Abstract

The Bighorn Basin of Wyoming and Montana contains several potential Cretaceous and Tertiary petroleum source rocks with the necessary types and amounts of organic matter, and thermal maturity levels for significant oil and gas generation. Potential petroleum source rocks include the Cretaceous Thermopolis and Mowry Shales, Frontier Formation, Cody Shale, and Mesaverde and Meeteetse Formations, and the Tertiary (Paleocene) Fort Union Formation. Vitrinite reflectance trends for these source rocks indicate good potential for petroleum generation, accumulation, and possible over-pressuring in "tight gas" reservoirs or basin-centered accumulations in the deeper parts of the basin.

Burial, thermal, and petroleum generation modeling was used to define the timing of oil and gas formation for the potential source rocks. In the deep trough of the basin the Lower Cretaceous Thermopolis Shale, the oldest unit considered, was buried to nearly 25,000 ft (7,625 m) at maximum burial 10 Ma, and reached temperatures of 327°F (164°C). Peak oil and gas generation in the Thermopolis occurred around 50 Ma. In contrast, at maximum burial 10 Ma, the Tertiary (Paleocene) Fort Union Formation, the youngest unit considered, was buried to around 17,000 ft (5,185 m), reached temperatures of 240°F (116°C), and peak hydrocarbon generation occurred around 10 Ma. Around the less thermally mature periphery of the basin, several of the source rocks have not achieved the proper thermal requirements for thermogenic gas generation but do have the potential for oil and biogenic gas generation.

Based on thermal maturity values and geothermal gradients, the Bighorn Basin is relatively "cool" compared to other Rocky Mountain foreland basins. Even in the deepest part of the basin, at reconstructed burial depths in excess of 20,000 ft (6,100 m), Cretaceous source rocks are still within the window for liquid hydrocarbon generation. Present-day geothermal gradients for the Bighorn Basin range from <1.0°F/100 ft (18.2°C/km) to >1.8°F/100ft (33°C/km). Relationships between vitrinite reflectance, depth of burial, and structure, indicate that geothermal gradients have remained fairly constant since the beginning of the Cretaceous.