The six assessment units in the lower Paleozoic of the Williston Basin Province. Base map from the National Park Service; USGS.
The estimate
Using a geology-based method, the U.S. Geological Survey estimates that the lower Paleozoic rocks of the Williston Basin Province — in North Dakota, Montana and South Dakota — hold undiscovered, technically recoverable conventional resources with mean totals of:
- 83 million barrels of oil (MMBO), with an F95–F5 range of 44 to 144 MMBO;
- 351 billion cubic feet of gas (BCFG), range 178 to 632 BCFG;
- 45 million barrels of natural gas liquids (MMBNGL), range 21 to 82 MMBNGL.
F95 means a 95-percent chance of at least that amount; F5, a 5-percent chance.
How the basin formed
The basin's present shape began in the Middle Ordovician, when a complex set of Proterozoic faults moved again and made room for the sandstone and shale of the Winnipeg Group. Continued faulting through the Late Ordovician and Silurian laid down shallow-sea carbonates: the Red River, Stony Mountain and Stonewall formations of the Bighorn Group, and the Interlake Formation.
The main source rocks are thought to be organic-rich shales in the Icebox Formation (Winnipeg Group) and the Red River Formation.
Six assessment units
The USGS defined six assessment units (AUs) within two total petroleum systems (TPSs): the Ordovician Winnipeg TPS and the Ordovician Red River TPS.
- Three fairway AUs — Winnipeg, Red River, and Interlake-Stonewall-Stony Mountain — cover the central basin, where oil and gas moved mostly vertically. All production from both systems so far comes from here. The model: oil and gas from the Icebox and Red River shales moved into Cambrian sandstones of the Deadwood and Winnipeg, or up faults into dolomite reservoirs in the Red River, Stony Mountain, Stonewall and Interlake formations.
- Three eastern margin AUs lie east and southeast of the fairway, where oil or gas might have moved sideways out of the same shales and pooled. The evidence: Lantry Field, found in 1970 about 100 miles southeast of Red River fairway production, which produced 140,000 barrels of oil before it was shut in.
Results by assessment unit
Fully risked means. "Oil fields" and "gas fields" are the accumulation types; oil fields also hold gas and liquids.
| Assessment unit | AU probability | Oil in oil fields (MMBO) | Gas in oil fields (BCFG) | Gas in gas fields (BCFG) | NGL in gas fields (MMBNGL) |
|---|---|---|---|---|---|
| Winnipeg Fairway | 1.0 | 3 | 3 | 129 | 10 |
| Winnipeg Eastern Margin | 0.5 | 1 | 1 | 12 | 1 |
| Red River Fairway | 1.0 | 42 | 42 | 64 | 13 |
| Red River Eastern Margin | 0.5 | 8 | 8 | 18 | 4 |
| Interlake-Stonewall-Stony Mountain Fairway | 1.0 | 24 | 26 | 36 | 7 |
| Interlake-Stonewall-Stony Mountain Eastern Margin | 0.5 | 5 | 5 | 7 | 1 |
The Red River Fairway holds the most undiscovered oil; the Winnipeg Fairway, the most gas in gas fields. Across all six units the F50 (median) totals are 74 MMBO, 307 BCFG and 37 MMBNGL.
More results are on the USGS Energy Resources Program website.
Sources
Based on Assessment of Undiscovered Conventional Oil and Gas Resources in the Lower Paleozoic of the Williston Basin Province, 2020, USGS Fact Sheet 2020–3063 (April 2021), by the Williston Basin Province Assessment Team, U.S. Geological Survey; a work of the United States government in the public domain. The table is taken from the fact sheet's Table 2.
Licence: CC0 1.0 (public domain) · Adapted from pubs.usgs.gov
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