T121 Frontiers in Coal Science: From Basic Research to Applied Technology (Posters)

Monday, 19 October 2009
9:00 AM - 6:00 PM
Hall A

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

Paper number: T121-3
Program: Technical Sessions
Time: 9:00 AM – 6:00 PM

Authors

Abstract

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.