Undiscovered Gas in the Williston Basin: USGS Assessment, 2020

Using a geology-based assessment method, the U.S. Geological Survey (USGS) estimated a mean of 2,438 billion cubic feet (2.4 trillion cubic feet) of undiscovered, technically recoverable gas in the Williston Basin Province of North Dakota, Montana and South Dakota.

What was assessed

The Williston Basin is a major petroleum-producing region. This assessment looked at its potential for gas not associated with oil — both conventional and continuous gas — and natural gas liquids (NGL), in three assessment units (AUs):

• one in the Ordovician Winnipeg Total Petroleum System — the Icebox Shale Gas AU;
• two in the Upper Cretaceous–Paleogene Biogenic Gas Total Petroleum System — a continuous coalbed gas AU and a conventional biogenic gas AU.

Figure 1. The three gas assessment units in the Williston Basin Province; adjacent lines show a shared boundary at the outermost line. Base map from the U.S. Department of the Interior, National Park Service. Credit: U.S. Geological Survey.

The geology

Shale gas in the Icebox Formation. Organic-rich shales in the Icebox Formation of the Ordovician Winnipeg Group are thermally mature enough, and have suitable properties, to hold a shale-gas accumulation. The key is the organic richness of the informally named Government Creek shale, the lower part of the formation: it contains Type I and Type II organic matter, total organic carbon up to 4.3 weight percent, and calculated hydrogen index values of up to 600 milligrams of hydrocarbon per gram of organic carbon. The Icebox Formation can be more than 120 feet thick, though the organic-rich part is much thinner.

Shallow biogenic gas. The basin's shallow strata include Upper Cretaceous and Cenozoic coals, lignites and carbonaceous shales. Bacteria can consume their organic matter and produce biogenic gas, a process common across the northern Great Plains of the United States and Canada.

Table 1. Key input data for the three assessment units. AU, assessment unit; EUR, estimated ultimate recovery; BCFG, billion cubic feet of gas. Credit: U.S. Geological Survey.

Results

The fully risked estimates (F95 is a 95-percent chance of at least the amount shown; the other fractiles likewise):

BCFG, billion cubic feet of gas; MMBNGL, million barrels of natural gas liquids. All three units had an assessment-unit probability of 1.

Table 2. Results for the three assessment units. Credit: U.S. Geological Survey.

Sources

• Schenk, C.J., Mercier, T.J., Woodall, C.A., Ellis, G.S., Finn, T.M., Le, P.A., Marra, K.R., Leathers-Miller, H.M., and Drake, R.M., II, 2021, Assessment of undiscovered gas resources in the Williston Basin Province, 2020: U.S. Geological Survey Fact Sheet 2021–3024. https://doi.org/10.3133/fs20213024
• The fact sheet draws on Nesheim and Nordeng (2013), Ulishney and others (2005), Shurr (1998), Shurr and Ridgley (2002), Rice and Claypool (1981), Payenberg and others (2003) and Anna (2010).
• The map and tables are taken from the fact sheet's PDF. Table 1 gives the coalbed gas unit's maximum average EUR as 0 — below its mode of 0.08 — apparently a misprint.
• Rewritten in hubnx's own words.