Heath Formation Oil, Central Montana and Western North Dakota: the 2016 USGS Assessment

Using a method based on geology, the U.S. Geological Survey estimates that the Heath Formation holds, on average, 884 million barrels of undiscovered oil and 106 billion cubic feet of undiscovered gas that could be recovered with current technology, in the North-Central Montana and Williston Basin Provinces of central Montana and western North Dakota.

The Big Snowy Trough

The assessment covers continuous (unconventional) oil and gas in the Big Snowy Trough, an east–west depression in Meramec time whose northern and southern edges were later sharpened by faulting and erosion. The Heath Formation, part of the Big Snowy Group, is found only within the trough.

The assessment rests on the idea of a total petroleum system: the source rocks, reservoir rocks and traps that belong together. The USGS defined a Heath system and two assessment units within it, each with a similar history of structure, layering and oil and gas charge.

The two continuous assessment units, the Big Snowy Trough and the Cedar Creek Anticline in the North-Central Montana and Williston Basin Provinces. USGS map, on a National Park Service base.

A self-sourced system

The Heath system is confined to the Heath Formation, a varied mix of black shale, limestone, sandstone, anhydrite and coal. Its main source rock is the Cox Ranch shale member in the middle of the formation. The formation sources its own oil: total organic carbon ranges from 0.5 to 26 percent by weight and averages 9 percent. Leaner shales in the upper and lower Heath may add a little.

Some oil appears to have migrated out of the Heath, and much of that has been produced from the Tyler Formation above it — more than 95 million barrels of the Tyler's production came from the Heath. But the main potential lies in the Heath Formation itself. Oil has been produced from the Heath since 1919, when it was found in the Devil's Basin oil field on the Central Montana Uplift.

Two assessment units

The units take in the areas where the Heath is thermally mature and not close to the surface.

• North-Central Montana Heath Continuous Oil: within the Big Snowy Trough of central and eastern Montana, up to 5,255,000 acres. Its mean potential production area was put at 2,231,067 acres, with 63.3 percent of wells expected to succeed.
• Williston Heath Continuous Oil: along the east side of the Cedar Creek anticline in the southwest of the Williston Basin. Here the Heath occurs only in a limited area; it is thickest along the anticline, at more than 300 feet. It was eroded away west of the structure in the Early Pennsylvanian and extends only a short way east of it. Its mean potential production area was put at 1,303,200 acres, with a 90 percent probability that the unit holds oil at all.

In both units, a successful well was expected to yield on average about 0.053 million barrels of oil. Drainage areas, success rates and yields were partly based on other U.S. shale-oil plays.

The results

The oil estimate ranges from 154 million barrels (a 95 percent chance of at least that much) to 1,965 million (a 5 percent chance); the gas from 17 to 246 billion cubic feet.

Sources

• Heath Formation Assessment Team (Ronald M. Drake II, Christopher J. Schenk, Timothy R. Klett, Phuong A. Le, Heidi M. Leathers-Miller, Michael E. Brownfield, Thomas M. Finn, Stephanie B. Gaswirth, Kristen R. Marra and Marilyn E. Tennyson), Assessment of Undiscovered Continuous Oil and Gas Resources in the Heath Formation, Central Montana and Western North Dakota, 2016, USGS Fact Sheet 2017–3032, June 2017. https://doi.org/10.3133/fs20173032
• Results: USGS Energy Resources Program, https://energy.usgs.gov