Origin, discovery, exploration and development status and prospect of global natural hydrogen under the background of “carbon neutrality”

1.   Introduction

A growing number of countries are adopting carbon neutrality as a national strategy, proposing a vision for a carbon-free future (Wang Y et al., 2021a, 2021b; Dai SX et al., 2022; Xu FY et al., 2022). At present, human-induced carbon dioxide emissions mainly come from the consumption of fossil fuels. The development of new energy and the realization of energy transformation are important measures to reduce carbon dioxide emissions. The world is currently facing the third major energy transition from fossil energy to non-fossil energy such as hydrogen (Zou CN et al., 2019, 2021; Gaucher E, 2020; Chen XJ et al., 2022; Wang FM et al., 2022). The purpose is to reduce the use of fossil energy, use more efficient energy, reduce environmental impact, cope with climate change, reduce greenhouse gas emissions, etc., and thus achieve the purpose of non-fossil energy utilization and carbon neutrality. Hydrogen generates only water steam when burning with no pollutants or greenhouse gases such as carbon dioxide (Gaucher E, 2020). At present, there are three main ways to produce hydrogen: (1) Green hydrogen production using renewable energy to electrolyze water; (2) blue hydrogen production from natural gas steam conversion or coal-to-gas combined with carbon capture and storage; (3) gray hydrogen production from fossil fuels without carbon capture and storage (Boreham CJ et al., 2021). Most of the hydrogen currently utilized is gray hydrogen. Global hydrogen use in 2020 is about 90×10⁶ t , which is produced almost entirely from fossil fuels and results in nearly 900×10⁶ t of CO₂ emissions. Producing green hydrogen by electrolysis of water is considered the main way of hydrogen production in the future. But now, the cost is relatively high and the output only accounts for 0.1% of the total hydrogen production (IEA, 2021a, 2021b). About 350 projects of hydrogen production by water electrolysis are being implemented worldwide, and it is expected that by 2030, the global supply of hydrogen produced by water electrolysis will exceed 8×10⁶ t, but it is still far below the 80×10⁶ t demand for hydrogen specified in the roadmap by the International Energy Agency (Hydrogen Council, 2021). Therefore, a better hydrogen source should be considered to meet the future demand for hydrogen. In the future energy pattern, it is necessary to find more economical and feasible methods for “low carbon” or even “zero carbon” hydrogen production, and the ubiquitous subsurface natural hydrogen may become the main pillar of the future energy revolution.

Twenty years ago, researchers were convinced that hydrogen does not exist in its natural state on the Earth’s surface (Par K, 2020). But in 2002, Nigel JP Smith of the British Geological Survey published in “first break” that “It’s time for explorationists to take hydrogen more seriously” (Smith NJP, 2002). Over the past few decades with the first use of natural hydrogen for power generation by Canadian company Hydroma in Mali, Africa, the understanding of natural hydrogen has gradually increased, and the situation that “From a geological perspective, hydrogen has been neglected” (Smith NJP et al., 2005) has also changed. Moreover, in the past 30 years, a large amount of observational data on hydrogen leakage are accumulated, and many papers on natural hydrogen have been published. This is especially true in the past two years, researches on natural hydrogen have been started all over the world. Under this background and based on a comprehensive literature review, this contribution focuses on explaining geological issues such as the origin and source of natural hydrogen from a geological point of view, analyzing the discovery of natural hydrogen, summarizing the exploration and development progress of natural hydrogen, and presenting the prospect of natural hydrogen exploration in China.

