate depositional fabrics. The principal diagenetic processes include dolomitization, anhydrite occlusion of primary porosity, and leaching.

McElroy, which was discovered in 1926, produces approximately 13,500 BOPD. This Chevron-operated field has 2.2 billion bbl of oil in place; 329 million bbl have been produced from nearly 1,600 wells. The field is currently under waterflood and is being evaluated for potential CO₂ injection.

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