Methanohalophilus is the Dominant Source of Biogenic Methane in Hydraulically Fractured Shales

Abstract

Hydraulic fracturing provides access to shale oil and gas, while also creating a microbial ecosystem that produces biogenic methane. Recent single-gene investigations from our laboratory and others have revealed a single methanogen type (Methanohalophilus) is conserved in produced fluids from several shale formations, including Antrim, Burket/Geneseo, Marcellus, and Utica-Point Pleasant. Here we reconstructed multiple Methanohalophilus genomes collected from produced fluids after hydraulic fracturing of Marcellus and Utica shales. Each of the recovered methanogen genomes encoded metabolic pathways for utilizing methylamines and methanol, and unlike other subsurface methanogens could not generate methane using hydrogen or acetate. Metabolite analyses of injected fracturing fluids revealed high concentrations of methylamines and methanol that support the growth of methanogens in fractured shales. Furthermore, our work uncovered that these substrates, specifically trimethylamine, could also be synthesized in situ by a network of interconnected microbial metabolisms commonly found across shales. Batch experiments demonstrated that microbially produced substrates could be a source of biogenic methane in the laboratory, producing 6.5 times more methane per day than unamended controls. From produced fluids sampled from multiple Utica and Marcellus wells, we cultivated several strains of this dominant shale methanogen. Laboratory cultivation techniques confirmed that shale-derived Methanohalophilus are halotolerant (growing at or above 80 g/L NaCl) and piezotolerant (>3,000 psi), demonstrating that these organisms are well adapted to growing and persisting in fractured shales. Given the prevalence of Methanohalophilus across hydraulically fractured shale formations, it is plausible that the gas isotope signatures widely used to assess biogenic versus thermogenic methane fail to account for contributions of biogenic methane produced from methyl-C1 compounds in these shales. We are currently focusing on assessing isotopic signature of methane produced from Methanohalophilus grown in the laboratory under shale conditions to quantify the contribution of newly produced biogenic methane. Our results leave open the possibility that, analogous to coal-bed methane, fractured shales could be managed to increase energy recovery and longevity through biostimulation.

AAPG Datapages/Search and Discovery Article #90258 © 2016 AAPG Eastern Section Meeting, Lexington, Kentucky, September 25-27, 2016