2.   Origin and classification of natural hydrogen

Hydrogen is ubiquitous in the subsurface,which can be defined as “Natural hydrogen” “Native hydrogen” “Geological hydrogen” (Шестопалов ВМ, 2020; Durham University, 2021; Hydrogeit, 2021). Natural hydrogen is considered sustainable hydrogen in the subsurface generated by geological processes. These processes occur in the shallow and deep parts of the Earth’s crust, and the generated hydrogen can be detected as they are leaking from the surface (Durham University, 2021; Hydrogeit, 2021). Native hydrogen is naturally occurring hydrogen; geological hydrogen is subsurface hydrogen with abiotic origins ( i.e. fully geological origin) (Шестопалов ВМ, 2020). This contribution discusses the concept from the perspective of natural hydrogen. In addition, “gold hydrogen” (Durham University, 2021) or “white hydrogen” (National Grid, 2022) is also used by some researchers to describe natural hydrogen, to distinguish it from “grey hydrogen”, “blue hydrogen” and “green hydrogen”.

Some researchers believe that natural hydrogen can be derived from the abiotic and biotic origin, the former including deep-seated hydrogen generation, water-rock reaction, and water radiolysis (Reeves EP and Fiebig J, 2020; Han SB et al., 2021); the latter including thermal action (Tissot BP and Welte DH, 1984; Hunt JM, 1996) and microbial action (Nandi R and Sengupta S, 1998; Hallenbeck PC and Benemann JR, 2002). Other researchers believe that hydrogen is derived from mixed origins. Different explanations for the origin of hydrogen (Angino EE et al., 1984; Vacquand C et al., 2018) suggest that hydrogen is ubiquitous, and researchers have not fully understood the nature and behavior of hydrogen in the crust. Moreover, natural hydrogen can also be divided into primordial hydrogen and secondary hydrogen (Zgonnik V, 2020; Boreham CJ et al., 2021; Han SB et al., 2021).

Natural hydrogen occurs in a variety of geological settings, such as in crystalline basements, volcanic ultramafic peralkaline igneous complexes, geothermal and mineral systems, graphite, evaporite deposits, anoxic sediments, conventional and unconventional oil and gas fields, and coalbed methane (Gregory SP et al., 2019; Laurent T et al., 2020; Boreham CJ et al., 2021). It is very difficult to classify natural hydrogen. A review paper published in 2020 mentioned two classification schemes (Zgonnik V, 2020). One is to divide natural hydrogen into primordial hydrogen and secondary hydrogen according to their geological origins in the Earth’s interior. Primordial hydrogen refers to the hydrogen stored in the mantle or core that is gradually released to the surface; secondary hydrogen refers to hydrogen generated by various chemical reactions in the mantle or crust. The other is to divide natural hydrogen into three categories according to their occurrence state, namely free hydrogen, hydrogen in inclusions, and dissolved hydrogen. Much of the literature adopts the latter classification. It was previously assumed that natural hydrogen is only generated from water, but latter investigations have found that natural hydrogen has a wide range of origins (Zgonnik V, 2020; Шестопалов ВМ, 2020).

2.1   Origin of natural hydrogen

2.1.1   Abiotic origins

(i) Deep-seated hydrogen

Deep-seated hydrogen hypothesizes that hydrogen is released from the core and the lower mantle, which is accumulated in the subsurface during the planetary accumulation process. This hypothesis was proposed and developed by researchers from the former Soviet Union (Шестопалов ВМ, 2020), followed by researchers from other countries suggesting to define hydrogen from the mantle or core as “deep origin” (Zgonnik V, 2020). The hypothesis of mantle-derived degassing was latter proposed after the role of hydrogen in the process of degassing and the Earth’s evolution are recognized through the study of the role of mantle fluids in the ore-generating process (Nivin VA, 2009). The overall analysis of the evolution of different fluids on Earth suggests that mantle-derived fluids are in a more reducing chemical condition than crust-derived fluids (Перчук ЛЛ, 2000). A large amount of hydrogen may exist in the mantle under a reducing condition (Smith EM et al., 2016). In addition, studies show that the upper mantle and most of the asthenosphere are saturated with metals, in which case the main fluid should be hydrogen-rich fluids (Rohrbach A et al., 2007). Hydrogen isotope studies show that the mantle contains hydrogen, and supplies hydrogen to crustal rocks (i.e. surface rocks ) (Deloule E et al., 1991). Recent studies point out that large amounts of hydrogen may be stored in the mantle in the form of hydrated minerals (Schmandt B et al., 2014). The latest study discovers natural hydrides for the first time, demonstrating the presence of hydrogen-dominated fluids in the mantle (Bindi L et al., 2019).

