The hydrogen color label "White" refers to natural (geological) reserves of underground hydrogen. 

Autor: Georgii Feodoridi

            Kseniia Feodoridi

Place: HYDITEX CORPORATION, Port Vila

Date: May 2024

Abstract: Natural hydrogen has been found in many geological environments, including ocean spreading centres, transform faults, passive boundaries, convergent margins and intraplate settings, etc.

Key Words: White hydrogen, geological hydrogen, natural hydrogen, underground hydrogen.

DOI: 10.13140/RG.2.2.22641.08805

The radiation released by decaying radioactive elements in marine rocks, such as uranium (238U and 235U), thorium (232Th), and potassium (40K), can produce α, β, and γ radiation, excite and ionize water, producing free radicals leading to hydrogen. Thus, the H–O bond in water is decomposed, and hydrogen and hydroxyl radicals are produced. Then, two hydrogen radicals react to form hydrogen 2H·→H2 [9, 10]. The production of hydrogen by radiolysis requires simple geochemical components (water and radionuclides) common on Earth and elsewhere in the solar system. Furthermore, radiolysis occurs under all temperature and pressure conditions where water is stable, even if water takes the form of ice, steam, or hydrated salts. H2O radiolysis differs from other forms of abiotic hydrogen production because the decomposition of neutrally charged H2O molecules yields complementary soluble oxidants (e.g., H2O2, O2, and O−) and reducing agents (H2). This is in contrast to serpentine, where the oxidation products precipitate to minerals such as magnetite, iron olivine, and radiolite. Radioactive hydrogen generation also occurs in the oceanic basalt crust and seafloor sediments. Therefore, the process of hydrogen generation is common in the crust. However, the radiation decomposition of hydrogen depends on the number of radioactive nuclide concentrations, the fracture pore space availability of H2O, and the fluid in the dissolved concentrations of anions and cations such as salt. The concentration of radionuclides in the continental crust is higher than that in the basaltic oceanic crust. Therefore, hydrogen production will be higher in the continental crust relative to the oceanic crust for the same porosity. When H2O radiolysis occurs in sediments or crystalline rocks, water is primarily confined to pore and fracture spaces. Thus, rock permeability and porosity are crucial for estimating hydrogen production from crustal regions with characteristic radionuclide concentrations [11, 12]. The radiolysis of hydrated salts also produces hydrogen. Specific minerals (such as zeolites) also increase hydrogen produced by radiolysis [6]. 

Alpha particle decay U238 to Th234 is in the form of He4 so H2 and He are sometimes found together. Therefore, helium produced during radiolysis can be a guide to understanding where migrating crustal hydrogen can be concentrated and easily captured (rather than consumed chemically or biologically) [13]. 

Rock Fracturing (Cataclasis)

Rock rupture breaks chemical bonds and generates free radicals that react with water to form hydrogen: 

2(≡ Si·) + 2H2O→2(≡ SiOH) + H2 [14, 15]. 

The unusually high concentrations of hydrogen in soil gases associated with tectonic faults are attributed to fault movement during rock crushing and its associated free radical formation [16]. Mechanical forces dissociate covalent Si–O bonds in silicate minerals to produce surface-free radicals ≡Si· and ≡SiO· (homolytic) and charged surface radicals ≡Si+ and −O -- Si ≡ (hetero). Once these surfaces are speciated, they either recombine to form siloxane bonds (Si–O–Si) or react with water via Si· + H2O→SiOH + H·, which releases hydrogen as a by-product as follows: H· + H·→H2. This process, known as mechanical or medical hydrogen generation, may be widespread in fault zones. Faults are standard geological features in orogenic belts, subduction zones, continental rifts, passive margins, spreading centers, transition faults, and fault zones. Hydrogen is generated when these faults are active. In creeping faults, hydrogen generation may continue, whereas in locked faults, hydrogen generation might be episodic and limited to slip events. Mechanical and medical hydrogen generation is not limited to tectonic faults. It can occur anywhere silicate rocks are crushed. A good example is subglacial bedrock crushing, where hydrogen generation can support microbial ecosystems near the freezing point of water, a process that may even have sustained life during global glaciation [17]. Other processes that cause rock fragmentation include freezing wedges (freeze–thaw), salt wedges, thermal shrinkage during cooling or expansion during heating, rock wear (sandblasting and erosion in rivers and surf zones), gravity impacts (landslides or rock falls), meteorite impacts, and reaction-driven fractures. However, the effectiveness of these processes in generating hydrogen is unclear [6]. 

