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

With the increasing importance of the carbon footprint of transport, new sources, closer to consumers, of $H_2$ 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 $N_2$-dominated free gas from two bubbling wells very close to the soil He hot-spot. Evidence for a water reservoir with high $N_2$-He gas bubbles in the very shallow basement-sediment cover interface is obtained through our geophysical data. In contrast, $H_2$ 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 $H_2$ results from these different transport processes. A simple geochemical model is proposed to explain the geochemical signature of bubble gas, rich in $N_2$ and He, through exsolution of air $N_2$ dissolved at recharge, where radiogenic He originates from the granitic basement.