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

Authors: S. C. Ruppel and R. G. Loucks

Venue: 2008 American Association of Petroleum Geologists (AAPG) Annual Convention and Exhibition, San Antonio, TX, April 19-24, 2008 “The Geology of Mudrocks”, session chaired by S. C. Ruppel and R. G. Loucks (http://www.aapg.org)

Abstract: The Woodford Formation, 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.

Related NETL Project
This presentation is related to the NETL project DE-FC26-04NT15509, “Integrated Synthesis of the Permian Basin: Data and Models for Recovering Existing and Undiscovered Oil Resources from the Largest Oil-Bearing Basin in the United States.” The objectives of the project are twofold: (1) to produce a detailed, comprehensive analysis and history of Paleozoic depositional and reservoir systems in the Permian Basin, and (2) to create spatially integrated databases of depositional, stratigraphic, lithologic, and petrophysical properties for selected reservoir plays and stratigraphic horizons. These objectives will be undertaken and completed sequentially during the 3 years of the project. The overall objective is to provide Permian Basin operators with (a) outcrop and subsurface reservoir specific data, data syntheses, and models to be applied to geological-, engineering-, and completion-based redevelopment of existing reservoirs, and (b) a detailed regional stratigraphic framework for applying such models to new exploration targets.

Project Contacts
NETL – Virginia (Ginny) Weyland (Virginia.WEYLAND@netl.doe.gov or 918-699-2041)
University of Texas at Austin – Stephen Ruppel (stephen.ruppel@beg.utexas.edu or 512-471-1534)