The U.S. Geological Survey has estimated the undiscovered, technically recoverable oil and gas in the Austin Chalk and the Tokio and Eutaw formations — Upper Cretaceous (Coniacian–Santonian) rocks buried beneath the Gulf Coast from south Texas to the Florida Panhandle. These units, onshore and in state waters, belong to the Upper Jurassic–Cretaceous–Tertiary Composite Total Petroleum System.
The geology
How the rocks formed. The Austin Chalk — chalk and marl — was laid down on a broad, gently sloping sea shelf that deepened to the south and west as the sea advanced during Coniacian–Santonian time. Toward the northeast, the land side, it passes into the sandstone and mudstone of the Tokio Formation in Arkansas and Louisiana and the Eutaw Formation in Mississippi and Alabama, deposited in shallow and marginal marine settings at the front of the advancing sea.
Where the oil and gas came from varies across the region:
- Most of the Austin Chalk's oil and gas is in Texas, sourced from the thermally mature mudstone of the Eagle Ford Group directly beneath it — a coupled Eagle Ford–Austin Chalk system.
- In northern Texas, where the Eagle Ford is too immature, the deep Upper Jurassic Smackover Formation is a source.
- In eastern Texas and western Louisiana, where the Eagle Ford is thin or absent, sources are in older Upper Cretaceous rocks.
Oil and gas moved into conventional reservoirs in the chalk and into the Tokio and Eutaw sandstones along faults and salt domes. Where the chalk lies on thermally mature Eagle Ford or equivalent source rocks, it acts as a continuous reservoir: oil and gas expelled under high temperature and pressure moved straight into the fractured base of the chalk.
Figure 3. Major geologic features of the onshore Gulf Coast where the rocks were assessed. Modified from Ewing and Lopez (1991), Schruben and others (1998) and Galloway (2008); base map from the National Park Service.
Eight assessment units
Rocks with similar layering, structure and history of oil and gas charge were grouped into eight assessment units (AUs): five conventional, three continuous. Vitrinite reflectance (Ro), a measure of how far source rocks have "cooked", marks several boundaries: about 0.6 percent where oil generation begins, and about 1.3 percent where gas generation begins.
Figure 1. The petroleum system and the five conventional assessment units.
| Conventional AU | Defined by |
|---|---|
| Western Updip Fault Zone Oil | Austin Chalk reservoirs; bounded on the northwest by the chalk and Eagle Ford outcrop and regional faults; west to the Mexican border, including parts of the Sabine Uplift and East Texas Basin; south to where the Eagle Ford reaches the oil threshold (Ro 0.6 percent) |
| Eastern Updip Fault Zone Oil | Tokio and Eutaw sandstones; bounded on the northeast by outcrop and faults, including parts of the Sabine Uplift and the North Louisiana and Mississippi salt basins; its southern edge is where the sandstones give way to the chalk, near the oil threshold in the source rocks beneath. It meets the western unit near the Texas–Louisiana line |
| Updip Salt Basins Oil and Gas | areas where movement of the Jurassic Louann Salt controlled how oil and gas migrated, were trapped and sealed — the Brazos Basin's diapir trend and parts of the East Texas, North Louisiana and Mississippi salt basins |
| Middip Fault Zone and Salt Oil | between the Lower Cretaceous shelf edge to the north and the Upper Cretaceous shelf edge to the south; accumulations tied to salt domes and the big faults of Louisiana's deep Tuscaloosa fault zone; its western edge is near the Louisiana–Texas line, where a quieter shelf margin gives way to a more active one |
| Downdip Salt Basins Gas | untested chalk reservoirs with no wells; from the shelf edges and the Eagle Ford gas wells of Texas south to the federal-waters boundary |
Figure 2. The petroleum system and the three continuous assessment units.
| Continuous AU | Defined by |
|---|---|
| Western Shelf Oil | from where the Eagle Ford begins to generate oil (Ro 0.6 percent) south to where it begins to generate gas (Ro 1.3 percent), west across the Maverick Basin to Mexico. The oil threshold matches the switch from conventional to continuous field development — confirmed by water-to-oil ratios, which rise over time in conventional wells but stay level in continuous ones |
| Eastern Shelf Oil | from the updip limit of the chalk and the oil threshold in the Turonian Tuscaloosa marine shale, down to the shale's gas threshold |
| Western Shelf Edge Gas | from the Eagle Ford's gas threshold to about the Lower Cretaceous shelf edge and the limit of productive Eagle Ford gas wells; no wells produce from the chalk here |
Results
The mean estimates for all eight units together:
| Mean | Range (F95–F5) | |
|---|---|---|
| Oil | 6,867 million barrels (6.9 billion) | 2,067–12,582 million barrels |
| Gas | 41,475 billion cubic feet (41.5 trillion) | 13,178–71,883 billion cubic feet |
| Natural gas liquids | 1,345 million barrels (1.3 billion) | 423–2,353 million barrels |
F95 is the amount with a 95-percent chance of being there; F5, a 5-percent chance. Nearly all of it is in the continuous units:
| Assessment unit | Oil, mean (million barrels) | Gas, mean (billion cubic feet) | NGL, mean (million barrels) |
|---|---|---|---|
| Western Updip Fault Zone Conventional Oil | 4 | 1 | 0 |
| Eastern Updip Fault Zone Conventional Oil | 8 | 1 | 0 |
| Updip Salt Basins Conventional Oil and Gas | 10 | 9 in oil fields, 17 in gas fields | 2 |
| Middip Fault Zone and Salt Conventional Oil | 5 | 17 | 1 |
| Downdip Salt Basins Conventional Gas | — | 1,324 | 9 |
| All conventional | 27 | 1,369 | 12 |
| Western Shelf Continuous Oil | 6,554 | 13,107 | 263 |
| Eastern Shelf Continuous Oil | 286 | 857 | 26 |
| Western Shelf Edge Continuous Gas | — | 26,142 | 1,044 |
| All continuous | 6,840 | 40,106 | 1,333 |
Estimates are fully risked. The Eastern Shelf unit was given a 0.5 chance of holding oil at all, so its F95 is zero; every other unit was rated 1.0.
Sources
- Pitman, J.K., Paxton, S.T., Kinney, S.A., Whidden, K.J., and others (the USGS Austin Chalk and Tokio and Eutaw Formations Assessment Team), 2020 (version 1.1, December 2020), Assessment of undiscovered oil and gas resources in the Upper Cretaceous Austin Chalk and Tokio and Eutaw Formations, U.S. Gulf Coast: U.S. Geological Survey Fact Sheet 2020–3045. https://pubs.usgs.gov/publication/fs20203045
- The maps and the per-unit results come from the fact sheet's PDF, which the web version lacks.
- Rewritten in hubnx's own words.
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