Evaluation of the deposition, lithology and diagenesis of the San Andres Formation in the Eastern Shelf of the Midland Basin, Tom Green County, Texas

The Eastern Shelf of the Midland Basin hosts extensive Permian-age strata that have been exploited for petroleum resources throughout the past century. Current activities in the region are focusing on secondary and tertiary recovery of hydrocarbons, which requires more detailed geologic characterization of producing zones. This study investigated the San Andres Formation across the western half of Tom Green County, Texas, encompassing approximately 1500 square miles or one million acres. Petrophysical logs and core across the study area were used to evaluate variations in lithology and rock composition. Data was analyzed to determine the structural nature of the San Andres strata in the study area and to develop isopach maps that evaluate spatial variability in depositional thickness on the Eastern Shelf. Cross-plot analyses of petrophysical logs were performed to differentiate lithological composition.;The analyses of this study concluded that San Andres deposition on the Eastern Shelf in Tom Green County, Texas, consisted of migrating, coarse-grained shoals along the margin of Permian Basin continental interior. Clastic inputs from the east were relatively constant, while wackestone and packstone carbonate shoals shifted in response to sea level change and wave energy in the subtidal to intertidal coastal zone along the margin of the Midland Basin. Diagenetic alteration of strata resulted in the replacement of anhydrite facies with calcite and extensive dolomitization of highly permeable wackstone and packstone facies.;The economic importance of the study area reveals that approximately one million barrels of oil have been already extracted from the region through conventional means. New technologies of drilling and well development may enable residual hydrocarbons still in place to be extracted by unconventional means. Studies like this one provide the necessary geologic insight to enable the development of unconventional oil extraction methods in long exploited oil fields.