SOURCE-ROCK POTENTIAL OF MIDDLE CRETACEOUS ROCKS IN SOUTHWESTERN MONTANA

The middle Cretaceous in southwestern Montana is composed of a marine and nonmarine succession of predominantly elastic rocks that were depo sited along the western margin of the Western Interior Seaway. In plac es, middle Cretaceous rocks contain appreciable total organic carbon ( TOC), such as 5.59% for the Mowry Shale and 8.11% for the Frontier For mation in the Madison Range. Most samples, however, exhibit less than 1.0% TOC. The genetic or hydrocarbon potential (S-1+S-2) of all the sa mples analyzed, except one, yield less than 1 mg HC/g rock, strongly i ndicating poor potential for generating commercial amounts of hydrocar bons. Out of 51 samples analyzed, only one (a Thermopolis Shale sample from the Snowcrest Range) showed a moderate petroleum potential of 3. 1 mg HC/g rock. Most of the middle Cretaceous samples are thermally im mature to marginally mature, with vitrinite reflectance ranging from a bout 0.4 to 0.6% R(o). Maturity is high in the Pioneer Mountains, wher e vitrinite reflectance averages 3.4% R(o), and at Big Sky, Montana, w here vitrinite reflectance averages 2.5% R(o). At both localities, hig h R(o) values are due to local heat sources, such as the Pioneer batho lith in the Pioneer Mountains.