PETROGRAPHIC AND GEOCHEMICAL EVALUATION OF THE LIMESTONE-DOLOSTONE TRAINSITION BEANTH THE RESIDUAL OIL ZONE OF THE LOWER PERMIAN SAN ANDRES FORMATION, ECTOR COUNTY, TEXAS

The Permian Basin reservoirs of West Texas and Southeast New Mexico contain some of the most prolific and critical hydrocarbon reserves in the United States. These reservoirs are not only important for oil and gas production which fuels our modern society, but also represent valuable targets for subsurface storage of carbon dioxide (CO₂). One of the largest targets for CO₂ storage is the early Permian San Andres Formation, which is responsible for approximately 40% of conventional oil production in the basin. Enhanced Oil Recovery (EOR) utilizing CO₂ accounts for about 10% of total production in the San Andres Formation. This tertiary production is from Residual oil zones (ROZ’s), where the original oil saturation was swept by mother natures waterflood. In the Goldsmith Landreth San Andres Unit (GLSAU) of the Goldsmith Field in Ector County, TX the ROZ in the San Andres Formation is associated with a transition from limestone to dolostone. This transition occurs over approximately two to three feet and is recognized petrologically in both core samples and in petrographic thin sections and the transition does not appear to reflect a change in sedimentary facies. This study utilizes drill core, petrophysical logs, detailed observations of petrographic thin sections, and geochemical data from portable X-ray fluorescence (pXRF), inductively coupled plasma optical emission spectrometry (ICP-OES), and isotope ratio mass spectrometry (IRMS) to constrain the diagenetic changes through the limestone-dolostone transition zone of the GLSAU 204R drill core. Rock from the transition zone is compared to the overlying rock in the main pay zone and from beneath the ROZ. Understanding the diagenetic changes associated with the formation of the ROZ will aid in evaluation of the San Andres Formation as a target for permanent subsurface CO₂ storage in the Permian Basin and elsewhere.