Abstract

Cyclic San Andres facies have been delineated for the Reeves field. Diagnostic facies criteria were established from thin-sections and core chips.

The Reeves field was discovered in 1957 and has reserves estimated at 20 million barrels. It is located on the northern shelf of the Midland Basin. The Reeves zone occurs about 500 feet below the top of the San Andres formation and consist of four sedimentary cycles, each between 10 and 20 feet thick. A complete cycle includes the following lithologies: 1) a thin, dark-gray, basal shale associated with nodular anhydrite, 2) carbonate mudstone, 3) wackestone, 4) packstone, and 5) oolitic grainstone. All the units of each cycle are not always present.

Numerous sedimentary structures can be recognized in the Reeves zone which are characteristic of supratidal deposition, such as stromatolites, vertical desiccation cracks and birdseye. The carbonate grain types are also characteristic of shallow-water deposition. Concentric oolites and coated grains are the most common, followed in abundance by pellets, skeletal grains, lumps and intraclasts.

Several varieties of anhydrite exist in the sediments of the Reeves zone. These include nodular anhydrite, void-filling anhydrite, replacement anhydrite, and euhedral anhydrite laths. No primary (bedded) anhydrite was found.

The diagenetic history of the Reeves zone is complex and can best be understood by using a reflux model under conditions similar to supratidal sebkhas of the Persian Gulf. Repeated diagenetic cycles involving both leaching and precipitation caused by changes in the water table complicated this relatively simple model.

Porosity variations in the Reeves zone are related to original sedimentary facies and their diagenesis. Oolitic grainstone porosity has been plugged with anhydrite. Productive porosity generally occurs in the wackestone facies, and it is either intercrystalline, leached or fracture origin. A northeast-southwest trending fracture system occurs in the Reeves field which enhances permeability and production in some areas.

Oil migrated into the Reeves trap after the late diagenetic processes were fairly complete.