Magma Degassing

Hydrogen degassing of the Earth, otherwise also called "Hydrogen Breathing of the Earth." [18] 

Numerous observations suggest that hydrogen derived from magmatic processes and the degassing of the Earth’s upper mantle and, hypothetically, the lower mantle and core [19] is prevalent, and routinely migrates through the crust to the Earth’s surface. These observations include: high concentrations of hydrogen in air and soil gases, hydrothermal fluids and volcanic gases associated with rift zones, mid-ocean ridges, volcanic and geothermal zones, and ophiolite complexes [3, 20, 21, 22, 23, 24], the increasing concentration of hydrogen in gases sampled at increasing depth from deep drillholes and deep-seated faults [14, 16, 25, 26],  inclusions from a variety of mantle-derived lithologies and minerals (e.g. kimberlites, olivine, diamonds and perovskite; [23, 26, 27] and Precambrian basement lithologies [30],  measurements of isotope ratios (δ2H) of hydrogen [4, 26, 29]. 

Theoretical studies indicate that hydrogen can be present in the mantle and core as natural metal hydrides (VH2) and appropriate pressure–temperature–redox conditions exist in the lower mantle to support the presence of water, and hydrogen-rich and methane-rich fluids [5, 23, 26, 30, 31]. 

Degassing of the core of the earth could generate a continuous flux of H2 (deep-seated or hydridic earth source). This theory is not universally supported by different academic scholars [6]. 

Crust Weathering

As seawater cools and ages, the oceanic crust changes, and the following reactions occur at cooler temperatures (<250 °C): 

2(FeO)rock + 4H2O→2Fe(OH)3 + H2, 2(FeO)rock + 2H2O→2FeOOH + H2, and 

 2(FeO)rock + H2O→Fe2O3 + H2 [32]. 

Samples from deep-sea and offshore drilling projects indicate that crustal weathering occurs within the ejecta and continues until the age of crust reaches ~10 to 20 Ma [32]. Alteration is typically limited to fractures and fracture margins, and the middle and lower oceanic crust is less exposed to weathering. For example, the gabbros samples recovered from active tectonic scarps of rapidly expanding ridges are young and typically have oxidation rates of less than 10%. Dredge samples from the more slowly expanding ridges and outcrops of the middle and lower ocean crust are older and more oxidized (50%) [6, 33, 34]. 

High-Temperature Basalt Alteration

The alteration of oceanic crust by seawater during high temperatures (350–400 °C) changes most ferrous silicates to ferrous minerals. However, a small fraction is converted into iron-bearing minerals to form hydrogen via 3(Fe2SiO4)rock + 2H2O→3SiO2 + 2Fe3O4 + 2H2 [35]. Although the depth at which hydrothermal fluids penetrate the oceanic crust is unknown, limited sampling indicates that the upper crust changes while the lower crust remains unchanged [34]. Pasquet et al. detected natural hydrogen gas in the Rift Valley of Djibouti, East Africa [36]. They performed linear sampling and collected altered/fresh basalt and gas from the area in situ. In their study locale, only small amounts of hydrogen exist at the surface. Their data suggest that natural hydrogen is transported through iron minerals in basalt and deep fluids at high temperatures (approximately 270 °C) in the Earth’s crust in the Rift region [6]. 

Lava–Seawater Interaction

The interaction of seawater and extruded lava produces hydrogen via 2(FeO)magma + (H2O)seawater→(Fe2O3)rock + H2 [37, 38]. Determining the extent of the lava–seawater interaction is challenging, but seawater interacts extensively with the jet surface, as evidenced by the fragmented and hardened crust [6]. 

Crystallization 

In the late crystallization process, hydrogen will be generated when the dissolved water in the magma oxidizes ferrous iron, 3(FeO)magma + (H2O)magma→(FeO·Fe2 O3)rock + H2 [6]. 

