Native H2 Exploration in the Western Pyrenean Foothills. (20th August 2021)

• Record Type: Journal Article
• Authors: Lefeuvre, Nicolas; Truche, Laurent; Donzé, Frédéric‐Victor; Ducoux, Maxime; Barré, Guillaume; Fakoury, Rose‐Adeline; Calassou, Sylvain; Gaucher, Eric C.
• Is Part Of: Geochemistry, geophysics, geosystems. Volume 22:Number 8(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-08-20
• Subjects: natural hydrogen -- radiolysis -- targeting -- soil gas -- serpentinization -- gas migration; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 550.5
• DOI: 10.1029/2021GC009917
• Languages: English
• ISSNs: 1525-2027

Abstract

Abstract: 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 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 H2 generation and drainage. On this basis, more than 1, 100 in situ soil gas analysis (H2, CO, CO2, CH4, H2 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 H2, CO2, 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 H2 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. Plain Language Summary: Native hydrogen (H2 ) could be considered as a possible energy resource for the development of a carbon‐free society. Throughout the world, and for over a century, numerous natural H2 ‐bearing geological fluids have been discovered, but to date, there is neither exploration strategy nor any resource assessment, as practical guidelines for hydrogen targeting are still missing. Here, we propose a new integrated approach dedicated to native H2 exploration, using the Pyrenean orogenic belt and its northern foreland basins as a test site. On this basis, a soil gas (H2, CO2, CH4, radon) exploration campaign, encompassing the major tectonic structures identified in the region has been carried out. This survey reveals several hotspots where H2, CO2, and radon concentrations are by two orders of magnitude above the regional background. These hotspots are mainly located along major faults deeply rooted in the mantle body (∼10 km depth) that is well imaged by geophysical data. Therefore, the combined presence of soil gases significantly enriched in H2, CO2, and radon, a dense mantle body below the foreland basin potentially subject to active hydrothermal alteration, and deep faults, represents a favorable geological setting for H2 generation and drainage. Key Points: A soil gas regional monitoring campaign dedicated to H2 exploration The western Pyrenees as H2 ‐fertile geological settings for production, migration and accumulation A multi‐gas analysis (H2, CO2, CH4, radon) carried out to detect gas migration