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The Heath Formation (Mississippian) in central Montana is a black calcareous shale containing low to moderate amounts of oil (Fischer assay) and is considered a petroleum source rock for the overlying Tyler Sandstone.
Seven core holes were drilled in the summer of 1982 by the Montana Bureau of Mines and Geology in cooperation with the Mineral Management Service. Thin coal seams from the core samples were studied using vitrinite reflectance analysis. Since vitrinite reflectance is a method of determining thermal maturation of organic material in sediments (in this case, a thin coal seam near the base of the Heath Formation), it was possible to construct an iso-reflectance map of the Heath Shale in this area, and estimate the minimum temperature of heating undergone by the organic constituents.
Reflectance values show a regional trend caused by burial and the geothermal gradient. Little variation is present in these reflectance values (0.49% to 0.55%). The lowest reflectance values are in the central portion of the study area, and increase to the east and west. However, substantially higher vitrinite reflectances were recorded in the far eastern portion of the area. These high reflectances probably are the result of heating by an igneous intrusion, which was cored during drilling.
The sediments heated by the normal geothermal gradient have immature vitrinite which is below the limits of the petroleum generation window. In the small area where the intrusive was discovered, the vitrinite is mature and there is a good possibility of oil generation.
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