Nouvelles méthodes d'exploration des ressources en hydrogène naturel (H2) : Le cas d'école du piémont Pyrénéen occidental

The existence of geological fluids rich in hydrogen (H2) 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 approach based on robust methodologies and pathfinders. The detection of H2 seepages at the surface is known to be the most ecient 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 focusing on surface gas monitoring, but also considering a deep geological approach 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 a strong link between putative deep H2 sources, crustal-scale migration pathways, fluid circulation dynamics, and sedimentary traps. Indeed, the northwestern Pyrenees and particularly the Maul´eon Basin is characterised by the presence of i) an ultramafic mantle body located less than 10 km depth and under pressure-temperature conditions favourable to serpentinisation ; 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 ; 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 measurments, carried out over more than 7,500 km2, revealed several hotspots with very high H2, CO2 and 222Rn concentrations anomalies around the Maul´eon Basin. This discovery allowed us to refine the mesh of the prospecting grid to the northern part of the Maul´eon Basin around the locality of Sauveterre-de-Béarn. Geochemical and geophysical measurements were carried out at Sauveterre-de-B´earn 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 comminution of quartz and rocks from the region 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 grinding of quartz 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.L’existence de fluides géologiques riches en hydrogène (H2) doit nécessairement faire l’objet de travaux d’exploration afin de statuer sur le potentiel énergétique de cette éventuelle ressource décarbonée. Depuis plus d’un siècle de nombreuses exhalations naturelles d’H2 ont été mis en évidence. Or à ce jour il n’existe aucun guide d’exploration basé sur une méthodologie et sur des indicateurs robustes. La détection d’occurrence gazeuse en surface correspond bien évidemment à l’approche la plus efficace et la plus rapide à mettre en œuvre pour identifier des flux. Il n’en reste pas moins qu’un flux ne constitue pas une ressource pour autant, puisqu’à ce jour, l’homme n’exploite que les stocks de ressources énergétiques fossiles. Il sera donc important de développer un guide d’exploration non pas orienté uniquement sur une problématique de surface, mais aussi sur des considérations géologiques profondes intégrant le système hydrogène dans son entier de la source au piège ou à la fuite dans l’atmosphère.Au cours de ce travail de thèse nous proposons d’utiliser le cadre géologique du piémont nord Pyrénéen pour élaborer un guide d’exploration. La compilation des données bibliographiques a révélé un contexte prometteur pour un système H2 du fait d’un lien entre sources profondes, chemins de migration crustale, dynamique de circulation de fluides, et pièges sédimentaires. En effet le nord-ouest des Pyrénées et plus particulièrement le Bassin Mauléon est caractérisé par la présence i) d’un corps mantellique (