Hydrocarbon Generation in Heath Formation (Mississippian) in Montana Thrust Belt in Response to Tectonic Burial

Mark R. Longden, Bill R. Banowsky, Lee A. Woodward

The Heath Formation of the Big Snowy Group (Mississippian) in Montana and North Dakota consists of dark-gray to black, petroliferous shale and limestone with up to 19.9% total organic carbon (TOC) and 20 gal of oil per ton. The Heath is considered a source rock for the overlying Tyler Formation as well as a possible oil-shale deposit. Regionally, the Heath is immature to marginally mature with respect to hydrocarbon generation, except in the deeper part of the Williston basin, in a few deeply buried areas in the central Montana trough, and in the Montana thrust belt.

Along the northern margin of the Helena salient of the Montana thrust belt, samples from the Heath contain 0.20-3.59 wt. % TOC, with most containing over 0.88%. Vitrinite reflectance ranges from 3.77 to 4.72% and the thermal alteration index is 4+ to 5, indicating that maturation proceeded beyond oil, wet gas, and dry gas preservation limits. Thus, the Heath Formation here passed through peak hydrocarbon generation zones and hydrocarbons may have been thermally destroyed.

Modeling of the depositional and thermal history prior to thrusting, using the Lopatin method, indicates maturation slightly below the oil generation window. This suggests that hydrocarbon generation occurred after thrusting, probably in response to tectonic burial beneath thrust sheets. This conclusion is also supported by vitrinite reflectance values from the Heath in the thrust belt compared with reflectance of 0.48-0.67% east of the thrust belt in central Montana.

AAPG Search and Discovery Article #91033©1988 AAPG Rocky Mountain Section, Bismarck, North Dakota, 21-24 August 1988