Pyrite Formation 

During the inorganic formation of pyrite and H2, the stoichiometric yield of H2 has been quantified [39, 40]. This reaction could occur in the characteristic high-temperature black smoke mouth of MOR. The pyrite contained in seafloor and chimney deposits primarily precipitate from hydrothermal fluids, with chemical reactions Fe2+ + 2H2S→FeS2 + H2 + 2H+ and Cu+ + Fe2+ + 2H2S→CuFeS2 + 0.5H2 + 3H+ [6, 41]. 

Decomposition of organic matter 

Through the processes of anaerobic decay, fermentation and nitrogen-fixing bacteria are also capable of generating natural hydrogen. These processes often take place in complex chemical and biological environments where the hydrogen produced is then taken up by hydrogen-consuming microorganisms or converted by complementary reactions in soil and sediments to produce hydrogen-fixing methane and nitrogenous compounds [4]. Overall, soils are a major hydrogen sink and it is likely that most hydrogen produced via biologically mediated processes is subsequently fixed in the soil, although [42] found soil hydrogen fluxes varied spatiotemporally and hydrogen could also be emitted from soil. An alternative scenario is where high hydrogen concentration is found associated with coal maturation and generation of hydrocarbons, a thermogenic process rather than microbial one [2, 43]. Recently, the kinetics of thermogenic hydrogen generation has been determined for lacustrine organic matter where free hydrogen becomes available from generation of overmature organic matter [44]. 

The take home message is there is evidence for geologically sourced production of natural hydrogen via a number of mechanisms. The hydrogen cycle is complex and poorly understood as a holistic system however [2, 21, 23, 26, 45]. The balance between subsurface hydrogen generation and consumption via organic (biologically mediated) and inorganic reaction processes, and hence the extent of hydrogen produced and stored as commercially viable natural accumulations, requires focused study [5]. 

Natural seeps 

"Free hydrogen" refers to hydrogen that migrates freely through the pores or fractures of rocks (or strata) [6]. Many naturally occurring hydrogen seeps have been identified on the seafloor and on the continents. Some continental examples, including Mt Chimaera (Turkey), Los Fuegos Eternos (Philippines), the Semail ophiolite (Oman), and other locations in Greece, Portugal, New Caledonia, and Bosnia and Herzegovina, have been linked to production of hydrogen from serpentinisation of ultramafic or ophiolitic complexes [3, 26]. Other diffusive seeps located in intracratonic settings (e.g. Sao Francisco Basin in Brazil, Bourakebougou in Mali, Africa, and Russia) have been attributed to fluid-rock chemical interactions such as the oxidation of Fe2+-rich minerals and lithologies in underlying rocks, or radiolysis of water as outlined above [3, 5, 24, 46]. 

These intracratonic diffusive seeps sometimes occur within shallow, circular to subcircular surficial depressions on the tens of metres to kilometre scale,very characteristic "subsidence ring structures" form in the areas where these leaks reach the surface and are referred to as ‘fairy circles’ by some researchers [5]. 

These "ring subsidence structures" are clearly visible on space images, they appear as light rings and circles in the places of hydrogen streams and jets outlets. In intensive places of primordial gas outlets, subsidence and formation of reservoirs are observed. Particularly noticeable on dry areas of land is a strip of more succulent and taller grass along the border of the circle. This is explained by the fact that streams of molecular hydrogen coming from the Earth's interior, passing through the fertile layer, destroy long molecules of black soil, discolouring the soil (Hydration of dark humus takes place). 

These "subsidence ring structures" often occur in clusters, and can be identified using statistical analysis of their surface geomorphology in conjunction with aerial photography, LiDAR and satellite imagery as a screening aid for early exploration targets [e.g. 21, 25, 47]. Using satellite imagery, geomorphological features of similar appearance have been identified in the Borisoglebsk area of Russia, on Kangaroo Island and Yorke Peninsula, South Australia, and the Yilgarn Craton, Western Australia [21, 25, 48, 49]. 

The formation of these subcircular depressions has been postulated as a diagenetic effect of hydrogen increasing rock porosity via chemical dissolution as it migrates toward the surface [25, 26, 47]. If so, it is possible that this process can enable the migration of hydrogen away from deep-seated conduits such as fault zones. Hence seeps may be directly connected to an actively producing hydrogen source or to a leaking reservoir. It is also noteworthy, however, that these shallow, subcircular surface features are not ubiquitously present in areas where hydrogen accumulations have been identified, and there are alternative biological and geomorphological processes, such as the mechanical weathering of soil by heavy rain, extreme heat and evaporation, which could also form these features in some locations [e.g. 50, 51]. 

