Undiscovered Oil and Gas of the Montana Thrust Belt, 2021

The six conventional oil and gas assessment units of the Montana Thrust Belt Province. USGS map.

The estimate

Using a geology-based assessment method, the U.S. Geological Survey estimated undiscovered, technically recoverable conventional resources in the Montana Thrust Belt Province, with means of:

F95 means a 95-percent chance of at least the amount shown; F5 is defined the same way.

The geology

The Montana Thrust Belt is a segment of the Cordilleran orogenic belt, which runs from Alaska to Mexico along the western edge of cratonic North America.

• The Belt Supergroup: extension along the Mesoproterozoic margin laid down kilometers of mainly clastic sediments; the Helena salient is interpreted as a failed Mesoproterozoic rift holding kilometers of these rocks.
• Paleozoic seas: the early Paleozoic was dominated by carbonate deposition on a passive margin. The Late Devonian and Mississippian Antler orogeny deepened the Williston and Sappington basins, where organic-rich marine shales of the Bakken Formation and the equivalent Sappington Formation were laid down beside carbonate platforms.
• Thrusting: renewed subduction in the Jurassic drove the eastward thrusts of the Sevier fold and thrust belt, with a broad foreland basin to the east that lasted through the Paleogene. The stacking of thrust sheets — including the several-kilometer-thick Lewis Thrust sheet during the Laramide orogeny — heated shales of the Sappington Formation and the Marias River Shale enough to generate oil and gas.
• Basin and range: late Eocene to Miocene extension formed west-facing normal faults and half grabens, filled with up to five kilometers of sediment, including organic-rich lake shales.

Petroleum systems and assessment units

The USGS defined two total petroleum systems and six conventional assessment units.

• Paleozoic–Mesozoic Composite: four units — Main Thrust Belt, Sawtooth, Foothills and Greater Helena Salient Structures. Oil and gas generated from Sappington (Bakken-equivalent) shales and the Marias River Shale as Laramide thrusting buried them, then migrated into fractured and karst reservoirs of the Madison Group and other Paleozoic carbonates, and into Mesozoic clastic reservoirs, in complexly faulted structural traps. Repeated deformation and fracturing may have let oil and gas escape.
• Oligocene Lacustrine: the Northern and Southern Cenozoic Basins units, defined by the presence and maturity of lake source rocks. Oil and gas from organic-rich lake shales are thought to have migrated into sandstone and conglomerate reservoirs; coal beds may add gas, while volcaniclastic debris may have hurt reservoir quality.

The assessment's inputs are summarized in the fact sheet and in Schenk (2022).

Sources

Based on the USGS Montana Thrust Belt Province Assessment Team (Christopher J. Schenk and colleagues), "Assessment of Undiscovered Conventional Oil and Gas Resources of the Montana Thrust Belt Province, 2021," Fact Sheet 2022–3048, September 2022, U.S. Geological Survey; a work of the United States government in the public domain. The unit-by-unit results and the map are taken from the fact sheet's PDF.