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Abstract

The Powder River Basin and San Juan Basin, U.S.A., are two of the most productive coalbed methane reserves in the world. Of particular interest is the microbial biodegradation of coal beds associated with this natural gas production. Biogenic methane production is indicated as a significant component to the total gas resources in the San Juan Basin, and as the nearly sole source for the shallow coals of the Powder River Basin. Molecular and isotopic signatures indicate a microbial origin for the gas. Geochemical characteristics of formation waters, such as elevated alkalinity and 13C-enriched dissolved inorganic carbon (DIC), further support extensive microbial degradation of coal organic matter associated with methanogenesis. Extractable organic matter isolated from coals in both basins point to patterns of hydrocarbon biodegradation in coals restricted to specific depths. To some extent, biodegradation patterns are similar to those observed in methanogenic, biodegraded black shales of the mid-continent of the United States. Specifically, both coals and shales exhibit near-quantitative removal of straight-chain and acyclic isoprenoid hydrocarbons. However, loss of aromatic hydrocarbons in the coals proceeds prior to the extensive removal of the saturated hydrocarbons, in contrast to what is conventionally observed in biodegraded petroleum systems or in black shales. In addition, previous thermal maturation histories in both the Fruitland and Fort Union coalbed methane systems have little impact on more recent hydrocarbon biodegradation. Instead, localized hydrologic conditions and subsurface geology likely play important roles in controlling the extents of biodegradation and methanogenesis. These results suggest that biodegradation of hydrocarbons coupled with methanogenesis may develop regardless of organic matter source across a range of inherited thermal maturities.