Scientific Investigations Report 2004–5091
The Crazy Mountains Basin in south-central Montana, a sparsely explored deep basin with a complex depositional and structural history, was investigated for basin-centered gas accumulations. The basin formed largely in response to thrust loading along the Helena salient, an eastward bulge in the Sevier orogenic belt just west of the deep basin trough, in Late Cretaceous (Maastrichtian) and Paleocene time. Drilling is too sparse to detect the presence of a basin-centered gas accumulation, so other Rocky Mountain basins serve as analogs as many contain Cretaceous and Paleocene source rocks similar to those in the Crazy Mountains Basin, and have demonstratable large basin-centered gas accumulations. Based on analogous comparisons, a basin-centered gas accumulation probably began to develop in the Crazy Mountains Basin during the later stages of basin subsidence, near the end of the Paleocene, due to burial heating. Thermal maturities were elevated further by an Eocene igneous event that produced intense heating in the vicinity of stocks and dikes as well as a low-grade, more regional heating across much of the basin. This heating event is similar to one that affected the Raton Basin in Colorado and New Mexico in Oligocene and Miocene time, which elevated thermal maturities but left the coalbed methane and basin-centered gas accumulation largely intact, and appears to have increased gas content in the coal beds. We suggest that a basin-centered gas accumulation is still present in the deeper parts of the Crazy Mountains Basin away from Eocene intrusions.
First posted February 14, 2011
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Johnson, R.C., Finn, T.M., Taylor, D.J., and Nuccio, V.F., 2005, Stratigraphic framework, structure, and thermal maturity of Cretaceous and lower Tertiary rocks in relation to hydrocarbon potential, Crazy Mountains Basin, Montana: U.S. Geological Survey Scientific Investigations Report 2004–5091, 95 p.
Geology of Basin-Centered Gas Accumulations
Regional Geologic Setting of the Crazy Mountains Basin
Cretaceous and Lower Tertiary Strata in the Crazy Mountains Basin
Fall River Sandstone
Telegraph Creek Formation
Claggett Shale Through Teapot Sandstone Member of the Mesaverde Formation and Its Equivalent Interval
Judith River Formation
Hell Creek Formation
Fort Union Formation or Group
Potential Hydrocarbon Source Rocks in the Crazy Mountains Basin
Hydrocarbon Production in the Crazy Mountains Basin
Implication of Present Investigation on the Structural Development of the Crazy Mountains Basin
Defining Basin-Centered Gas Accumulations in Rocky Mountain Basins
Key Deep Wells in the Crazy Mountains Basin