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Relationship of Permian San Andres Facies to Porosity and Permeability Distribution in ODC Field, Gaines County, Texas

Bryce J. McKee

The ODC field, operated by American Petrofina Company of Texas, is located in Gaines County, Texas, on the northwestern shelf of the Midland basin (Central Basin platform). Consisting of 53 wells, the ODC field produces from 80,000 net ac involved in a waterflood injection program that was initiated in 1969. The primary reservoirs are dolomitic carbonates in the Permian San Andres Formation. Since flooding was initiated, 836,043 bbl of crude oil has been produced from an average reservoir depth of 5,500 ft subsea.

To maximize production using a proposed carbon dioxide injection program, porosity and permeability trends were analyzed, detailing lateral and vertical continuity, and the depositional and diagenetic processes controlling porosity and permeability were investigated, using data from five well cores and corresponding laboratory reports.

Detailed logs were made, to define depositional facies and their lateral and vertical relationships. These data were related to corresponding laboratory data that revealed porosity and permeability variations resulting from diagenetic modifications of the lithofacies.

As a result, 12 depositional facies were recognized, which could be divided into three groups: supratidal, intertidal, and subtidal facies. Higher energy intertidal and subtidal facies (e.g., algal packstone, skeletal packstone) were found most frequently at the western edge of the study area. The remaining lower energy intertidal and subtidal facies were found more frequently to the east. This overall trend follows water energy levels, which are related to bathymetry of the eastward-deepening margin of the Midland basin western shelf.

The general diagenetic pattern is porosity inversion, similar to Dune field in west Texas. Within the entire San Andres section of the ODC field, rocks were completely altered to dolomite. Anhydrite is prominent in the supratidal facies, and authigenic clays, quartz, and chert occur in places in the more porous subtidal facies.

Porosity in the ODC field occurs as three types: (1) predominant intercrystalline, (2) microvugular pinpoint, and (3) fracture porosity.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.