IDENTIFICATION OF LITHOFACIES, DEPOSITIONAL ENVIRONMENT AND CYCLIC STRATIGRAPHY: UPPER SAN ANDRES FORMATION, VACUUM FIELD, LEA COUNTY NM USA

The Vacuum Field is located in Lea County New Mexico, on the Northwestern Shelf of the Permian Basin, and produces from numerous units including the Leonardian/Guadalupian (Permian) San Andres and Grayburg Formations. This thesis project involved the description and interpretation of two Vacuum Field wells, the VGSAU # 157 and the CVU #72, which were cored primarily in the San Andres Formation. The main purposes of this study are to describe the shallow water platform carbonates of the San Andres Formation as developed in the Vacuum Field and to fully interpret the lithofacies, depositional environments, cyclicity, and their correlation to reservoir continuity.

Despite extensive dolomitization and karsting within the cored intervals, several lithofacies were identified from grain types, fabrics and sedimentary structures in the upper San Andres of the Vacuum Field. Tidal flat-capped cycles contained stromatolitic algal laminations, mud cracks and predominantly nonskeletal grains and represented the shallowest water deposits. Shoal-capped cycles formed in slightly deeper water, but with high-energy cycle tops marked by cross-bedding and mostly non-skeletal grains, and open marine-capped cycles formed in the deepest water and displayed burrowing and skeletal grains. In overall shallowing upward sequences, the depositional environments identified include; open marine subtidal, shoal/intertidal, restricted subtidal, restricted peritidal and supratidal/tidal flat.

Three High Frequency Sequences of the upper San Andres (Guad 4, Guad 8 {Guad 12} and Guad 9 {Guad 13}) identified by previous workers were observed in core. Exposure and karsting occur in the upper part of these HFS’s. Karst caves and collapse breccias are cemented with anhydrite and/or quartz sand and thus are not porous. The original depositional textures of the San Andres Formation exerted a major control on resultant diagenetic fabrics and porosities identified in the study. These diagenetic features interrupt the cycle set/flow units in both Guad 4 and Guad 8, therefore affecting both reservoir continuity and quality.