Hydrogeochemical controls on microbial coalbed methane accumulations in the Williston Basin, North Dakota.

摘要

Extensive research has been conducted in numerous coalbed methane (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 has been reported, but there has been no systematic study on gas origin and distribution, or hydrogeochemical controls on gas generation to date. This study aims to determine differences in chemistry between groundwaters with and without the presence of CH4 to better understand factors affecting CBM generation. Results reveal that shallow gas accumulations in WB coalbeds are microbial in origin and formed via CO2 reduction. CBM is associated with Na-HCO3 type groundwater with SO4 concentrations 1 mmole/L due to cation exchange and sulfate reduction, respectively. These groundwaters occur in deeper units of the Fort Union Formation, underlying multiple coalbeds, suggesting that CH4 is present in waters that have reacted extensively with formations containing low-rank (lignite) coals.