First Steps Toward Evaluation of Ultra-Shallow Gas Accumulations in the Northern Great Plains

George W. Shurr¹, Layne D. Schulz², and Richard H. Hammond^3
1 GeoShurr Resources, LLC, Ellsworth, MN
² South Dakota Geological Survey, Vermillion, SD
³ Hammond Wetmore Drilling, Vermillion, SD

Ultra-shallow gas shows and historic production from bedrock and glacial drift in the Northern Great Plains are an untested potential resource. Organic rich Cretaceous shales, siltstone, and chalks subcrop beneath glacial drift on the eastern margins of the Williston and Alberta basins. This regional geologic setting is similar to the northern margin of the Michigan basin where microbial methane is produced from the Devonian Antrim Shale. High total organic carbon (TOC) in the shale constitutes the “pasture” for microbes to “graze”. Ground water in the fractured shale helps maintain a favorable environment for microbes.

A glacial aquifer on the southeastern margin of the Williston basin provides a “research farm” to investigate an ultra-shallow microbial methane system at field scales. The Dolton Aquifer in southern McCook County, South Dakota, is locally in contact with Cretaceous bedrock that furnishes the feedstock for methanogens. Analyses of TOC and micropaleontology from recently acquired bedrock cores are used to characterize this microbe pasture.

The Dolton Aquifer is penetrated by a grid of closely spaced observation wells and test holes. Gas shows are clustered within an elongate sweetspot where water compositions have high bicarbonate and low sulfate values. Concentrations of headspace gas are higher in the sweetspot than in surrounding areas. Similar patterns are documented using a gas detection meter to directly measure methane concentrations in observation wells.

On-going investigations at the Dolton Aquifer “research farm” will provide the information framework needed to carry out evaluation, exploration, and development of ultra-shallow microbial methane systems.