New Measurements from Appalachian and Michigan Sedimentary Basins Imply that Geologic Methane Emissions Are Less Widespread than Previously Assumed.

Methane is an important greenhouse gas with both natural and anthropogenic sources. The current global estimate for the magnitude of natural geologic emissions (e.g., emissions of fossil methane from mud volcanoes, offshore and onshore gas and oil seeps, microseepage, volcanoes, and geothermal areas) is very uncertain, with large disagreement between bottom-up estimates from flux measurements and top-down constraints from ice core 14C measurements. Defining the natural contribution to fossil methane emissions is critical for improved top-down quantification of anthropogenic methane emissions from fossil fuel extraction and use. Previous research suggested that the largest contribution to natural geologic methane emissions is from microseepage, which is the diffuse methane flux from soils over large areas of productive hydrocarbon basins. Our project aims to quantify microseepage in onshore hydrocarbon basins in the United States (U.S.) to improve the bottom-up methane emissions estimates. Results from over 750 measurements taken during both summer and winter seasons across the Appalachian and Michigan Basins suggest that microseepage is highly localized rather than widespread, with null or negative methane fluxes observed in most sampling locations. In the Appalachian Basin, we only observed geologic methane microseepage in proximity to macroseeps (visible oil and/or gas seeps with elevated levels of hydrocarbon emissions), decreasing with increased distance from the seep. Even areas with predictive features for microseepage (e.g., shale or sandstone outcrops, prior measurements of elevated hydrocarbons in soil or groundwater, and recent seismic activity) mostly yield null or negative fluxes. We did not observe any geologic seepage in the Michigan Basin. The widely observed negative fluxes are likely due to the microbial soil methane sink in temperate forest environments common to the Eastern U.S., with strongest negative fluxes observed in the summer season (methanotrophs, methane-consuming bacteria, are inactive at colder temperatures). Measurements were taken in the summer and winter in order to understand whether this microbial sink could be consuming microseepage of methane and preventing emission to the atmosphere.