Abstract

The San Andres Formation (Upper Permian) has been a prolific oil producer in the Permian Basin of West Texas for many years. Located on the Eastern Shelf, the Garza field has produced oil from San Andres (Guadalupian) sediments for nearly fifty years. Production has been enhanced by a waterflood program initiated in 1967. While the majority of the field remains to be water-flooded, additional recovery enhancement with CO₂ injection is planned for the southwest portion of the field within the next two years. This investigation should enable reservoir engineers and production geologists working the Garza field to better understand the distribution of porosity, permeability, and facies.

Due to fluctuations in sea level, Permian sedimentation in the northern Midland Basin was cyclic. This cyclicity, combined with the relatively low relief of the Permian shelf depositional surface, resulted in considerable vertical facies variability and excellent lateral facies continuity. Within the carbonate and evaporitic sediments of the Garza field, three main environments and fifteen facies were identified. The environments identified were supratidal, intertidal, and subtidal. The major facies within the supratidal environment include: dolomitized mudstone, dolomitized intramicrite, algal laminated dolomudstone, and nodular anhydrite. Sediments of the intertidal environment are burrowed dolomudstone, burrowed molluscan wackestone, and wispy laminated facies. Subtidal facies include dolomitized skeletal packstone and dolomitized skeletal wackestone.

A relationship exists between facies and the distribution of porosity and permeability in the Garza field; the facies present affect the type and extent of diagenesis, which in turn, affects the distribution of porosity and permeability.