There have been different views on the formation of hydrogen in the Earth’s core. Some studies suggest that the Earth may have accumulated a large amount of hydrogen during its formation and they hypothesize that hydrogen has existed on the Earth since then. Hydrogen may be bound to iron in the Earth’s early evolution (Gilat AL et al., 2012). Other studies suggest that iron reacts with water to form separate iron oxide and iron hydride (Yagi T and Hishinuma T, 1995; Mao HK et al., 2017). Calculations based on the correlation of the element’s ionization potential with the element’s distribution in the solar system suggest that hydrogen is very abundant in the original composition of Earth. Therefore, primordial hydrogen may be stored inside the Earth. This hypothesis was proposed by Vladimir Larin in 1973 (Ларин В, 1973), and was applied to the study of the Earth rich in primordial hydrogen many years later. Recently, researchers have revisited this idea using statistical physics methods, confirming that the original composition of Earth is rich in hydrogen (Toulhoat H and Zgonnik V, 2022).

(ii) Water-rock reaction

Water-rock reaction generally refers to the interaction between fluids and rocks in all geological processes. Water-rock reactions associated with hydrogen generation mainly include serpentinization, water reactions with newly exposed rock surfaces, and hydroxyl reactions in minerals (Ding K, 1989; Han SB et al., 2021).

Serpentinization is the most studied, most important, and most common in the study of hydrogen generation by the water-rock reaction. Serpentinization occurs in the hydrothermal cycle of mafic-ultramafic rocks in deep-sea hydrothermal vents and continental ophiolite, and hydrogen ions (H⁺) are released during the serpentinization of fayalite (Boreham CJ et al., 2021). In general, as the magnesium content of olivine and magnesium silicate increases, the fluids generated tend to be alkaline. Therefore, hydrational alteration of minerals such as olivine and pyroxene generates serpentine, brucite, magnetite, and hydrogen (McCollom TM and Seewald JS, 2013; Worman SL et al., 2016). The reaction rate of serpentinization is fastest at temperatures between 200°C and 310°C (Mccollom T and Bach W, 2009). The reaction rate will slow down to the above temperature range (McCollom TM and Donaldson C, 2016). Mantle rocks also undergo serpentinization. At low temperatures (<100°C), hydrogen generation is related to the spinel reactant and the cubic crystal structure. The amount of hydrogen produced by serpentinization is directly related to the proportion of reduced iron in the reactants, such as olivine or pyroxene with different Fe/Mg ratios, and the conversion to ferric iron (Fe³⁺) (McCollom TM and Seewald JS, 2013; Murray J et al., 2020). The key factor for serpentinization is a strong reducing condition, that is, a setting below the low oxygen fugacity/high hydrogen activity associated with fayalite-magnetite-quartz solutions (Orcutt B et al., 2019). Serpentinization of ultramafic rocks typically produces hydrogen with a higher content than basalt-water reaction (Hao Y et al., 2020). Serpentinization may only occur in the shallow part of the crust, that is, at a depth of about 4 km to 6 km in the upper crust, corresponding to the 400°C isotherms (Andreani M et al, 2007), but the optimal temperature for serpentinization may correspond to a depth of 10 km to 12 km (Donze FV et al., 2020).

