Alain Prinzhofer

The development of natural hydrogen confronts numerous challenges. This paper sum marizes the status of research and exploration in the natural hydrogen field and outlines future directions and trends. The accumulation of natural hydrogen is a dynamic process, and current research on its origins, migration pathways, reservoir and cap rocks, chemical and biological consumption and final occurrence state is still at the exploratory stage. This lack of comprehensive understanding hinders the exploration and commercial development of natural hydrogen. Due to the unique characteristics of natural hydrogen, there is currently a shortage of effective exploration technologies and commercial development cases for natural hydrogen. The dynamic enrichment process of natural hydrogen and the consumption reaction mechanisms during this process will be the main aspects of future theoretical research on natural hydrogen. Additionally, the exploration of natural hydrogen needs to be combined with more practical applications of theory to enrich experience. It is necessary to establish and standardize work processes for different geological environments to achieve commercial development of natural hydrogen. Document Type: Perspective Cited as: Han, S., Huang, J., Prinzhofer, A., Wang, C. Advances in global natural hydrogen research and exploration. Advances in Geo-Energy Research, 2025, 15(2): 91-94. https://doi.org/10.46690/ager.2025.02.01 References Couzin-Frankel, J., Hand, E., Langin, K., et al. 2023 Breakthrough of the Year. Science, 2023, 382(6676): 1228-1233. Etiope, G. Massive release of natural hydrogen from a geological seep (Chimaera, Turkey): Gas advection as a proxy of subsurface gas migration and pressurised accumulations. International Journal of Hydrogen Energy, 2023, 48(25): 9172-9184. Han, S., Tang, Z., Wang, C., et al. Hydrogen-rich gas discovery in continental scientific drilling project of Songliao Basin, Northeast China: New insights into deep Earth exploration. Science Bulletin, 2022, 67(10): 1003-1006. Jin, Z., Zhang, P., Liu, R., et al. Discovery of anomalous hydrogen leakage sites in the Sanshui Basin, South China. Science Bulletin, 2024, 69(9): 1217-1220. Leong, J.A., Nielsen, M., McQueen, N., et al. H2 and CH4 outgassing rates in the Samail ophiolite, Oman, Implications for low-temperature, continental serpentinization rates. Earth and Planetary Science Letters, 2023, 347: 1-15. Maiga, O., Deville, E., Laval, J., et al. Characterization of the spontaneously recharging natural hydrogen reservoirs of Bourakebougou in Mali. Scientific Reports, 2023, 13: 11876. Milkov, A.V. Molecular hydrogen in surface and subsurface natural gases: Abundance, origins and ideas for deliberate exploration. Earth-Science Reviews, 2022, 230: 104063. Prinzhofer, A., Tahara Cissé, C.S., Diallo, A.B. Discovery of a large accumulation of natural hydrogen in Bourake bougou (Mali). International Journal of Hydrogen Energy, 2018, 43: 19315-19326. Truche, L., Donz, F-V., Goskolli, E., et al. A deep reservoir for hydrogen drives intense degassing in the Bulqizë ophiolite. Science, 2024, 383(6683): 618-621. Zgonnik, V. The occurrence and geoscience of natural hydrogen: A comprehensive review. Earth-Science Reviews, 2020, 203: 103140.

Offshore the emissions of dihydrogen are highlighted by the smokers along the oceanic ridges. Onshore in situ measurements in ophiolitic contexts and in old cratons have also proven the existence of numerous H₂ emissive areas. When H₂ emanations affect the soils, small depressions and vegetation gaps are observed. These depressions, called fairy circles, have similarities with the pockmark and vent structures recognized for long time in the sea floor when natural gas escapes but also differences. In this paper we present a statistic approach of the density, size, and shape of the fairy circles in various basins. New data from Brazil and Australia are compared to the existing database already gathered in Russia, USA, and again Brazil. The comparison suggests that Australia could be one of the most promising areas for H₂ exploration, de facto a couple of wells already found H₂, whereas they were drilled to look for hydrocarbons. The sum of areas from where H₂ is seeping overpasses 45 km² in Kangaroo Island as in the Yorke Peninsula. The size of the emitting structures, expressed in average diameter, varies from few meters to kilometers and the footprint expressed in % of the ground within the structures varies from 1 to 17%. However, globally the sets of fairy circles in the various basins are rather similar and one may consider that their characteristics are homogeneous and may help to characterize these H₂ emitting zones. Two kinds of size repartitions are observed, one with two maxima (25 m and between 220 m ± 25%) one with a simple Gaussian shape with a single maximum around 175 m ± 20%. Various geomorphological characteristics allow us to differentiate depressions of the ground due to gas emissions from karstic dolines. The more relevant ones are their slope and the ratio diameter vs. depth. At the opposite of the pockmark structures observed on the seafloor for which exclusion zones have been described, the H₂ emitting structures may intersect and they often growth by coalescence. These H₂ emitting structures are always observed, up to now, above Archean or Neoproterozoic cratons; it suggests that anoxia at the time the sedimentation and iron content play a key role in the H₂ sourcing.

Proterozoic sedimentary basins very often emit natural hydrogen gas that may be a valuable part of a non-carbon energy infrastructure. Vents in the Sao Francisco Basin in Brazil release hydrogen to the atmosphere mainly during the daylight half of the day. Daily temperature and the regular daily tidal atmospheric pressure variations have been suggested as possible causes of the pulsing of H₂ venting. Here, we analyze a ~550 m-diameter depression that is barren of vegetation and venting hydrogen mainly at its periphery. We show that daily temperature changes propagated only ~1/2 m into the subsurface and are thus too shallow to explain the H₂ variations measured at 1-m depth. Pressure changes could propagate deeply enough, and at the depth at which the cyclic variations are measured hydrogen concentration will have the observed phase relationship to atmospheric pressure changes provided: (1) the pressure wave is terminated by geologic barriers at about 25% of its full potential penetration distance, and (2) the volume of gas in the vents is very small compared to the volume of gas tapped by the venting. These constraints suggest that there is a shallow gas reservoir above the water table under the ~550 m-diameter barren-of-vegetation depression. The 1D-analytical and finite-element calculations presented in this paper help define the hydrogen system and suggest the further steps needed to characterize its volume, hydrogen flux and resource potential.