Carbon Isotopic Composition of Crude Oils from Ellenburger Group (Lower Ordovician), Permian Basin, West Texas and Eastern New Mexico

Abstract

Carbon isotopic compositions have been determined for 37 crude oils from the Ellenburger Group (Lower Ordovician) in the Permian basin of West Texas and eastern New Mexico. These isotopic values have been compared with those obtained from 66 Permian basin crude oils ranging in age from Middle Ordovician (post-Ellenburger) through Early Permian (Wolfcamp). Carbon isotope data are reported as concentrations of C13 in parts per million of total carbon. Crude oils in the Ellenburger can be arranged into five groups based on carbon isotopic composition, geographic position, and stratigraphic relations. One group of Ellenburger crude oils has C13 values ranging from 10,732 to 10,753 ppm. which correspond to ^dgrPDBC13 ratios of --34.6 to --32.6. These isotopic values are unusually low compared with isotopic values usually found in all other crude oils of the world. The Ellenburger Group serves as a reservoir for oils of many ages, from Early Ordovician through Early Permian. Carbon isotopic compositions are useful in relating and distinguishing crude oils on a regional basis.