Understanding the resource potential of natural hydrogen on Earth: Scientific gaps, uncertainties and recommendations

A comprehensive scientific research roadmap is essential to bridge knowledge gaps and deepen the understanding of key geological, geochemical, and geophysical aspects of natural hydrogen (H₂) as a potential new energy resource. This paper reviews major scientific uncertainties on natural H₂, suggesting research priorities, as a guide for defining exploration strategies, techniques, and data interpretation. The uncertainties concern all phases of the natural H₂ cycle, from generation (source rocks) through migration (advection and diffusion) and accumulation (reservoir and cap rocks) to the application and interpretation of subsurface and surface geochemical and geophysical exploration techniques. Understanding H₂ sources and generation rates (the amount of H₂ generated by a given volume of rock over time) is crucial for determining whether a geological H₂ system operates as a short-term dynamic system with rapid H₂ production and release, or as a conventional gas system with long-term accumulations, analogous to petroleum reservoirs. Preliminary estimates for serpentinisation, radiolysis, and organic matter degradation suggest that H₂ generation is not inherently fast, especially for non-hydrothermal continental systems (crystalline basement of shields, ophiolites, peridotite massifs, sedimentary basins), and long-term accumulations, like those of fossil natural gas systems, represent the most likely scenario. The mechanisms of H₂ migration through geological formations require application of fundamental principles of fluid-flow physics, distinguishing advection and diffusion, as well as their forms (from gas-phase, bubble flows to aqueous solutions). Additional studies of H₂ accumulation and retention in subsurface reservoirs could improve understanding of mechanisms of H₂ migration by focusing on the rock fluid-bearing properties and the factors affecting H₂ preservation, such as the presence of cap rocks impermeable to H₂, pressure conditions, residence times, and microbial or abiotic consumption. Advanced techniques, including reservoir modelling, flow simulations, 3D imaging (micro-CT) of H₂-bearing rocks, and extraction and analysis of gas occluded in rocks, can provide insights into the stability and potential recoverability of H₂ accumulations. The interpretation of surface exploration techniques, including gas geochemistry, geophysics, and remote sensing, long employed in mineral and energy resource exploration, is now being adapted for natural H₂ studies, but challenges remain in the data interpretation. Distinguishing H₂ seepage due to geological degassing from H₂ produced near the surface by modern microbial processes or artificial sources, such as hammering or drilling for soil-gas sampling, drilling into aquifers, and corrosion in boreholes, is an essential step in exploration. The simple detection of H₂ in soils, even in morphological structures like sub-circular depressions or “fairy circles”, cannot be cursorily interpreted as a signal of natural H₂ seepage from a deep source. A holistic geochemical approach, including isotopic analyses of gases associated with H₂, is recommended to distinguish among the variety of possible H₂ origins. Observations of H₂ in wells should be interrogated to rule out possible artifacts such as corrosion and drill bit metamorphism. The integration of multiple geophysical methods, including seismic, gravimetric, magnetic, and electro-magnetic surveys, is recommended to mitigate interpretation ambiguities regarding the structure of a subsurface H₂ system (source and reservoir rocks, including fluid and gas storage), due to the non-uniqueness of rock-specific physical properties.