Diffusive seeps have generally been confirmed and characterised through the deployment of soil gas chemistry monitoring sensors, which indicate that hydrogen fluxes tend to vary spatiotemporally. In the case of the circular to subcircular topographic depressions, hydrogen is present within the depressions, but absent in the areas outside of and surrounding the depressions [21, 24, 25, 47]. At this stage there are no direct soil gas data indicating an active hydrogen flux at the circular structures documented in South Australia; however, gas analyses from drillholes drilled in the 1920s–30s on Kangaroo Island (American Beach Bore 1) and Yorke Peninsula (Ramsay Oil Bore 1), provide alternative evidence that a hydrogen resource may be present in these locations [52, 53, 54]. A recent study [49] demonstrated that a series of similar circular structures, spatially related to the Darling Fault in the North Perth Basin, Western Australia, are natural hydrogen seepages. In this location, the Darling Fault juxtaposes Precambrian ultramafic and mafic rocks and Proterozoic Fe-rich granite of the Yilgarn Craton against layered Permo-Mesozoic sedimentary rocks which include multiple aquifers and aquitards. This suggests that the natural hydrogen emitted from the circular structures at surface could be sourced from the oxidation of Yilgarn basement rocks by anoxic fluids, mobilised along zones of transmissivity associated with the Darling Fault system [5, 49]. 

High hydrogen fluxes in drillholes and mines

There are also numerous examples of groundwater, oil and gas drillholes, and mines intersecting significant hydrogen fluxes or accumulations in a range of cratonic settings including: Russia [25]; Brazil [42]; Bourakebougou, Mali [24]; northeastern Kansas, USA [27, 46]; the Witwatersrand, South Africa; Canada; Finland [20]; the Otway Basin (Robe 1), Yorke Peninsula, Kangaroo Island and the western flank of the Cooper Basin (Ralgnal 1), South Australia [52, 53, 54]; the Amadeus and McArthur basins, Northern Territory; and many other Australian basins [4, 29]. However, only a few have been comprehensively studied with a view to understanding the hydrogen system. A number of authors [3, 4, 21, 26, 42] have noted the similarity between the development of the hydrocarbons industry and the current state of knowledge about natural hydrogen. A possible analogue and reasonable starting point to developing an exploration strategy then might be the petroleum systems (source – migration pathway – reservoir–seal–preservation) approach that underpins hydrocarbon exploration [5, 55]. 

Adsorbed Hydrogen

Natural hydrogen can be trapped inside various rock types as inclusions or adsorption product. Truche et al. determined that the clay rocks of the Cigar Lake uranium deposit in northern Saskatchewan, Canada, were dominated by illite, chlorite, and kaolinite and contained 500 ppm hydrogen. During the 1.4 Ga lifetime of the Cigar Lake uranium deposit, 4–17% of the hydrogen produced by water radiolysis was sequestered in the surrounding clay alteration halo, with chlorite being the primary mineral adsorbing hydrogen [56 ]. 

A recent study reported mapping hydrogen and other gases at multiple (165) sites in Ukraine, primarily in oil and gas fields, coal mines, celestial bodies, and offshore shelf areas [26, 57]. The flow of hydrogen through aqueous porous media was reduced by a factor of 10 compared to that through pure water. The obstruction of hydrogen diffusion by saturated water sediments could be related to the van der Waals radius (the distance at which van der Waals forces are effective). The study’s authors note that molecular hydrogen and hexadecane inert gases have similar radii. Therefore, the migration characteristics of these gases in porous media are similar. Traps and aquifers might impede hydrogen migration [7]. However, the van der Waals radius of hydrogen molecules varies with pressure, the presence of other molecules, and other factors [6, 59]. 

