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U.S. Geological Survey, Denver, CO

Abstract: Thermal Maturity of Cretaceous and Tertiary strata, Crazy Mountains Basin, South-Central Montana

As part of a DOE-funded study to characterize the potential for a basin-centered gas accumulation in the Crazy Mountains Basin, southcentral Montana, vitrinite reflectance (Ro) was used to determine levels of thermal maturity of Cretaceous and Tertiary strata. The Crazy Mountains Basin of south-central Montana is one of the deepest basins in the Rocky Mountain Region with a maximum of about 20,000 ft of Cretaceous and Paleocene age rocks preserved along the deep basin trough. Little deep drilling has occurred in the basin, in part due to uncertainties related to Paleocene-age stocks and dike swarms of intermediate composition which intrude the deep basin trough.

Vitrinite reflectance analyses on twenty-four surface samples around the margins of the basin range from 0.35 to 0.63 percent, indicating relatively low levels of thermal maturity. These values are typical of most Rocky Mountain basins. Interestingly, however, two samples collected seven and sixteen miles southeast of the Big Timber stock, the largest stock in the basin, indicate elevated levels of thermal maturity having Ro values of 1.29 and 1.35 percent respectively. Four subsurface samples from a well fifteen miles southeast of the stock indicate high levels of thermal maturity reaching an Ro of 2.59 percent at a depth of 6,025 ft.

None of the samples collected were directly adjacent to mapped intrusions suggesting that regional heating, rather than contact metamorphism is responsible for the elevated maturity levels. These preliminary results suggest that much of the deep trough of the Crazy Mountains Basin may have obtained high levels of thermal maturity (favorable for gas generation to possibly exceeding the gas window) at relatively shallow depths.

AAPG Search and Discovery Article #90919©1999 AAPG Rocky Mountain Section Meeting, Bozeman, Montana