Geophysical and Biogeochemical Indicators for Detecting Coal-Bed Methane Seepage in the San Juan Basin

Mills, Christopher T.¹, Kevin W. Mandernack¹, Robert F. Dias², Gregory F. Slater³, Alena L. Leeds⁴, Chris M. Reddy^5
1 Colorado School of Mines, Golden, CO
² Old Dominion University, Norfolk
³ McMaster University, Hamilton
⁴ United States Geological Survey, Golden, CO
⁵ Woods Hole Oceanographic Institution, Woods Hole

We are investigating the potential of using near-surface biogeochemical and geophysical measurements for detecting methane seeps. At our study site in the San Juan Basin, soil gas profiles show steep vertical and horizontal gradients of methane and oxygen. These profiles suggest that oxygen is being actively consumed by microbially-mediated methane oxidation. Furthermore, magnetic anomalies measured at the site correlated well with areas of high methane and low oxygen (>15% CH4 and <1% O₂ at 1-meter depth). We are examining the association between methane seepage and these anomalies using analyses of bacterial phospholipid-derived fatty acid (PLFA) biomarkers and their delta¹³C and ¹⁴C values. ¹³C and ¹⁴C depleted (-65 ‰ and 6 pMC, respectively) PLFA biomarkers for methane-oxidizing bacteria (methanotrophs) were detected in these soils, further suggesting microbial consumption of methane (delta¹³CCH₄ = -42 ‰ and ¹⁴CH₄ = 7 pMC). Soil incubation experiment results also indicate the potential for microbial sulfate reduction in these soils. Others have hypothesized this anaerobic process, likely enhanced due to the depletion of O₂ by methane oxidation, as the origin of magnetic anomalies associated with other hydrocarbon seeps. The anomalies likely result from mineralogy changes due to the reduction of iron oxides by sulfide. Because soil surface methane fluxes may be absent due to complete consumption of methane by near-surface methanotrophs, detection of both magnetic anomalies and bacterial PLFA biomarkers may serve as important prospective tools for the discovery of coal-bed methane.