Introduction

The U.S. Geological Survey (USGS) assessed undiscovered, technically recoverable continuous and conventional oil and gas resources in the Middle to Late Devonian Woodford Shale (Ruppel and others, 2020a) and Mississippian Barnett Shale of the Permian Basin Province in western Texas and southeastern New Mexico (fig. 1A, B). For this study, the USGS defined nine total assessment units (AUs): four continuous AUs and one conventional AU for the Woodford Shale and four continuous AUs for the Barnett Shale (fig. 1A, B). The Woodford and Barnett Shales were previously assessed by the USGS in 2007 (Schenk and others, 2008).

Geologic Summary

A widespread Devonian marine transgression across the North American craton created anoxic bottom-water seas and resulted in the deposition of organic-matter-rich biosiliceous mudstones of the Woodford Shale (Ruppel, 2019). Subsequent sea-level fall during the Early Mississippian led to exposure, erosion, and periods of nondeposition. Renewed sea-level rise during the Middle to Late Mississippian resulted in the development of a large, rimmed carbonate platform and coeval deposition of basinal, organic-rich mudstones of the Barnett Shale (Ruppel, 2019). After the deposition of the Barnett Shale, the Permian Basin region transitioned to an active tectonic phase in which there was renewed collision of Gondwana and Laurentia, causing uplift, deformation, and erosion of both units (Ruppel, 2019).

The Woodford Shale has a maximum thickness of 900 feet (ft) and is variably porous (as much as 10 percent) and highly quartzose (50−90 percent; Ruppel and others, 2020a). In the distal southern Permian Basin Province, the quartz is mostly biogenic in origin, and clay content is low (10−35 percent; Ruppel and others, 2020a). In the proximal north, detrital quartz and clays are more prevalent (20−50 percent; Ruppel and others, 2020a). The Barnett Shale has a maximum thickness of 700 ft, is generally less porous (3−6 percent) than the Woodford Shale, and comprises mainly quartz and clays, with biogenic silica more prevalent to the south and detrital quartz to the north (Ruppel and others, 2020b). Quartz origin and clay content in both units affect facies and AU definitions: biosiliceous quartz and low clay content are representative of euxinic environments and higher organic richness, whereas the presence of clay-rich detrital quartz facies reflects an oxic, organic-poor environment. The presence of fractures, potentially from Late Mississippian to Pennsylvanian tectonism, likely enhances the reservoir quality of both formations.

The Woodford and Barnett Shales serve as both primary source rock and reservoir. Given their depositional histories, both the Woodford and Barnett Shales are considered Type II marine organic systems, with total organic carbon (TOC) content as high as 8 weight percent (wt. pct.) in the Barnett Shale and 14 wt. pct. in the Woodford Shale (Cicero, 2022), and both have a range of thermal maturities from immature oil to dry gas. As of November 2025, production is concentrated along the edges of the Delaware and Midland Basins and structural highs of the Central Basin Platform. Biomarker studies have shown that hydrocarbons generated from the Woodford and Barnett Shales have also migrated into overlying conventional reservoirs (Echegu, 2013; Spence, 2024).

Total Petroleum System and Assessment Units

The USGS defined four continuous and one conventional AUs for the Woodford Shale: the Woodford Alpine High Continuous Gas AU, Woodford Delaware Basin Continuous Gas AU, Woodford Midland Basin Continuous Oil AU, Woodford Central Basin Platform Continuous Oil AU, and Woodford Central Basin Platform Conventional Oil AU (fig. 1A, B). Four continuous AUs were defined for the Barnett Shale: the Barnett Alpine High Continuous Gas AU, Barnett Delaware Basin Continuous Gas AU, Barnett Midland Basin Continuous Oil AU, and Barnett Central Basin Platform Continuous Oil AU (fig. 1A, B). All the AUs are within the Permian Basin Paleozoic Composite Total Petroleum System (Schenk and others, 2008). Criteria used to define these AUs include the following: thickness of the source or reservoir greater than (>) 50 ft, favorable gross depositional environment or facies, TOC content >2 wt. pct., a range of thermal maturation windows that define oil and gas (vitrinite reflectance [R_o] between 0.6 and 1.3 percent for oil, and >1.3 percent for gas), and a drilling depth cutoff of 18,000 ft true vertical depth (TVD), which corresponds to the approximate limit of current (2010–24) successful onshore horizontal drilling, based on data from S&P Global Commodity Insights (2024).

Undiscovered Resources Summary

The USGS quantitatively assessed undiscovered continuous and conventional oil and gas resources in nine AUs in the Woodford and Barnett Shales of the Permian Basin Province (table 2). The fully risked, estimated mean totals are 1,625 million barrels of oil (MMBO), or 1.6 billion barrels of oil, with an F95–F5 range from 470 to 2,928 MMBO; 28,259 billion cubic feet of gas (BCFG), or 28.3 trillion cubic feet of gas, with an F95–F5 range from 8,098 to 51,706 BCFG; and 813 million barrels of natural gas liquids (MMBNGL), with an F95–F5 range from 236 to 1,469 MMBNGL.

For More Information

Assessment results are also available at the USGS Energy Resources Program website, https://www.usgs.gov/programs/energy-resources-program.

Permian Basin Province Assessment Team

Andrea D. Cicero, Christopher J. Schenk, Jenny H. Lagesse, Benjamin G. Johnson, Tracey J. Mercier, Heidi M. Leathers-Miller, Sarah E. Gelman, Jane S. Hearon, and Phuong A. Le