Deep crustal production of hydrogen (H ₂ ) is a potential source of primary energy if recoverable accumulations in geological formations are sufficiently large. We report direct measurements of an elevated outgassing rate of 84% (by volume) of H ₂ from the deep underground Bulqizë chromite mine in Albania. A minimum of 200 tons of H ₂ is vented annually from the mine’s galleries, making it one of the largest recorded H ₂ flow rates to date. We cannot attribute the flux solely to the release of paleo-fluids trapped within the rocks or to present-day active and pervasive serpentinization of ultramafic rocks; rather, our results demonstrate the presence of a faulted reservoir deeply rooted in the Jurassic ophiolite massif. This discovery suggests that certain ophiolites may host economically useful accumulations of H ₂ gas.

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, H₂S, and ²²²Rn) were performed at ∼1 m depth at the regional scale along a 10 × 10 km grid spanning over 7,500 km². The analysis campaign reveals several areas of high occurrence to the north of the Mauléon Basin where H₂, CO₂, and ²²²Rn 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.

Hydrogen gas is seeping from the sedimentary basin of São Franciso, Brazil. The seepages of H₂ are accompanied by helium, whose isotopes reveal a strong crustal signature. Geophysical data indicates that this intra-cratonic basin is characterized by (i) a relatively high geothermal gradient, (ii) deep faults delineating a horst and graben structure and affecting the entire sedimentary sequence, (iii) archean to paleoproterozoïc basements enriched in radiogenic elements and displaying mafic and ultramafic units, and (iv) a possible karstic reservoir located 400 m below the surface. The high geothermal gradient could be due to a thin lithosphere enriched in radiogenic elements, which can also contribute to a massive radiolysis process of water at depth, releasing a significant amount of H₂. Alternatively, ultramafic rocks that may have generated H₂ during their serpentinization are also documented in the basement. The seismic profiles show that the faults seen at the surface are deeply rooted in the basement, and can drain deep fluids to shallow depths in a short time scale. The carbonate reservoirs within the Bambuí group which forms the main part of the sedimentary layers, are crossed by the fault system and represent good candidates for temporary H₂ accumulation zones. The formation by chemical dissolution of sinkholes located at 400 m depth might explain the presence of sub-circular depressions seen at the surface. These sinkholes might control the migration of gas from temporary storage reservoirs in the upper layer of the Bambuí formation to the surface. The fluxes of H₂ escaping out of these structures, which have been recently documented, are discussed in light of the newly developed H₂ production model in the Precambrian continental crust.

During fluid injection tests performed inside the clay-rich Tournemire fault zone, the monitored fault displacement exhibited limited amplitudes whereas a sharp increase of the permeability occurred as the fluid pressure reached a threshold value. The injected fluid channeled through the fractured zone or at the interface between the host rock and the fault core over decameter distances without triggering large irreversible deformation of the fault zone. To assess the underlying hydro-mechanical mechanisms of this sudden increase of permeability, a numerical model was set up to simulate an injection test inside a fracture. When the fluid pressure outweighs the mechanical contribution of the heterogeneities along the fracture planes, the sudden increase of permeability appears. The existence of the channeling process, occurring below a critical fluid pressure at the fracture scale, could explain why it is possible to generate fault leakage inside a non-active fault zone crossing shale rocks. Based on the in-situ and laboratory monitoring, an analytical expression based on TPHM (Two-Part Hooke’s Model) can reproduce the stress-dependent permeability evolution. For low values of the effective stress, a simple cubic law formulation does not reproduce correctly the sharp increase of permeability identified once the Fracture Opening Pressure (FOP) is reached.