Hydrogen in Inclusions

Natural fluid inclusions in minerals (commonly in quartz) provide unique data on drainage systems. Fluid inclusions containing hydrogen have been recognized in numerous geological environments. Hydrogen has been found in inclusions of samples from ultrabasic, Precambrian, igneous, and volcanic rocks tubular kimberlite, ore bodies, coal basins, sedimentary or metamorphic rocks, and rock salt deposits [60, 61, 62]. Analysis of samples has shown that the hydrogen concentrations in the inclusions are not always uniform, ranging from 0.2% to 100%. Smith et al. probed mineral inclusions from large diamonds and detected abundant slivers of iron metal surrounded by reducing gases [63]. This indicates that the large diamonds grew from liquid metal in the Earth’s mantle. These inclusions provide direct evidence of a long-suspected metal precipitation reaction requiring a more reductive mantle. In 13 samples, hydrogen was also detected, accompanying intense CH4 signals. Furthermore, the study of Raman maps showed CH4 and H2 concentrated at the inclusion nucleus [63]. A study by Klein detected CH4 and H2 fluid inclusions trapped in a calcite crystal from ophicarbonates in the Lanzo peridotite massif (Italian Alps) [64]. 

Minerals in the rift zone contain gas inclusions with a high hydrogen content. In the oceanic rift, the average hydrogen concentration found in inclusions was 21.4%, and more hydrogen detections were reported for inclusions in rocks of Precambrian age [6, 65]. 

If we talk about hydrogen sources at the bottom of the World Ocean, they can be divided into two main groups: 

The first is represented by local anomalies in seawater associated with active high-temperature (up to 400o C) hydrothermal sources ("black smokers"). These sources are widespread within mid-ocean ridges and in back-arc spreading centres. They are comprehensively characterised in numerous publications and summarised in a recent summary [66, 67]. 

The second group is represented by intense methane anomalies (CH4 content up to 50 nmol/kg), revealing elevated hydrogen contents (up to 13 mmol/kg) and confined to outcrops of massive mantle hyperbasites near some transform faults of the Mid-Atlantic Ridge. Methane and hydrogen generation in these anomalies is attributed to serpentinisation of mantle peridotites [67, 68, 69, 70, 71]. 

During serpentinisation of mantle peridotites, intense hydrogen emission occurs. Hydrogen is a substrate of life activity of various prokaryotes, primarily methane-generating archaea [72]. Their active hydrogen metabolism is favoured by a high concentration of heavy metal ions (Fe, W, Ni and other activators of hydrogen metabolism enzymes - hydrogenases) in hydrothermal discharge zones and in pore waters. Thus, the total volume of generated methane is formed from two sources - abiotic (reaction of hydrogen with carbon dioxide dissolved in seawater) and biotic (methanogens). Inhabited by bacteria and archaea (hyperthermophiles), hydrotherms create a huge biomass not only on the surface of the seafloor but also in the interior of the ocean at considerable depths. This organic matter has been and remains the main source of biogenic hydrocarbons since the emergence of the oceans on Earth. Biotic degradation of this biomass and its chemical transformations lead to the formation of simple gaseous hydrocarbons and petroleum. The bioproductivity of prokaryotic biota (biomass and biogenic methane) of the above biotopes largely determines the hydrocarbon potential of porous rocks and sediments located above the subduction zone. In this case, the main factor in the transformation of methane, hydrogen and hydrogen sulfide into more complex hydrocarbons is the vital activity of bacteria consuming these gases, since abiogenic methane serves as a food base for bacteria, and the latter create organic substances from which normal hydrocarbons are formed in the future [73, 74]. 

The natural degassing of hydrogen from the Earth occurs constantly. Emanations of Hydrogen have been observed in many places. Many hydrogen emergences have been identified on mid-ocean ridges. Geological hydrogen degassing is distributed between mid-ocean ridges and faults with account 90% , 2% by volcanoes and 8% by ring structures.

Future natural hydrogen exploration should focus more on the distribution state of natural hydrogen (natural hydrogen exists in different states at different temperatures and pressures), underground migration mode (direction, strength and resistance of migration) and hydrogen accumulation mode. Based on the above, it can be concluded that further research into the sources and mechanisms of hydrogen production will be the key to solving problems with hydrogen exploration, storage and transport as an energy resource.

HydITEx is developing a geological exploration concept for detailed searches over large areas for hydrogen deposits, where the likelihood of their geological discovery has received sufficient geological justification, by analyzing the biomass of hydrogen bacteria, the presence of noble gases and their isotopes, as well as their composition.

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