The Devonian Woodford Fm of the Permian Basin: Complex Depositional and Temporal Variations Across from Anaerobic Marine Basin

Stephen C. Ruppel and Robert G. Loucks
Bureau of Economic Geology, University of Texas at Austin, Austin, TX

The Woodford Fm, a key oil and gas source rock in the Permian Basin of Texas and New Mexico, is part of an extensive, platform marginal, organic-rich, mudrock succession that formed along the southern and western margins of Laurussia during the Devonian and Mississippian. Studies of >35 Woodford cores reveal wide variability in facies, organic content, and mineralogy that can be related to age and paleogeographic setting. Woodford facies include silt-rich mudstones (detrital silica), siliceous mudstones (biogenic silica), calcareous mudstones, and claystones. Recent studies show that facies are partitioned between two temporally distinct successions: a Middle Devonian silt- and carbonate-rich section that is irregularly distributed across the basin, and an Upper Devonian siliceous claystone/mudstone section that is widespread and separated from underlying successions by a significant hiatus. All Woodford rocks contain mixtures of illite, kaolinite, chlorite, and mixed layer clays; total clay and chlorite abundance is lowest in distal Upper Devonian rocks. Although silica content is variable, Upper Devonian mudrocks typically contain more abundant biogenic silica, especially in distal parts of the basin, whereas Middle Devonian rocks are dominated by detrital silica. The two successions display consistent differences in depositional facies. The silt-rich Middle Devonian section is cross-laminated, locally graded, and commonly bioturbated. Upper Devonian mudrocks, by contrast, are dominated by fine-scale, parallel laminations and show no evidence of infaunal activity. These rocks also contain common conodonts, radiolarians, spore bodies, and deep-water brachiopods. The data suggest that the lower Woodford was deposited by deep water, turbid flow, whereas the upper Woodford accumulated under more distal, low energy, poorly oxygenated, hemipelagic conditions.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas