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The Bakken Formation's black shales, which also occur in the Williston Basin and, as the Exshaw Formation, in Alberta, lie beneath northwest Montana too. The U.S. Geological Survey's 2022 assessment estimates that they still hold, undiscovered but technically recoverable, a mean of 220 million barrels of oil and 1,030 billion cubic feet of gas — about 1 trillion cubic feet.
The estimate
| Resource | Mean | F95–F5 range |
|---|---|---|
| Oil | 220 million barrels (0.2 billion) | 62–431 million barrels |
| Gas | 1,030 billion cubic feet (about 1 trillion) | 274–2,056 billion cubic feet |
| Natural gas liquids | 24 million barrels (0.02 billion) | 7–48 million barrels |
F95 means a 95 percent chance of at least that much; F5, a 5 percent chance. These are continuous resources — oil and gas held in the source rock itself rather than pooled in a conventional trap — and fully risked. All of the oil, 250 billion cubic feet of the gas and 21 million barrels of the liquids are in the oil assessment unit; the gas unit holds the other 780 billion cubic feet of gas and 3 million barrels of liquids.
How the rocks formed
| When | What happened |
|---|---|
| Late Devonian – Early Mississippian | east-dipping subduction (the Antler orogeny) squeezed the crust into a north–south foredeep, slope and platform — the Antler foreland in the United States, the Prophet trough in Canada. A carbonate ramp formed to the east; organic-rich mudstones of the Bakken and Exshaw Formations settled to the west, along the slope and foredeep |
| Jurassic – Paleogene | renewed subduction drove the eastward thrusts of the Sevier fold and thrust belt |
| Laramide orogeny | thrust sheets of the Montana thrust belt, including the several-kilometer-thick Lewis thrust sheet, stacked up and buried the Bakken deep enough to generate oil and gas |
The formation has three informal units:
| Unit | Organic carbon (TOC) | Hydrogen index | Thickness |
|---|---|---|---|
| Upper black shale | up to 14 weight percent | up to 1,060 mg HC/g TOC | up to 7 m |
| Middle siliciclastic unit | — | — | up to 13 m |
| Lower black shale | up to 20 weight percent | up to 906 mg HC/g TOC | up to 20 m |
Where the oil went — and what may remain
Once generated, oil and gas migrated out into conventional reservoirs: fractured carbonate and karst reservoirs of the Mississippian Madison Group and other Paleozoic carbonates, and Mesozoic clastic rocks in structural and stratigraphic traps, as at Cutbank Field. The assessment's model is that part of it stayed behind in the Bakken. Shows of oil in production tests show that movable oil does exist in part of the oil unit. Toward the thrust belt in the west, though, fracturing may become more pervasive, and without a good top seal it may have let oil and gas escape.
The two continuous assessment units in the Bakken Formation of northwest Montana. Credit: USGS.
The two assessment units
Both run from the Canadian border south to the Scapegoat-Ballantyne structural trend; south of it the same rock is called the Sappington Formation. Their east–west limits follow vitrinite reflectance, a measure of how far the rock has been heated:
| Continuous oil unit | Continuous gas unit | |
|---|---|---|
| East boundary | 0.5 percent vitrinite reflectance | 1.3 percent vitrinite reflectance |
| West boundary | 1.3 percent vitrinite reflectance | footwall of the frontal thrust of the Montana Thrust Belt |
| Potential production area (mean) | 834,833 acres | 815,333 acres |
| Average drainage area of a well (mode) | 280 acres | 160 acres |
| Success ratio (mode) | 60 percent | 30 percent |
| Untested area (mode) | 99.8 percent | 100 percent |
Sources
Based on "Assessment of continuous oil and gas resources in the Bakken Formation of northwest Montana, 2022," USGS Fact Sheet 2023–3016 by Christopher J. Schenk and the Northwest Montana Bakken Assessment Team (June 7, 2023), a work of the United States government in the public domain. Input data are in Schenk (2023), a USGS data release.
Licencia: CC0 1.0 (dominio público) · Adaptado de pubs.usgs.gov
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