Identification of Depositional Facies, Cyclic Stratigraphy and Diagenetic Components: Upper San Andres Formation, Vacuum Field, Lea County NM USA

Abstract

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 diagenetic components and their effects on reservoir quality. Three main cycle types were identified: tidal flat/exposure capped cycles, ooid shoal capped cycles and open-marine capped cycles. These cycles exhibit a progradational pattern from shallow open marine depositional environments to very shallow, high-energy shoals to tidal flat environments that formed on the topographically highest part of the platform (landward). In overall shallowing upward sequences, the depositional environments identified include; open marine subtidal, shoal/intertidal, restricted subtidal, restricted peritidal and supratidal/tidal flat. Post-depositional diagenetic features are superimposed on most of the observed lithofacies, and are interpreted to have formed as a result of subaerial exposure and karsting. Features observed in the core include caves filled with anhydrite, carbonate debris, and laminated cave floor deposits. The entire core shows a complex history of extensive dolomitization that produced a variety of dolomite fabrics/textures within the previously discussed lithofacies and depositional environments. Paragenesis plays an important role in the porosity evolution of the Upper San Andres lithofacies of the Vacuum field; hence a detailed study of distinctive diagenetic facies observed would help to better predict porosity trends and flow units within the reservoir. The main controls on porosity types and development appear to be lithology, dolomitization and anhydritization. The main association between primary pore types and depositional environments can be found in the supratidal/tidal flat lithofacies, which generally have low porosity. Major pore types identified are interparticle, moldic, intercrystalline and fenestral, and are variably occluded by anhydrite. Other diagenetic features post-dating dolomitization are fractures and stylolites.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015