The Permian Basin's Deep Shales: USGS Estimates 1.6 Billion Barrels of Oil and 28.3 Trillion Cubic Feet of Gas in the Woodford and Barnett

The U.S. Geological Survey (USGS) has reassessed the undiscovered, technically recoverable oil and gas in two deep shales of the Permian Basin Province in western Texas and southeastern New Mexico: the Middle to Late Devonian Woodford Shale and the Mississippian Barnett Shale. Both were last assessed in 2007.

The estimate

These are fully risked estimates across nine assessment units. F95 means a 95% chance of at least that much; F5, a 5% chance.

Assessment units in the Woodford Shale (A) and the Barnett Shale (B), Permian Basin Province. USGS

How the shales formed

1. Devonian: the sea spread widely across the continent, and oxygen-starved bottom waters let organic-rich, silica-rich muds pile up — the Woodford.
2. Early Mississippian: the sea fell, exposing and eroding the land, with gaps in deposition.
3. Middle–Late Mississippian: the sea rose again. A large carbonate platform grew, while organic-rich muds settled in the basin — the Barnett.
4. Afterwards: renewed collision between Gondwana and Laurentia uplifted, deformed and eroded both.

Silica from organisms and low clay mark oxygen-free, organic-rich settings; clay-rich detrital quartz marks oxygenated, organic-poor ones — which is how the assessment units were drawn. Fractures, perhaps from Late Mississippian to Pennsylvanian tectonics, likely improve both as reservoirs. Both are Type II marine source rocks — and their own reservoirs — with maturities from immature oil through to dry gas. Chemical fingerprinting shows their oil and gas have also migrated up into overlying conventional reservoirs. As of November 2025, production was concentrated along the edges of the Delaware and Midland basins and on the structural highs of the Central Basin Platform.

The nine assessment units

All lie within the Permian Basin Paleozoic Composite Total Petroleum System. To qualify, rock had to be more than 50 ft thick, in favorable facies, with TOC above 2%, and within the oil window (vitrinite reflectance 0.6–1.3%) or gas window (above 1.3%) — and shallower than 18,000 ft, roughly the limit of successful onshore horizontal drilling in 2010–24.

Some details:

• Alpine High — a structurally high, productive area — is treated separately from the surrounding Delaware Basin gas units, bounded on the west by gas maturity and facies and elsewhere by faults marking a steep dip into the basin.
• The Woodford Central Basin Platform continuous oil unit has several producing wells, but earlier work suggests it is less likely to be productive, given low maturity, low pressure and thick, viscous oil.
• The Woodford conventional unit has just one producer — the Angel 2 well at Gladiola Field, the only Woodford producer in all of New Mexico — so it was assessed with considerable risk.
• In the Barnett Midland Basin oil unit, production has risen steadily since 2013, mostly along the basin's western edge where maturity, thickness, depth and facies suit oil. Carbonates interleaved with the shale in the north, unique to the Barnett, may make it easier to fracture. Both shales thin sharply eastward across the Midland Basin.

More results are on the USGS Energy Resources Program website.

Sources

Based on "Assessment of undiscovered continuous and conventional oil and gas resources in the Woodford and Barnett Shales of the Permian Basin Province," U.S. Geological Survey Fact Sheet 2026-3059, by the USGS Permian Basin Province Assessment Team (Andrea D. Cicero and others); a work of the United States government in the public domain.