2007 GSA Denver Annual Meeting (28–31 October 2007)

Paper No. 2
Presentation Time: 1:45 PM

MOLECULAR, MICROBIAL AND GEOCHEMICAL SIGNATURES OF HYDROCARBON BIODEGRADATION AND METHANOGENESIS IN COALS OF THE POWDER RIVER BASIN


PETSCH, Steven T., Department of Geosciences, University of Massachusetts-Amherst, Morrill Science Building, Amherst, MA 01003, FORMOLO, Michael J., Department of Geosciences, University of Massachusetts-Amherst, Amherst, MA 01003, MARTINI, Anna M., Department of Geology, Amherst College, Amherst, MA 01002 and NÜSSLEIN, Klaus, Department of Microbiology, Univ of Massachusetts-Amherst, Amherst, MA 01003, spetsch@geo.umass.edu

The Powder River Basin of eastern Wyoming and SE Montana is estimated to contain over 14 trillion cubic feet of natural gas that has been generated and stored in extensive coals deposited in the early Cenozoic. Along the eastern margin of the basin, incursion of meteoric water into the subsurface is associated with methane that is classified as microbial in origin based on isotopic characteristics of gas and co-produced waters, including a linear relationship between hydrogen isotope ratios of formation waters and co-produced CH4. Geochemical characteristics of formation waters and gases, such as elevated alkalinity (up to 22 meq L-1) and 13C-enriched DIC (up to +20 ‰), further support extensive microbial degradation of coal organic matter associated with methanogenesis. A limited number of wells associated with poor CH4 production exhibit measurable sulfate concentrations, and 13C-depleted DIC (as low as -20 ‰). Extractable organic matter isolated from coals across the basin points to more extensive biodegradation in shallow coals compared with material retrieved from deeper in the basin. Patterns of hydrocarbon biodegradation are broadly similar to anaerobic petroleum biodegradation and to methanogenic environments in Paleozoic black shales of the mid-continent of the USA, including extensive loss of straight chain and acyclic isoprenoid hydrocarbons. However, unlike typical petroleum biodegradation, concurrent biodegradation of aromatic hydrocarbons such as methylated naphthalenes and phenanthrenes is also indicated. Viable cells and nucleic acids recovered from water-producing wells suggest an active subsurface community within shallow Powder River Basin coals, with cell counts ranging between 103-104 cells mL-1 formation water. Sequence analysis of Bacterial and Archaeal 16S rRNA genes reveals a diverse assemblage of mainly fermentative bacteria coupled with acetoclastic and CO2-reducing methanogenic Archaea.