REAL-TIME METHANE PRODUCTION FROM COAL: IMPLICATIONS FOR A SUSTAINABLE ENERGY RESOURCE IN SUBSURFACE COALBEDS

Methane desorbed from subsurface coalbeds supplies about 9% of the natural gas produced in the contiguous United States. Isotopic analyses of samples from several coalbeds reveal a biogenic origin of the gas, suggesting indigenous microbes produced methane in situ. Despite the potential importance of this process to the supply of domestic energy, our current understanding of the biogeochemical factors influencing microbial methanogenesis in coal is limited. We assessed the ability of indigenous microbial populations to produce methane in freshly-collected samples of Miocene-age lignite obtained from the Yukon Flats Tertiary Composite, and investigated factors influencing the rate and extent of methanogenesis. Samples were incubated in the presence of an inorganic salts medium alone or amended with methanogenic substrates such as hydrogen (4 kPa), formate (20 mM), or acetate (25 mM). Microbial methanogenesis and abiotic desorption were distinguished via replicate incubations that contained bromoethanesulfonic acid (5 mM), an inhibitor of methanogenesis. A relatively shallow (400 m) sample exhibited a rate of 700 nmole CH₄*day^-1*g coal^-1, although methanogenesis was negligible in a deeper (650 m) coal from the same borehole. Methane production in the latter coal could be stimulated (80 nmole CH₄*day^-1*g coal^-1) by inclusion of formate, indicating methanogens were present despite the lack of activity in unamended incubations. Furthermore, a filter-sterilized warm water extract of coal renewed methanogenesis in incubations no longer generating methane, suggesting the cessation of activity was due to exhaustion of suitable substrates. Cell membrane lipids and 16S ribosomal DNA were extracted from the samples and demonstrated the presence of microbes from the Kingdoms Bacteria and Archaea. This information suggests the existence of a complete microbial food chain capable of methanogenic coal degradation. These preliminary findings indicate the potential for real-time methanogenesis in subsurface coalbeds. The availability of suitable substrates is likely the primary factor influencing the rate and extent of methane production.