METHANOGENESIS IN THE FORT UNION FORMATION, POWDER RIVER BASIN, WYOMING

Unconventional natural gas resources, such as coalbeds and organic rich shales, are becoming increasingly important for meeting U.S. energy needs.  Often the gas produced in these deposits is a mixture of thermogenic and biogenic methane.  One such site is the Powder River Basin.  The coalbeds of this basin are significant producers of natural gas.  The Fort Union Formation, specifically the Canyon, Anderson and Smith coal zones of northeastern Wyoming and southeastern Montana have been the focus of development.  The methane has been produced at relatively shallow depths along the eastern margin of the Powder River Basin,and has been identified as microbial in origin. 

Water and gas compositional and isotopic data has been determined from 11 wells.  DIC concentrations of samples range from 12.22 meq/kg to 18.31 meq/kg which is higher than groundwater (~4 meq/kg).  δ¹³C_DIC values range from -28.51% VPDB to 20.02% VPDB.  This data can be divided into two groups, those indicative of methanogenesis with δ¹³C_DIC values of 10 meq/kg and greater, and those that suggest sulfate reduction with δ¹³C_DIC values less than -10 meq/kg.  δD_CH4 values, when plotted against δD_H2O follow a linear trend along which CO₂ reduction is the methanogenic pathway. 

Incubations were inoculated under anoxic conditions with ground water from the Canyon, Anderson, and Smith coal zones.  These samples will be analyzed for methane production over time via gas chromatography in order to obtain idealized rates for methanogenesis. 

The research done on this project will help determine formation pathways of methane within the Powder River Basin as well as provide accuracy for future well location selections by emphasizing the specific environmental conditions needed for methanogenesis.