Soil gas analysis, western Pyrenees

Native hydrogen (H2) may represent a new carbon free energy resource, but to date there is no specific exploration guide to target H2-fertile geological settings. Here, we present the first soil gas survey specifically designed to explore H2 migration in a region where no surface seepage has been documented so far. We choose the Pyrenean orogenic belt and its northern foreland basin (Aquitaine, France) as a playground to test our strategy. The presence of a mantle body at shallow depth (<10 km) under the Mauléon Basin connected to the surface by major faults is considered as a preliminary pathfinder for H2 generation and drainage. On this basis, more than 1,100 in situ soil gas analysis (H2, CO, CO2, CH4, H2S, and 222Rn) were performed at ~1 m depth at the regional scale along a 10 × 10 km grid spanning over 7,500 km2 25 . The analysis campaign reveals several hot spots to the north of the Mauléon Basin where H2, CO2 and 222Rn concentrations exceed 1000 ppmv, 10 vol% and 50 kBq m-3 , respectively. Most of these hot spots are located along the North Pyrenean Frontal Thrust and other related faults rooted in the mantle body. These results, together with evidence of fluid migration at depth, suggest that H2 may be sourced from mantle rocks serpentinization and carried to the surface along major thrusting faults. Hydrogen traps remain unidentified up to now but the presence of salt-related structures (diapirs) near these hot spots could play this role.