Lithofacies and chemostratigraphy of the upper Wolfcampian in the southeastern Delaware Basin, Pecos County, Texas

During the deposition of Wolfcampian sediments, the southern portion of the Delaware Basin was subject to active tectonism and an evolving basin configuration which led to a complex depositional environment. Intense interest exists in the petroleum geology of this region for hydrocarbon exploration. An increase in the understanding of the petrophysical properties, lithofacies and depositional processes are beneficial to their exploitation. Subsurface basinal Wolfcamp lithofacies were described from three cores in the Gomez gas field and analyzed for elemental abundance, bulk mineralogy and total organic carbon. Seven lithofacies were observed in the study area cores. Oligomictic limestone paraconglomerate, crinoid-fusulinid packstone, silty crinoid wackestone/packstone, structureless to laminated fine sandstone, bioturbated siliceous siltstone, argillaceous siliceous silty mudstone, laminated argillaceous siliceous silty mudstone and bioclastic argillaceous siliceous silty mudstone were identified and described. Two distinct lithostratigraphic features were defined by wireline log correlation; an upper carbonate debris flow unit and lower siliciclastic sandstone channel and lobe unit. An eight-mile long by nine-mile wide (at a minimum), 260-foot thick carbonate debris flow unit was interpreted as successive debris flows, concentrated density flows and hyperconcentrated density flows with argillaceous siliceous silty mudstone deposition separating the two units. The five-mile-long by three-mile-wide (at a minimum), 270-foot thick lower siliciclastic sandstone channel and lobe unit was interpreted as a sandy turbidite-filled channel with fine-grained to muddy interchannel configuration. Both of these units thickened towards the Coyanosa structure which was likely a local control of sediment transport. These units were vertically separated by hundreds of feet of high-gamma ray mudrock. Only four of 145 total organic carbon measurements were greater than two percent, which is commonly cited as the minimum organic carbon enrichment necessary for unconventional mudrock hydrocarbon generation and is lower than similar Wolfcampian mudrock successions in other areas of the Delaware Basin. Poor TOC preservation was likely to be at least partially caused by terrigenous quartz sediment dilution as well as microscopic and megascopic bioturbation.