Native H 2 Exploration in the Western Pyrenean Foothills

Native hydrogen (H 2 ) may represent a new carbon free‐energy resource, but to date there is no specific exploration guide to target H 2 ‐fertile geological settings. Here, we present the first soil gas survey specifically designed to explore H 2 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 test site for our strategy. The presence of mantle rocks at shallow depth (<10 km depth) under the Mauléon Basin connected to the surface by major faults is considered as preliminary requisites for H 2 generation and drainage. On this basis, more than 1,100 in situ soil gas analysis (H 2 , CO, CO 2 , CH 4 , H 2 S, and 222 Rn) were performed at ∼1 m depth at the regional scale along a 10 × 10 km grid spanning over 7,500 km 2 . The analysis campaign reveals several areas of high occurrence to the north of the Mauléon Basin where H 2 , CO 2 , and 222 Rn concentrations exceed 1,000 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 from the literature of fluid migration at depth, suggest that H 2 may be sourced from mantle rocks serpentinization and carried to the surface along major thrusting faults. Traps containing hydrogen remain unidentified up to now but the presence of salt‐related structures (diapirs) near these hot spots is considered encouraging.