Anaerobic oxidation of methane from abandoned oil and gas wells leaking into aquifers

As oil and gas wells in sedimentary basins worldwide are increasingly abandoned, the legacy wells can remain as conduits for deeper fluids to reach shallow aquifers. To investigate impacts of mixing between the deep and shallow subsurface, we characterized the geochemistry of artesian flows and groundwater discharges associated with abandoned wellbores in the oldest commercially developed oil and gas basin (Appalachian Basin, U.S.A.). Across 18 sites, we observed evidence that basin brines and gases migrate upwards from oil- and gas-producing formations and mix with meteoric waters at shallow depths. All the observed groundwater chemistries were methane-rich and reducing, but some sites were also characterized by high concentrations of dissolved metals, which we hypothesized to derive from the anaerobic oxidation of methane near leaking wellbores. To explore this, we characterized microbial assemblages in groundwaters, conducted laboratory incubation experiments, and developed a reactive transport model integrating our results. We detected evidence for both brine contamination and anaerobic methanotrophic archaea in field samples. Our observations of these archaea in field samples, alongside observations from laboratory incubations of the consortia, provide evidence for active anaerobic oxidation of methane (AOM) in aquifers affected by gas and oil extraction. We also discovered that dissolved gas compositions and the microbial community assemblages are consistent with both methanogenesis and methanotrophy affecting fluids that migrate via abandoned wellbores. We observed complex patterns in the carbon isotopes of hydrocarbons, including divergent trends in the isotopic composition of methane and ethane. These patterns may derive from non-traditional isotopic effects of AOM at low energetic yields in freshwater environments, particularly where aquifer geochemistry or long fluid residence times produce low concentrations of electron acceptors. Simulating AOM coupled to iron- or sulfate-reduction within methane migration pathways using a reactive transport model, we show that metal-rich and metal-poor waters can be easily explained by a chemical divide driven by availability of electron acceptors (iron versus sulfur respectively) in the presence of AOM. Our results emphasize that legacy oil and gas operations not only leak natural gases that contaminate the atmosphere, but also release salts and reducing equivalents into groundwater that deleteriously affect water quality.