The U.S. Geological Survey (USGS) has assessed how much undiscovered, technically recoverable conventional oil and gas lies in the Buda Limestone of Texas, a rock unit from the Upper Cretaceous (Cenomanian).
The answer
| Resource | Mean estimate | Range (F95–F5) |
|---|---|---|
| Oil | 12 million barrels | 4–26 million barrels |
| Gas | 184 billion cubic feet | 55–439 billion cubic feet |
| Natural gas liquids | 2 million barrels | 0–4 million barrels |
F95 means a 95% chance of at least that amount; F5, a 5% chance. Estimates are fully risked.
Two assessment units
The USGS split the Buda into two assessment units (AUs), within the Upper Jurassic–Cretaceous–Tertiary Composite Total Petroleum System. They expand the 1995 Buda Fault Zone Oil Play and Buda Downdip Oil Play units to take in the East Texas Basin Province, and separate the Buda from the Fredericksburg-Buda Carbonate Platform-Reef Gas and Oil AU used before.
The two assessment units in the Buda Limestone of Texas. USGS
| AU | Bounded by |
|---|---|
| Buda Limestone Oil and Gas AU | the Lower Cretaceous shelf margin (southeast); the updip limit of the petroleum system, near where the Buda outcrops (north); the U.S.–Mexico border (west); the Balcones and Mexia-Talco Fault Zones, the East Texas Basin and the edge of the Buda (northeast) |
| East Texas Basin Buda Limestone Oil and Gas AU | the shape of the basin; the Mexia-Talco Fault Zone (west), included to catch any oil or gas migrating out of the basin and trapped along the faults; the edge of the Buda (north and east) |
Key inputs
| Buda Limestone AU (min / median / max, mean) | East Texas Basin AU (min / median / max, mean) | |
|---|---|---|
| Oil fields | 1 / 5 / 10, 5.1 | 1 / 2 / 6, 2.1 |
| Gas fields | 1 / 10 / 30, 10.6 | 1 / 2 / 8, 2.2 |
| Oil field size (million barrels) | 0.5 / 1 / 20, 1.5 | 0.5 / 1 / 40, 1.8 |
| Gas field size (billion cubic feet) | 3 / 6 / 500, 13.9 | 3 / 6 / 120, 8.9 |
| AU probability | 1.0 | 1.0 |
Results by unit (means)
| AU | Oil (million barrels) | Gas (billion cubic feet) | NGL (million barrels) |
|---|---|---|---|
| Buda Limestone, oil accumulations | 8 | 11 | 1 |
| Buda Limestone, gas accumulations | — | 147 | 1 |
| East Texas Basin, oil accumulations | 4 | 6 | 0 |
| East Texas Basin, gas accumulations | — | 20 | 0 |
The rock
The Buda is a widespread, 30–160-foot-thick, low-energy limestone rich in coccoliths, laid down when shallow seas flooded the Comanche shelf. It sits on the Del Rio Formation under the Eagle Ford Group. North of the San Marcos Arch and into the East Texas Basin, the Woodbine Formation lies between the Buda and the Eagle Ford; in the East Texas Basin the Maness Shale (absent in the north of the basin) comes between the Buda and the Woodbine. Its boundaries are usually described as unconformable, though some authors report conformable contacts in places.
It formed in different settings across the state:
| Region | Setting and rock |
|---|---|
| Western Texas | a broad, low-relief, partly restricted shelf above storm-wave base; informally three members — the lower Lechuguilla and upper Love Station (micritic limestone) and the middle Red Light (clay-rich micritic limestone) |
| Southern Texas | deeper subtidal water, nearly open-marine toward the Lower Cretaceous shelf margin |
| Central Texas | a broad, shallow, semi-restricted shelf; a lower chalky, glauconitic member dominated by wackestone and an upper packstone member |
| East Texas Basin | less known; at the Schneider field in Wood County, a reef complex that grew on the south flank of a high raised by the Hainesville salt dome — wackestones, packstones, grainstones, and coral and stromatoporoid boundstones |
How the oil and gas got there
- Source: mostly the Eagle Ford Group, whose hydrocarbons migrated into the Buda through open faults and fractures; the Upper Jurassic Smackover Formation may have added a little wet gas.
- Storage: the Buda is a tight limestone, so oil and gas collect only where there are extensive fractures, microfractures, fault-shattered rock or some matrix porosity — usually in the upper Buda.
- Traps: stratigraphic (sealed against impermeable rock such as the Eagle Ford), structural (growth faults, folded anticlines) or both. Production generally comes where stress and compaction over buried structures have fractured the rock.
- In central Texas, the middle Buda may act as a carrier bed, moving Eagle Ford oil and gas updip along faults.
- Downdip of the Lower Cretaceous shelf edge in southern Texas, the potential declines; faults on the reef trend suggest Eagle Ford gas has moved into the Edwards Formation instead.
- In the East Texas Basin, oil produced in the north may come from Jurassic rocks; to the south, undiscovered oil may come from the Eagle Ford and gas from Jurassic rocks. Salt diapirs from the Middle Jurassic Louann Salt that pushed up into the Buda may form additional traps.
Sources
Based on Lohr CD, Doolan CA, Merrill MD, Craddock WH, Gardner R, Anderson CP, Le PA, Mercier TJ, Schenk CJ, "Assessment of undiscovered conventional oil and gas resources in the Buda Limestone of Texas, 2025," U.S. Geological Survey Fact Sheet 2026–3015, USGS (doi:10.3133/fs20263015); a work of the United States government in the public domain. Results are also at the USGS Energy Resources Program. The fact sheet's stratigraphic column, modified from published figures, is not reproduced here.
Licence: CC0 1.0 (public domain) · Adapted from pubs.usgs.gov
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