Contrasting Biogenic Gas Systems in the Eastern Great Plains of North America

George W. Shurr
GeoShurr Resources, LLC, Ellsworth, MN

Biogenic gas systems in Cretaceous rocks and overlying glacial sediments are distributed throughout the eastern Great Plains from Nebraska northward into the Dakotas and Canada’s southern prairie provinces. Accumulations are demonstrated by anecdotal information, historic production, direct detection in observation water wells, and a few gas tests. These accumulations are an essentially untested resource and there is no commercial production as yet.

There are two separate types of biogenic gas systems in the eastern Great Plains. Early generation biogenic gas systems are similar to extensive production in Cretaceous rocks of Montana and Alberta. Late generation biogenic gas systems have the same fundamental characteristics as commercial accumulations of microbial methane in the Devonian Antrim Shale of Michigan.

Eastern Great Plains biogenic gas systems illustrate the distinctive environments and contrasting histories of early and late generation biogenic gas systems. The food source, metabolic pathways, available living space, aqueous environments, and even the microbial consortia are different in the two systems. Early generation gas forms by microbial activity at the time of host rock deposition. The ancient microbial communities were especially active in shallow marine waters at the margins of depositional basins. Late generation gas forms by microbial activity in the relatively recent geologic past, especially during Pleistocene glaciation. Favorable environments for the methanogens are found in water-filled fractured shales on the margins of structural basins. Although both systems are present in the eastern Great Plains, it is the late generation microbial methane that constitutes the most promising unconventional energy resource.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas