Facies, stratigraphic architecture, and faults - The controls on the cement distribution in the Devonian Sappington Formation in southwestern Montana

Assessment of undiscovered conventional oil and gas resources of the Montana Thrust Belt Province, 2021

“…The Paleozoic-Mesozoic Composite TPS was defined to include the Main Thrust Belt Structures Conventional Oil and Gas AU, the Sawtooth Structures Conventional Oil and Gas AU, the Foothills Structures Conventional Oil and Gas AU, and the Greater Helena Salient Structures Conventional Oil and Gas AU. The geologic model for these four AUs is similar; oil and gas were generated from shales of the Sappington Formation (Bakken Formation equilavent) and the Cenomanian-Turonian Marias River Shale as these rocks entered the thermal-generative windows for oil and gas during Laramide thrust loading (Clayton and others, 1982;Dolson and others, 1993;Di Pasquo and others, 2019;Browne and others, 2020;Schultz and Hofmann, 2021). Generated and expelled oil and gas migrated into fractured carbonate and karst reservoirs of the Madison Group and other Paleozoic carbonates and Mesozoic clastic reservoirs within complexely faulted and fractured structural traps.…”

“…The lower Paleozoic was dominated by passive margin carbonate deposition. Subduction during the Late Devonian and Mississippian Antler orogeny accentuated the existing structure of the Williston and Sappington basins (Schultz and Hofmann, 2021), within which organicrich marine shales of the Bakken Formation and the stratigraphically equivalent Sappington Formation were deposited coeval with adjacent carbonate platforms. Renewed subduction during the Jurassic led to the progressive eastward-directed thrusts of the Sevier fold and thrust belt that formed an extensive foreland basin to the east, which persisted through the Paleogene.…”