Undiscovered Oil and Gas in North-Central Montana: the 2017 USGS Assessment

A U.S. Geological Survey assessment from 2017, published as Fact Sheet 2017–3092 in February 2018.

Using a method built on the geology of the rocks, the U.S. Geological Survey (USGS) estimates that the North-Central Montana Province holds a mean of 55 million barrels of undiscovered, technically recoverable oil and 846 billion cubic feet of gas, in both conventional and continuous (unconventional) accumulations.

Figure 1. The six assessment units in the North-Central Montana Province. Base map from the National Park Service. USGS.

Eight episodes that built the province

The province's geology is the product of a series of tectonic events along the western edge of North America:

1. Neoproterozoic rifts and a regional system of lineaments.
2. A passive margin in the early and middle part of the lower Paleozoic (Cambrian–Silurian).
3. The east–west central Montana trough, forming at the same time as the Williston Basin.
4. Subduction and compression in the Late Devonian through Mississippian, during the Antler orogeny.
5. Pennsylvanian uplift and erosion linked to the Ouachita event.
6. High-angle subduction and compression from the Jurassic to the Early Cretaceous, during the Sevier event.
7. Nearly flat subduction and compression from the Late Cretaceous into the early Tertiary, during the Laramide orogeny.
8. Uplift, erosion and the formation of structural traps as intrusive rocks were emplaced in the Eocene.

These events shaped the province's structure and rock layers, and the petroleum systems within them.

Four petroleum systems, six assessment units

Similar Ordovician organic-rich shales in the Williston Basin have total organic carbon up to 6 weight percent and hydrogen index values up to 800 milligrams of hydrocarbon per gram of total organic carbon. The Heath's continuous oil was assessed separately, in an earlier study.

The undiscovered oil and gas of the Ordovician and Devonian–Mississippian systems is concentrated in the central Montana trough, which appears to have been connected to the Williston Basin from the Ordovician through the Mississippian.

Figure 2. The rock layers of the province, coloured by the petroleum system each belongs to. Modified from Dyman and others (1995). USGS.

For the one continuous unit, the coalbed gas, well drainage areas, success ratios and estimated ultimate recoveries came from coalbed-gas plays in the United States.

Results

The estimated means are:

• oil: 55 million barrels, with an F95–F5 range from 0 to 139 million barrels;
• gas: 846 billion cubic feet, from 145 to 2,144 billion cubic feet;
• natural gas liquids: 6 million barrels, from 0 to 15 million barrels.

Biogenic gas makes up about 90 percent of the estimated mean undiscovered gas: 171 billion cubic feet in the conventional biogenic gas unit (range 35 to 459) and 585 billion cubic feet in the coalbed gas unit (range 110 to 1,454).

F95 means a 95 percent chance of at least the amount shown, and the other columns are read the same way. The results are fully risked.

Sources

• Schenk, C.J., Mercier, T.J., Brownfield, M.E., Tennyson, M.E., Woodall, C.A., Le, P.A., Klett, T.R., Gaswirth, S.B., Finn, T.M., Pitman, J.K., Marra, K.R., and Leathers-Miller, H.M., Assessment of Undiscovered Oil and Gas Resources in the North-Central Montana Province, 2017, U.S. Geological Survey Fact Sheet 2017–3092, February 2018. https://doi.org/10.3133/fs20173092 — the map, the stratigraphic column and the results table recovered from the PDF, which the imported page had lost.
• The fact sheet draws on Anna (2010), Anna and others (2011), Aram (1993), Baker and Johnson (2000), Drake and others (2017), Dyman and others (1995), Fishman and others (2012), Jarvie (2001), Jarvie and others (2016), Maughan (1989), Nesheim (2017), Payenberg and others (2003) and Petersen (1986).
• Rewritten in hubnx's own words.