Abstract

The Somerset Field is located within southwestern Bexar and northern Atascosa counties in south central Texas. This portion of the northeastern Maverick Basin has not been well studied. For over 100 years, the Upper Cretaceous Olmos Sandstone has been the primary target out of three separate producing zones. The primary structure of the region is the graben system of the Balcones-Luling-Mexia fault system, in which, the Somerset Field lies partially within a southern half graben of the Luling Fault system. There are conflicting reports of the Olmos Formation being truncated by the famous Bigfoot Unconformity, down dip of the study area, and that the underlying San Miguel sandstone has been misidentified as the Olmos Formation. Through core and well log analysis, a fluvial deltaic and barrier bar system was identified. A volcano, of the Uvalde Igneous Province was discovered as well. Thin sections were cut from one well, the rest were studied as is, due to the swelling clay content of the core samples, and their reactivity to our thin sectioning fluids. The sandstone is comprised of very fine-grained igneous grains and copious amounts of clay. The igneous clasts have been heavily altered and many of the original sedimentary structures, as well as permeability has been destroyed by bioturbation. The sandstone contains Skolithos, foraminifera, and aragonite oyster shells, as well as pyrite. With the data that has been analyzed, it is entirely possible that the Olmos formation of the Somerset Oil Field is in reality the underlying San Miguel Sandstone, in a depositional setting that is analogous to the Texas Gulf coast of today. More study is needed to come to a definitive conclusion however, since no known subsurface San Miguel samples had been collected for comparison.