INFLUENCE OF GROUNDWATER RECHARGE RATES, NUTRIENT SUPPLY, AND SULFATE REDUCTION ON METHANOGENIC PATHWAYS: POWDER RIVER BASIN, WY

Approximately 20% of natural gas resources in coalbeds and organic-rich shales have been generated by microorganisms, which leads to the intriguing hypothesis that these microbial communities may be stimulated to generate new methane on human timescales. However, to begin to test this hypothesis, we need a better understanding of the dominant controls on methanogenesis. This study investigates the importance of groundwater recharge rates, nutrient supply, and sulfate reduction on methanogenic pathways in the Powder River Basin (PRB) coalbeds.

Previous studies of the PRB, using C and H isotopes of CO₂, CH₄ and associated waters, observed that methane was generated via acetate fermentation in select areas near the basin margins, and via CO₂ reduction at depth. To determine if this geographic distribution of metabolic pathways is a function of groundwater recharge rates, nutrient cycling, and/or the presence of sulfate reduction, we collected co-produced waters and natural gas from 27 coalbed methane wells, along two transects across the PRB. Gas samples were analyzed for molecular composition and compound specific isotopes. Water samples were analyzed for major ion chemistry, nutrients, stable isotopes and age tracers (¹⁴C and ³H).

Initial results show detectable SO₄ (up to 6 mg/L), high δ³⁴S-SO₄ values (>27‰), and low δ¹³C values of dissolved inorganic carbon (DIC; -12.7 to 4.3‰), indicative of sulfate reduction, in areas associated with acetate fermentation. These samples also contain high amount of O₂ (2-15 mole %), which may promote methane oxidation. In contrast, wells with gas isotopic signatures indicative of CO₂ reduction have high δ¹³C-DIC values (>12.3‰), no detectable O₂, and no sulfate. There was no correlation between PO₄, total nitrogen, NH₄, NO₃, or NO₂ concentration and dominant metabolic pathways. Detectable carbon-14 contents (0.39-4.13 pmc) suggest that coal waters throughout the basin were recharged less than 50 k yrs BP. There is no clear trend in ³H or ¹⁴C related to metabolic pathways.

Together, these results suggest that: 1) nutrient supply and groundwater residence times are not controlling factors of methanogenic pathways; 2) isotopic signatures used to distinguish acetate fermentation from CO₂ reduction may have been altered by sulfate reduction and/or methane oxidation.