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San Andres Dolomite Reservoir, Emma Field, Andrews County, Texas: Depositional Facies and Diagenesis

Stephen C. Ruppel

Although nearly 100% (19 million bbl of oil) of the estimated recoverable oil in the Emma San Andres reservoir has been produced, indications are that as much as 15 million bbl of recoverable, movable oil remains. Detailed study of facies and diagenesis in the reservoir reveals considerable lateral and vertical variability, which has important implications for recovery and injection efficiencies in the field.

The upper San Andres Formation in the Emma field comprises a shallowing-upward sequence of pervasively dolomitized, shallow-water subtidal to supratidal carbonates that accumulated on a gently east-sloping ramp. The lowermost part of the reservoir interval is composed of laterally extensive fusulinid packstone deposited in an open-marine ramp setting. Abruptly overlying this facies is a sequence of interbedded fusulinid/algal grainstone, formed in a migrating sand-shoal complex, and burrowed, skeletal mudstone deposited on a restricted inner ramp. The grainstone is restricted to the western and central parts of the field. These deposits are overlain by pisolitic and cryptalgal mudstones and terrigenous siliciclastics of supratidal origin. Anhydrite is abundant throughout the section.

Porosity and permeability are facies controlled. Highest permeabilities are developed in shoal complex graintone (100 md) and in fusulinid packstone (25-50 md). Lateral and vertical facies variations resulted in considerable reservoir heterogeneity.

Trace-element and isotope data indicate that most of the San Andres dolomite and associated sulfate (anhydrite) precipitated from downward-moving, seawater-derived brines during shallow burial in the late Guadalupian. Subsequent anhydrite dissolution has locally enhanced original porosity.

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