PETROGRAPHY AND GEOCHEMISTRY OF THE LIMESTONE-DOLOSTONE TRANSITION IN RESIDUAL OIL ZONE OF THE EARLY PERMIAN SAN ANDRES FORMATION, PERMIAN BASIN, TEXAS

The early Permian San Andres Formation accounts for ~40% of all oil production in the Permian Basin from conventional carbonate reservoirs that have been partially or pervasively dolomitized. A smaller, yet still significant, portion of San Andres oil production (~10%) is from enhanced oil recovery (EOR) through carbon dioxide (CO₂) flooding of the residual oil zone (ROZ) beneath the main dolomite reservoirs. Oil was swept from the ROZ by natural waters that flowed through the rock before present day. The ROZ in the GLSAU Field on the Central Basin Platform in Ector County, TX is located above a transition zone, where petrographically well-preserved limestone abruptly converts to fabric-destructive dolostone over an interval of a few feet. In the GLSAU 204R core, the limestone interval is characterized by fusilinid-rich wackestone and packstone with well-defined intraparticle porosity. The base of the ROZ at -4406.5 ft is marked by oil stain from the original oil-water contact and a transition from dolomitic limestone to fabric destructive dolostone. This transition from limestone to dolostone in the core does not appear to be associated with a change of sedimentary facies. An outstanding question associated with this transition is to what extent does the geochemistry of the San Andres Formation reflect interaction with post-depositional and diagenetic fluids. A penecontemporaneous origin for dolomitization in the shallow-water deposits of the San Andres Formation is consistent with a reflux model for dolomitization in the Cowden Field located to the northeast of the GLSAU Field. Here we present new elemental (Ca, Mg, Fe, Mn, and Sr), carbon isotope, and oxygen isotope values of carbonate in the GLSAU 204R core. The elemental and isotopic data gathered in this study provides additional constraints on the diagenetic framework of the San Andres Formation, the nature of the limestone-dolostone transition, and the development of ROZs in the Permian Basin.