Deciphering Degassing Mechanisms of He and H2 ${\mathbf{H}}_{\mathbf{2}}$ at the Sedimentary Basin‐Basement Interface by Surface Geophysics and Gas Geochemistry

Abstract

International audienceWith the increasing importance of the carbon footprint of transport, new sources, closer to consumers, of and He are explored. Within this context, we present a combined near‐surface geophysical imaging, soil gas sampling, and bubbling well gas sampling approach to study fluid and gas pathways near a fault system in the Morvan massif, part of the French Massif Central, in the south‐east of the Paris Basin. Electrical resistivity and seismic refraction tomography profiles allowed identifying a fault network. The co‐located soil gas sampling shows a He hot‐spot clearly linked to a section of one fault, suggesting a preferential pathway via water advection. Very high He concentrations, are also measured in ‐dominated free gas from two bubbling wells very close to the soil He hot‐spot. Evidence for a water reservoir with high ‐He gas bubbles in the very shallow basement‐sediment cover interface is obtained through our geophysical data. In contrast, spreads more widely, pointing to biological production and consumption coupled to soil aeration, as well as a possible geological seep with diffusion processes controlled by clay/marls. The very distinct spatial variability observed for He and results from these different transport processes. A simple geochemical model is proposed to explain the geochemical signature of bubble gas, rich in and He, through exsolution of air dissolved at recharge, where radiogenic He originates from the granitic basement