Geochemical characterization and classification of crude oils of the Permian Basin, west Texas and southeastern New Mexico

Fifty crude oils from reservoirs ranging in age from Cambrian to late Permian and distributed widely over the Permian Basin were geochemically characterized to assess their stratigraphic interrelationships and provenance. Detailed characterization using stable carbon isotopic composition and biomarker characteristics recognized five genetic groups. Genetic group I oils originate from a type II kerogen, Ordovician marine shale (Simpson Group) source rock deposited under anoxic conditions. Genetic group II oils, believed to be sourced from two kitchens, originate from type II/III kerogen Devonian marine shale (Woodford Shale) source rock deposited under anoxic to suboxic environments. Genetic group III oils are similar in many respects to group II oils. This group, however, shows evidence of significant terrigenous organic matter input. Genetic group III oils, single kitchen-derived oils, are inferred to be sourced from the Mississippian shale (Barnett Shale). Oils of genetic group IV are sourced from two kitchens and generated from the Cisuralian Wolfcamp shale source rock having type II/III kerogen deposited under anoxic-suboxic environments. Group V oils are derived from carbonate-rich shales (marls) of the Cisuralian Bone Spring Formation, having type II kerogen deposited under extremely anoxic environments. With the exception of Devonian- and Mississippian-sourced oils, which occur across a large stratigraphic interval (Cambrian-Ordovician to Permian), the rest of the genetic oil groups occur in close proximity to their candidate source rocks. Although no source rock samples were used in this study, the detailed geochemical study of oils has provided an understanding of the stratigraphic relationships between oils and their inferred source rocks.