Extensive research has been conducted in numerous CBM basins; however, the Williston Basin (WB) remains largely unexamined due to the absence of CBM production despite large coal reserves. CBM in WB coalbeds have been reported, but there has been no systematic study of the gas origin and distribution, or hydrogeochemical controls on gas generation to date. This study aims to determine differences in water chemistry between groundwaters with and without the presence of CH4 to gain better understanding on the potential environmental factors affecting microbial methanogenesis. Gas composition and isotope results reveal that shallow gas accumulations in WB coalbeds are microbial in origin and likely formed via CO2 reduction. CBM is commonly associated with Na-HCO3 type groundwater with SO4 concentrations less than 1 mmole/L. These groundwaters typically occur in deeper units of the Fort Union Formation, underlying multiple coalbeds, suggesting that CH4 is present in waters having relatively long contact times with organic strata.

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