Deciphering Intermittently Bubbling Degassing Mechanisms of He‐Rich N2 ‐Bubbles at theSedimentary Basin‐Basement Interface by Surface Geophysics and Gas Geochemistry

With the growing emphasis on reducing the carbon footprint of transport, there is increasing interest in identifying local sources of hydrogen (H₂) and helium (He) closer to consumers. In this context, we present an integrated approach combining near-surface geophysical imaging, soil gas sampling, and bubbling well gas sampling to investigate fluid and gas pathways near a fault system in the Morvan massif, located in the southeastern Paris Basin. Using electrical resistivity and seismic refraction tomography, we mapped a fault network in the area. Soil gas sampling along these faults revealed a helium hotspot, strongly linked to a specific fault segment, indicating a preferential pathway likely driven by water advection. Additionally, exceptionally high helium concentrations were detected in nitrogen (N₂)-dominated free gas from two nearby bubbling wells, closely associated with the soil helium hotspot. Our geophysical data further suggest the presence of a shallow water reservoir at the basement-sediment interface, containing N₂-He gas bubbles. In contrast, hydrogen (H₂) exhibits a broader spatial distribution, likely due to biological production and consumption processes, as well as soil aeration. A potential geological seep, with diffusion controlled by clay and marls, may also contribute to H₂ dispersion. The distinct spatial patterns observed for He and H₂ reflect their differing transport mechanisms. We propose a simple geochemical model to explain the N₂- and He-rich signature of the bubble gas, attributing it to the exsolution of dissolved atmospheric N₂ during recharge, while radiogenic He originates from the underlying granitic basement.