ASSESSMENT RESULTS OF UNDISCOVERED OIL AND GAS RESOURCES OF THE HEATH FORMATION, CENTRAL MONTANA AND WESTERN NORTH DAKOTA

The U.S. Geological Survey completed a geology-based assessment of undiscovered, technically recoverable petroleum resources in the Heath Formation in central Montana and western North Dakota. The Upper Mississippian Heath Formation is a self-sourced, heterogeneous mix of grey to black mudstone, dolomite, limestone, anhydrite, and coal. The primary source rock, informally called the Cox Ranch oil shale bed, lies within the middle section of the Heath Formation. In this study, one total petroleum system and two assessment units (AUs) were defined, these are the: (1) North-Central Montana Heath Continuous Oil AU; and (2) the Williston Heath Continuous Oil AU.

The Heath Formation produces both oil and gas within the North-Central Montana Heath Continuous Oil AU. The AU boundary is, in part, defined by the extent, depth, and thermal maturity of the source rocks within the Heath Formation, which is confined to the Big Snowy trough of central Montana and the western portion of the Williston Basin. Within the North-Central Montana Heath Continuous Oil AU, the estimated mean total volume of undiscovered oil is 637 million barrels of oil (MMBO), with a fractile (F95–F5) range from 154 to 1,371 MMBO respectively. The mean assessed volume of gas is 76 billion cubic feet of gas (BCFG), with a fractile range from 17 to 172 BCFG.

The Williston Heath Continuous Oil AU is located along the east side of the Cedar Creek anticline in the southwest portion of the Williston Basin. The Heath Formation is present only in a limited area and is thickest along the Cedar Creek anticline. The mean assessed volume of technically recoverable undiscovered oil is 247 MMBO with a fractile (F95–F5) range from 0 to 594 MMBO respectively. The mean assessed volume of gas is 30 BCFG, with a fractile range from 0 to 74 BCFG. Overall, in this assessment, there are 2 continuous (unconventional) AUs with estimated mean total volumes of 884 MMBO and 106 BCFG in central Montana and western North Dakota.