McElroy Field: Development of a Dolomite Reservoir, Permian Basin of West Texas

S. D. Walker, P. M. Harris

McElroy field, located in west Texas along the boundary between Crane and Upton Counties, lies along the eastern edge of the Central Basin platform. Production is from the Permian Grayburg Formation. The structure of the reservoir in McElroy field is a north-south-trending asymmetrical anticline with a steep east flank and gently dipping west flank. A stratigraphic permeability barrier defines the western boundary of the field, whereas the eastern edge of the reservoir is limited by a gradual reduction of permeability coupled with an increase in water saturation. Wells in McElroy field range in depth from 3,000 to 4,100 ft and produce from a gross section that averages 275 ft in thickness. Average reservoir porosity is 14% with permeability ranging from 0.01 to 2,000 md. otal areal extent of the field exceeds 50 mi². Since the field was discovered in 1926, more than 1,800 wells have been drilled. Secondary recovery processing of the reservoir began in 1960.

Prior to deposition of the Grayburg Formation, a regression exposed the underlying San Andres carbonate platform. During subsequent reflooding of the platform, a shallowing-upward sequence of Grayburg carbonate shelf deposits accumulated. Open-shelf deposits at the base of the sequence are dolowackestones and packstones. The overlying shallow shelf facies is formed of burrowed dolowackestones and packstones. The capping regressive part of the sequence comprises shallow-water shelf to intertidal sediments of the upper Grayburg, which form the main part of the reservoir. The upper Grayburg deposits with the overlying supratidal anhydrites, siltstones, and tidal-flat dolomites of the Queen Formation, which form the seal for the reservoir, prograded from west to east across the field.

Oil/water production and reservoir pressure conform to the development and orientation of permeability within the pay interval. Recovery varies between the central and flanking areas of the field. Cumulative production data indicate that maximum production has been from wells in the central part of the field that are situated along the crest of the structure. Reservoir zones thin toward the eastern flank of the field and pinch out to the west. Interbedded nonporous zones separate the reservoir vertically throughout the entire field and make correlation of pay zones and prediction of continuity between wells difficult.

AAPG Search and Discovery Article #91037©1987 AAPG Southwest Section, Dallas, Texas, March 22-24, 1987.