Seepage of natural hydrogen: geochemical uncertainties and the need of a holistic approach

Natural hydrogen (H₂) exploration is particularly focused on surface emissions (seeps) and soil-gas prospections, similar to early petroleum reservoir explorations. However, defining the amount of H₂ at the surface that indicates a potentially economic resource is impossible, and identifying its origin is challenging due to the fact that geological H₂ concentrations and isotopic composition can overlap with the in-situ biological signature. The potential for H₂ generation in surface environments as a result of microorganisms, corrosion of iron particles or minerals, or even drilling (artificial H₂) must be extensively evaluated. Despite these limitations, similarities to hydrocarbon systems suggest that certain seeps, which typically occur in correspondence with faults, are an expression of advection (not diffusion), driven by pressure gradients, and thus can disclose the existence of pressurized pools (Etiope, 2023). The fluid dynamics of gas seeps can therefore reveal the nature of the H₂ supply system. Comparing seep flux rates with potential H₂ generation rates, either via radiolysis or serpentinization, is a fundamental exercise for determining whether the H₂ system must include a reservoir, similar to conventional natural gas systems, or if the H₂ observed at the surface originates from continuous flow crossing short-term accumulations or directly from the source rock, as some scholars have hypothesized. However, analyses of the gases associated with H₂ (such as CO₂, CH₄, N₂ and He) are always advised. Assessing the potential of a geological H₂ resource necessitates a holistic approach that integrates multiple geochemical, ecosystem, and geological data.

References

Etiope G., (2023) https://doi.org/10.1016/j.ijhydene.2022.12.025