Exploration Strategies for Ultra-Shallow Microbial Methane on the Eastern Margin of the Williston Basin

George W. Shurr¹, Thomas Haggar², and Sarah A. Chadima^2
1 GeoShurr Resources, LLC, Ellsworth, MN
² South Dakota Geological Survey, Vermillion, SD

All of the requirements for generation of microbial methane are present on the eastern margin of the Williston basin in North and South Dakota. Organic rich Cretaceous bedrock, water-filled fractures, and favorable microbial environments have resulted in gas shows and historical production from depths of less than 1000 ft in bedrock and overlying glacial drift. The “pasture” of organic material, the fracture “plumbing”, and the microbe ecology are all critical components of an exploration strategy for late generation, microbial methane on shallow basin margins.

Total organic carbon (TOC) has been analyzed in samples from four separate Cretaceous formations in cores taken near Iroquois and Sisseton, South Dakota. Values of TOC for more than 60 analyses ranged from .18% to 10.08%, with an average of 3.84%. This is the “pasture”.

Corridors of increased fracturing and ground water movement bound regional structural blocks that are rooted in the Precambrian basement. Specific features in the Precambrian basement such as the Great Lakes Tectonic Zone, Trans-Hudson Orogen and Sioux Ridge have expression as block mosaics outlined on the surface by Landsat lineament zones. This is the “plumbing”.

A water quality data base maintained by the state of South Dakota for shallow observation wells, provides information on the environmental conditions that will support methanogenic microbes. Ideally bicarbonate values greater than about 500 ppm in conjunction with sulfate values of less than 500 ppm are found in areas that have gas shows or historic production. Other attributes such as concentration of selenium or arsenic may also provide proxies useful in exploring for microbial methane. Observation wells with favorable water chemistry are further screened using a gas detection meter to directly measure methane concentrations.