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

Even if measurements of high H₂ concentrations in continental rocks have significantly increased in the last decade, the origin of H₂ remains enigmatic in this context. Here we show that the localities in continental rocks where H₂-rich gases have been reported are mainly located near orogenic gold deposits. Two types of geomorphological features were identified near orogenic gold deposits on satellite images. They consist in both barren ground depressions and high densities of self-organized, small (< 20 m in diameter) circular- and comet-shaped white spots in 32 and 7 localities, respectively. Fe-carbonates commonly occur near gold deposits since gold is transported in CO₂-rich fluids. Thermodynamic modelling reveal here that they can further dissolve in the presence of aqueous fluid to produce magnetite and up to ~ 1 mole of H₂ per kg of rock. This reaction leads to a volume decrease of ~ 50 %. Based on these findings, we propose that Fe-carbonate dissolution could be the primary source of H₂ in orogenic gold deposit areas, and involved in the formation of the geomorphological structures reported here. The association between H₂-rich gas and ground depressions was also observed near other formations containing Fe-carbonates such as iron formations and carbonatites. This suggests that H₂ production through Fe-carbonate dissolution is not restricted to gold deposits. The global H₂ production in crustal rocks associated with Fe-carbonate alteration is estimated to 3 x 10⁵ mol/yr.

H₂-bearing fluids (pH 10 – 12) issued in alkaline springs found in several ophiolitic complexes worldwide are believed to result from the alteration of ultramafic rocks by infiltration of meteoric waters. The mineralogical fingerprint of the reactive percolation of such an alkaline fluid is revealed by veinlet mineralization occurring in the New Caledonian ophiolite (Massif du Sud). In two localities separated by ~ 15 km (Georges Pile and GR2H mines), late veins in a partially serpentinized peridotite contain magnetite crystals younger than 2 Ma as inferred from (U-Th)/He geochronometry. While the serpentinite host at Georges Pile is largely overprinted by lateritic weathering, primary parageneses are preserved at GR2H. There, magnetite occurs along with dolomite and Fe-poor lizardite as filling in millimeter sized veins cross-cutting the mesh texture of the partially serpentinized dunite. Temperature of the aqueous fluid from which the vein material precipitated is estimated to be ~95°C from in situ δ¹⁸O data on the magnetite-dolomite pair, indicating a low-temperature alteration process. Thermochemical calculation shows that this aqueous fluid was alkaline and most likely H₂-bearing. Chemically, it strongly resembles waters that are issued today in H₂ and CH₄ – bearing (hyper)alkaline springs of the Massif du Sud. δ¹³C isotopic composition of dolomite is exceptionally high, between 7.1 and up to 17.3 ‰ and is interpreted as evidence for low-temperature methanogenesis.

The alteration of ferroan brucite, a common by-product of serpentinization, has been proposed as a H₂ source at low temperature. Here, synthetic ferroan brucite with Fe/(Fe+Mg) = 0.2 was reacted with pure water at temperatures ranging from 348 to 573 K in 29 experiments either conducted in gold capsules or in Ti-based reactors. H₂ production monitoring with time and characterization of the reaction products revealed the occurrence of the following reaction: 3 Fe(OH)₂^brucite = Fe₃O₄ + H₂ + 2 H₂O. This reaction proceeded completely in ~ 2 months at 378 K and was thermally activated. The small grain size of the synthetic brucite (40-100 nm) was similar to observations in natural samples, and was probably responsible for the high reaction rate measured. H₂ production reached a plateau and Fe-bearing brucite also precipitated as a reaction product, suggesting the achievement of equilibrium. The thermodynamic properties of Fe(OH)₂ were refined based on the experimental dataset and differ by less than 5 % from previous estimates. However, ferroan brucite is predicted to be stable at an hydrogen activity one order of magnitude lower than previously calculated. As a result, significant H₂ production during ferroan brucite alteration at low temperature requires efficient fluid renewal. Such a mechanism strongly differs from olivine serpentinization which can occur even at high activity in H₂ and thus with limited water renewal.