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Source rock potential of middle Cretaceous rocks in southwestern Montana

January 1, 1996

The middle Cretaceous in southwestern Montana is composed of a marine and nonmarine succession of predominantly clastic rocks that were deposited along the western margin of the Western Interior Seaway. In places, middle Cretaceous rocks contain appreciable total organic carbon (TOC), such as 5.59% for the Mowry Shale and 8.11% for the Frontier Formation in the Madison Range. Most samples, however, exhibit less than 1.0% TOC. The genetic or hydrocarbon potential (S1+S2) of all the samples analyzed, except one, yield less than 1 mg HC/g rock, strongly indicating poor potential for generating commercial amounts of hydrocarbons. Out of 51 samples analyzed, only one (a Thermopolis Shale sample from the Snowcrest Range) showed a moderate petroleum potenti l of 3.1 mg HC/g rock. Most of the middle Cretaceous samples are thermally immature to marginally mature, with vitrinite reflectance ranging from about 0.4 to 0.6% R0. Maturity is high in the Pioneer Mountains, where vitrinite reflectance averages 3.4% R0, and at Big Sky, Montana, where vitrinite reflectance averages 2.5% R0. At both localities, high R0 values are due to local heat sources, such as the Pioneer batholith in the Pioneer Mountains.

Citation Information

Publication Year 1996
Title Source rock potential of middle Cretaceous rocks in southwestern Montana
DOI 10.1306/64ED8CD2-1724-11D7-8645000102C1865D
Authors T. S. Dyman, J. G. Palacas, R.G. Tysdal, W. J. Perry, M. J. Pawlewicz
Publication Type Article
Publication Subtype Journal Article
Series Title American Association of Petroleum Geologists Bulletin
Index ID 70018413
Record Source USGS Publications Warehouse