Oil explorers have worked the sandstones of South Texas since at least the 1930s. Horizontal drilling and hydraulic fracturing have since made each well far more productive, so the U.S. Geological Survey took a fresh look. Its 2023 assessment of the Escondido, Olmos and San Miguel Formations found that most of what remains lies in deeper, "continuous" deposits — an estimated mean of 361 million barrels of oil and 10,978 billion cubic feet of gas.
The nine assessment units in the Western Gulf Basin Province. Credit: U.S. Geological Survey.
The formations
The San Miguel Formation (Campanian Taylor Group) and the Olmos and Escondido Formations (Campanian–Maastrichtian Navarro Group) are shale and sandstone beds beneath the southern Texas Gulf Coastal Plain, from the San Marcos Arch to the U.S.–Mexico border, including the Maverick Basin. They were laid down on a continental margin in settings ranging from land to shallow and deep sea. Net sandstone thickness runs to tens or hundreds of feet, and whole formations to hundreds of feet or about a thousand. Minimum cumulative production from the three ranges from 6.8 to 170 million barrels of oil and from 120 to 2,800 billion cubic feet of gas per formation.
Where the oil came from. The main source rock is the Eagle Ford Shale beneath them. Oil gravity and gas-to-oil ratios change with the Eagle Ford's thermal maturity, and biomarkers in San Miguel oils point the same way. Petroleum probably moved up along regional faults — the Balcones, Luling and Charlotte-Jourdanton fault zones — or through fractures, mostly during the Paleogene, with a later phase of migration when the Maverick Basin was uplifted that may have formed the province's heavy oil deposits.
Two kinds of deposits
| Updip | Downdip | |
|---|---|---|
| Accumulation | Conventional: oil-water and gas-water contacts visible in well logs | Continuous: no contacts can be shown |
| Porosity | Field averages of 20 to 30 percent | Lower; the highest reported are 15 to 20 percent |
| Traps | Structural and stratigraphic | Stratigraphic |
| Seals | Shale beds interfingered with the sandstones | The fine-grained beds that make up most of the rock |
USGS defined nine assessment units, three per formation: an updip conventional unit, a middle continuous oil unit (extending to about where Eagle Ford vitrinite reflectance reaches 1.3 percent, with a mix of oil and gas expected), and a downdip continuous gas unit extending to about the Lower Cretaceous shelf margin plus a roughly 20-kilometer buffer. Earlier assessments treated the downdip deposits as conventional while noting the line between the two was unclear.
The estimates
| Mean undiscovered, technically recoverable | Oil (million barrels) | Gas (billion cubic feet) | Natural gas liquids (million barrels) |
|---|---|---|---|
| Conventional (3 updip units) | 5 | 25 | 0 |
| Continuous (6 Maverick Basin units) | 361 | 10,978 | 60 |
The continuous estimates span a wide range: a 95 percent chance of at least 57 million barrels of oil and 1,369 billion cubic feet of gas, and a 5 percent chance of at least 916 million barrels and 29,539 billion cubic feet. The Olmos continuous units hold the largest shares — 177 million barrels of oil in the oil unit and 3,265 billion cubic feet in the gas unit.
The last USGS assessment, in 2003, estimated means of 33.22 million barrels of oil, 1,682 billion cubic feet of gas and 34.26 million barrels of natural gas liquids, mostly downdip.
Sources
Based on "Assessment of Undiscovered Conventional and Continuous Oil and Gas Resources in the Escondido, Olmos, and San Miguel Formations," U.S. Geological Survey Fact Sheet 2025–3015; a work of the United States government in the public domain.
Licence: CC0 1.0 (public domain) · Adapted from pubs.usgs.gov
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