Stratigraphic Analysis of the Upper Devonian Woodford Formation, Permian Basin, West Texas and Southeastern New Mexico

The Upper Devonian Woodford Formation is an organic-rich petroleum source rock that extends throughout West Texas and southeastern New Mexico and currently is generating oil or gas in the subsurface. The Woodford is a potential hydrocarbon reservoir in areas where it is highly fractured; the most favorable drilling targets are fractured siltstone or chert beds in densely faulted regions such as the Central Basin Platform, southernmost Midland Basin, and parts of the Northwestern Shelf. Stratigraphic analysis was undertaken to determine how the Woodford was deposited and why its petroleum source potential is so great.The Woodford consists of two lithofacies, black shale and siltstone. Black shale, the most widely distributed rock type, is very radioactive and contains varvelike parallel laminae, abundant pyrite, and high concentrations of marine organic matter. Siltstone, typically a basal facies, in deep basin and proximal shelf settings, exhibits disrupted stratification, graded layers, fine-grained Bouma sequences, and a subequal mixture of silt-sized quartz and dolomite. Black shale is mostly pelagic and represents an anaerobic biofacies, whereas siltstone is the result of bottom-flow deposition and represents a dysaerobic biofacies.The depositional model developed herein for the Woodford was based on stratigraphic sequence, patterns of onlap, and lithologic variations, together with published information about global paleogeography, paleoclimate, and eustasy. During the Late Devonian, the Permian Basin was a low-relief region located on the western margin of North America in the arid tropics near 15 degrees south latitude. Worldwide marine transgression caused flooding of the craton and carried water from a zone of coastal upwelling into the expanding epeiric sea. Strong density stratification developed, due partly to accumulation of hypersaline bottom water that formed locally in the arid climate. Anaerobic conditions resulted from poor vertical circulation and from high oxygen demand, which was caused by the decay of abundant organic matter produced in the nutrient-rich surface waters. Continuous, slow deposition of pelagic material was interrupted by episodic, rapid deposition of silt and mud from bottom flows generated during frequent tropical storms.This report documents the composition, distribution, and structure of the Woodford Formation in a major hydrocarbon-producing basin. Petrologic and organic geochemical data helped explain the origin of the unit and provided information necessary for predicting potential locations and lithologies of commercial petroleum reservoirs within the Woodford. Combining comprehensive stratigraphic, petrologic, and geochemical data was useful for developing a depositional and exploration model of Devonian black shale in West Texas and New Mexico. Similar studies should be conducted elsewhere to enable discovery of unconventional hydrocarbon reserves in black shales.