The Raton Basin, in Colorado and New Mexico, sits on rocks laid down in the Western Interior Seaway — a sea that at its largest ran more than 3,000 miles from the Arctic Ocean to the Gulf of Mexico. A U.S. Geological Survey assessment estimates that two of those marine shale formations still hold, on average, 1.04 billion barrels of oil, 3.9 trillion cubic feet of gas and 11 million barrels of natural gas liquids that are undiscovered but technically recoverable.
The continuous oil and gas assessment units in (A) the Pierre Shale and (B) the Niobrara Formation, Raton Basin–Sierra Grande Uplift Province. Credit: U.S. Geological Survey.
The rocks
| Formation | What it is | Source rock | Possible reservoir |
|---|---|---|---|
| Niobrara Formation | a lower Fort Hays Limestone Member (limestone and calcareous shale) and an upper Smoky Hill Shale Member (calcareous and sandy shale and limestone) | the Smoky Hill: up to 6 percent organic carbon, hydrogen index up to 586 mg HC/g TOC | carbonate-rich zones |
| Pierre Shale | mostly medium-dark gray marine shale with thin limestone, siltstone and sandstone | the Sharon Springs Member: up to 4 percent organic carbon, hydrogen index up to 387 mg HC/g TOC | more brittle sandy and silty zones |
Natural fractures should improve how well the reservoirs produce. The seaway filled a long north–south basin east of the mountain belt of western North America; the Laramide orogeny, from the latest Cretaceous into the early Eocene, broke it into smaller basins such as the Raton, which filled with thick sediment. That uplift and later burial cooked the organic-rich shales: they are mature for oil around the shallow edges of the basin and for gas in its deeper center.
Four assessment units
The shales are both source and reservoir, part of the Upper Paleozoic–Cenozoic Composite Total Petroleum System. Oil and gas are thought to have formed in oil-prone source rocks and been partly held there after moving into neighboring reservoir layers. The USGS assessed four continuous (unconventional) units:
| Unit | Where | Mean potential production area | Mean well recovery |
|---|---|---|---|
| Raton Niobrara Shale Oil | the oil window | 528,833 acres | 0.132 million barrels |
| Raton Niobrara Shale Gas | where oil has cracked to gas | 583,333 acres | 0.508 billion cubic feet |
| Raton Pierre Shale Oil | the oil window | 686,833 acres | 0.132 million barrels |
| Raton Pierre Shale Gas | where oil has cracked to gas | 494,333 acres | 0.508 billion cubic feet |
All the units are untested; expected recoveries per well are based on an analog, the Sand Wash Basin of southwestern Wyoming.
Sources
Based on "Assessment of Undiscovered Continuous Oil and Gas Resources in Upper Cretaceous Marine Shales of the Raton Basin–Sierra Grande Uplift Province, Colorado and New Mexico," U.S. Geological Survey Fact Sheet 2023–3049, a work of the United States government in the public domain. Input data: Finn (2024).
Licence: CC0 1.0 (public domain) · Adapted from pubs.usgs.gov
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