Coal deposits of the Powder River Basin (PRB) in Wyoming, United States contain significant reserves of natural gas proven to be of biogenic origin. The timing of gas generation in this basin (and many others) is either unknown or questionable. Luca Technologies evaluated the prospect that methanogenesis is ongoing in PRB coals using a combined laboratory and field approach. The anaerobic bioconversion of coal to methane in laboratory experiments was observed in multiple samples indicating that indigenous microorganisms can generate methane from PRB coal. Acetate was found to be a quantitatively important intermediate in methane formation. The detection of microbial metabolites including acetate in coalbed methane formation waters and the ability of PRB methanogenic consortia to convert the detected metabolites to methane is also consistent with in situ methanogenesis. Field tests revealed in situ microbial acetate utilization and significant dilution, dispersion, and migration of injected water and solutes in the coal seam. This is believed to be important for contacting significant coal surface area and important for the measurement of in situ microbial activities. Collectively, this information indicates that methanogenesis is active in the coal seams evaluated in the PRB and that all of the methane present in the coals need not be a remnant of past methanogenesis. (C) 2008 Elsevier B.V. All rights reserved.