Biogeochemistry of the Forest City Basin coalbed methane play

Hydrogeochemical and microbial analyses of co-produced formation waters and gas in the Forest City Basin were coupled to determine the origin of methane in shallow coal seams and make comparisons to commercial shale and coal gas plays in the adjacent Illinois, Michigan and Cherokee basins. Forest City Basin coals contain dilute meteoric waters (Cl-<411 mM; average delta O-18 and delta D values=-6.9 parts per thousand and -45.6 parts per thousand VSMOW, respectively), no detectable SO42-, and high alkalinity concentrations (similar to 10 meq/L) with elevated delta C-13 values (up to 13.1 parts per thousand VPDB). The delta C-13 values of coalbed methane (average=-64.7 parts per thousand VPDB) are approximately 72 parts per thousand depleted relative to the potential dissolved inorganic carbon source, and the delta D values of CH4 (average= -221 parts per thousand VSMOW) are approximately -175 parts per thousand depleted compared with ambient formation waters. Together, these molecular and isotopic signatures of Forest City Basin waters and gas point to a microbial origin for methane. Enrichment cultures of microbial cells inoculated from Forest City Basin coal waters confirm the presence of a microbial community of fermentative bacteria, and both CO2-reducing and acetate-utilizing forms of methanogenic Archaea, similar to what has been observed in the Antrim Shale in the Michigan Basin. (C) 2008 Elsevier B.V. All rights reserved.