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Similarity of Grayburg/San Andres Dolomite Reservoirs--Eastern Edge of Central Basin Platform, Permian Basin

Raymond A. Garber, Paul M. Harris

Porous dolomites of the Grayburg/San Andres Formations (Permian, Guadalupian) form major hydrocarbon reservoirs along the eastern edge of the Central Basin platform of the Permian basin, Texas. A comparison of the lithologic descriptions and depositional models for these reservoirs shows that although substantial local variation exists, the overall generalized vertical sequence and, particularly, the occurrence of porous shoal grainstones are strikingly similar.

Data presented in this pilot study comes from the Gulf Oil 223-R J. T. McElroy Cons. well in McElroy field, the Gulf Oil 35 E. N. Snodgrass well in Waddell field, and the Gulf Oil 1359-56 C. A. Goldsmith et al well in Goldsmith field. Grayburg/San Andres reservoirs in all three fields occur mainly in the middle part of a shallowing-upward carbonate-shelf sequence that is 250-300 ft thick. The sequence consists of: (1) basal open-shelf, bioturbated dolowackestones/packstones; (2) middle shallow shelf fusulinid dolowackestones overlain by nonskeletal shoal dolopackstones/grainstones; and (3) upper variable intertidal/supratidal deposits that grade into the Queen Formation.

The depositional sequence formed in a regional facies tract of shallow-water shelf and shoal environments with related tidal-flat and evaporitic (sabkha) sediments. The facies developed during easterly progradation across a deeper water open shelf. Porosity was formed by near-surface dolomitization and minor dissolution. Early and late evaporite plugging reduced porosity, so reservoir zones average 9% intercrystalline and moldic porosity with 10-md permeability.

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