New exploration methods for natural hydrogen (H₂): a study case of the western Pyrenean foothills

The existence of H₂-rich geological fluids rich rise the question about the energy potential of this carbon-free resource. For more than a century and throughout the world, numerous natural H₂ seepages have been discovered. However, to date there is no exploration strategy based on robust methodologies and pathfinders. The detection of H₂ seepages at the surface is probably the most efficient and straight forward approach to identify H₂ flows. However, a gas flow does not necessarily constitute a resource, since to date, mankind only exploit reservoirs of fossil energy resources. Therefore, it is important to develop an exploration guide that is not only focused on surface gas monitoring, but also considered a deep geological settings integrating the entire hydrogen system from source to trap or leakage into the atmosphere. Here, we propose to use the geological framework of the North Pyrenean foothills as a case study to develop this exploration guide. A literature review of the area revealed a promising geological setting for a H₂ system due to strong links between putative deep H₂ sources, crustal-scale migration pathways, fluid circulation dynamics, and sedimentary traps. Indeed, the northwestern Pyrenees and particularly the Mauleon Basin is characterised by the presence of i) an ultramafic mantle body located less than 10 km depth and under pressure-temperature conditions favorable to serpentinization; ii) major faults such as the North Pyrenean Frontal Thrust (NPFT) constituting large-scale flow convergence and drainage, iii) hydraulic gradients due to the presence of strong reliefs, combined with temperature and pressure gradients that trigger fluids migration, and iv) impermeable sedimentary formations or caprocks such as evaporites or claystones overlying porous reservoir rocks that can constitute traps for accumulating H₂. Following this preliminary geological survey, we set up a soil gas exploration campaign (H₂, CO₂, ²²²Rn, O₂, CH₄) at the regional scale. These field measurements, carried out over more than 7,500 km², revealed several hotspots with very high H₂, CO₂ and ²²²Rn concentration anomalies around the Mauleon Basin. This discovery allowed us to refine the mesh of the prospecting grid to the northern part of the Mauleon Basin around the locality of Sauveterre-de-Bearn. Geochemical and geophysical measurements were carried out at Sauveterre-de-Bearn to determine the source and migration path of the gases at the origin of this anomaly. Based on soil gas analysis and electromagnetic surveys, we confirmed the existence of a fault draining deep fluids. In addition, the study of historical data from drilling carried out in the region more than fifty years ago, combined with the latest geological and geophysical knowledge of the region, enabled us to highlight zones where H₂ could accumulate. Finally, an experimental part of quartz and rocks comminution was carried out in order to explore the reaction mechanisms of H₂ production in active fault zones. We reveal the very strong influence of the water/rock ratio (W/R) and pH on H₂ production. These findings shed new light on the mechano-radical mechanisms of H₂ production where the efficiency of grinding as well as the speciation of mineral surface sites are key parameters controlling H₂ production. We reveal for the first time that quartz grinding in the presence of carbonate solutions induces the formation of carboxylate species (formate, acetate, oxalate). In addition to producing H₂, the mechano-radical mechanisms thus allow the production of reduced carbon species that can constitute an energy source for subsurface lithotrophic microbial ecosystems. (author)