ORGANIC SUBSTRATES SUPPORTING MICROBIAL METHANOGENESIS IN COAL: LAB AND FIELD STUDIES

Organic substances play a key role in the microbial degradation of macromolecules in coal and other complex geopolymers to methane. The microbial biodegradation pathway involves many steps starting with the breakdown of geopolymers and release of monomeric organic compounds, and ending with the production of simple molecules (e.g., acetate, hydrogen) that support methanogenic Archaea and production of methane. In order to better understand this process and the intermediates involved, we examined organic substances present in: (1) solutions from laboratory biodegradation experiments on coal, (2) formation water from coal beds producing biogenic methane, and (3) formation water from field experiments involving stimulation of microbial methanogenesis.

Our lab experiments on coals of different rank from the USA, Pakistan, and other areas suggest that biodegradation proceeds by: (1) release of alkanes, aromatic hydrocarbons, and long chain fatty acids (LCFA) from geopolymers in coal, (2) biodegradation to intermediate chain fatty acids, (3) further biodegradation to substrates for methanogenesis (acetate and H₂), and (4) production of methane gas. Buildup of acetate in incubation fluids occurs after the accumulation of LCFA. Acetate continues to accumulate until methane production commences (typically 20-30 days) at which point acetate concentrations decline precipitously.

Organic compounds similar to those found in lab incubations of coal have been observed in formation water from coal deposits that produce biogenic methane. For example, LCFA intermediates have been identified in Powder River Basin (USA) formation waters associated with microbial natural gas. Acetate also appears to be an important driver of methane production in areas of the Powder River Basin where biogenic methane production seems to be stimulated by nutrients supplied by recharge water. A field experiment at a USGS test site in eastern Montana demonstrated that yeast extract and nutrients injected into a Powder River Basin coal successfully stimulated microbial methanogenesis. Under these enriched conditions, acetate produced via biodegradation of intermediates also appears to drive microbial methanogenesis. We hypothesize that microbial methanogenesis under enriched conditions is dominated by syntrophic acetate fermentation.