(iii) Radiolysis of water

The radiolysis of water in the lithosphere is another important mechanism for the generation of natural hydrogen. A large number of radioactive elements such as uranium, thorium, and potassium in the Earth’s crust can release alpha, beta, and gamma rays during radioactive decay, and the releasing energy decomposes water molecules into oxygen and hydrogen. It is worth noticing that compared with pure water, brine can generate more hydrogen (Wang W et al., 2019). Previous experimental studies show that the oxidant generated during radiolysis of water is mainly hydrogen peroxide, which rapidly decomposes to oxygen and the oxygen content can be as high as 30% to 35% (Вовк И, 1979). But this is not true. For example, pure hydrogen is detected from the fluid inclusions in quartz of the Oklo Precambrian uranium deposit in Gabon by Raman spectroscopy (Dubessy J et al., 1988). High levels of hydrogen are systematically detected in potash deposits, and researchers generally believe that hydrogen may be generated by the radiolysis of water due to the radioactive decay of potassium and rubidium (Parnell J and Blamey N, 2017). However, the observed hydrogen/argon ratio and the carnallite/potashite hydrogen content ratio do not support this view (Вовк Н, 1978). Therefore, the radiolysis of water is by no means a single reaction, but a series of complex and interrelated reaction processes. Researchers seem to forget or ignore another factor when explaining the origin of hydrogen, that oxidizing chemicals and hydrogen are produced at the same time. If the amount of hydrogen is very large, it is unlikely that the hydrogen is produced by the radiolysis of water. In fact, studies have shown that radioactive decay is not sufficient to generate the measured amount of hydrogen (Zgonnik V, 2020).

2.1.2   Biogenesis

Biological activity is often used to explain the origin of natural hydrogen in natural gas samples, which is generated by the anaerobic decomposition of organic matter, fermentation, and nitrogen-fixing bacteria (Morita RY, 1999). In nature, hydrogen-generating microorganisms coexist with hydrogen-consuming microorganisms, and all biogenesis hydrogen is rapidly converted into other compounds (Gregory SP et al., 2019). Hydrogen-generating microorganisms cannot exist without hydrogen-consuming microorganisms, because hydrogen inhibits the former’s activity (Hoehler TM, 2005). There are two possible mechanisms for hydrogen decomposition: Microbial decomposition and soil decomposition (Constant P et al., 2010). The rate of hydrogen generation is negligible in soil compared to the rate of hydrogen consumption. The rate of hydrogen generation is very fast in wetland, but the generated hydrogen is immediately converted to methane by methanogenic bacteria. Hydrogen is mainly generated by nitrogen-fixing bacteria in dryland under an aerobic condition, and then consumed by abiotic enzymes (Conrad R. 1996). The amount of hydrogen generated by the fermentation of organic matter is not much (Lin LH et al., 2005).

Aromatization of residual kerogen is gradually enhanced during sedimentary metamorphism, which generates large amounts of methane but also some hydrogen. Alkanes and methane are pyrolyzed at temperatures 200°C and above 500°C, respectively, resulting in hydrogen release (Boreham CJ et al., 2021). However, at such high temperatures in nature, the generated hydrogen may be immediately consumed in the reaction with oxygenates to become water that is more thermodynamically stable (Zgonnik V, 2020).

2.2   Classification of natural hydrogen by its occurrence state

2.2.1   Free hydrogen

Free hydrogen refers to hydrogen that can freely migrate in the pores or fractures of rocks (or formations), which is one of the main forms of natural hydrogen. It was first discovered near the city of Antalya in Turkey, where leaking gas continues to burn and these active vents have existed for at least 2500 years (Hosgörmez H et al., 2008; Zgonnik V, 2020). About two centuries ago, similar leaks were found in the Philippines, where the gas burned with flame, and the measured hydrogen content remain stable to be 41.4% to 44.5% (Vacquand C, 2011). At present, researchers have monitored nature hydrogen from ophiolites, Precambrian rocks, igneous rocks, volcanic gases, hydrothermal systems like geysers and hot springs, tubular kimberlites, ore bodies, oil and gas fields, coal basins, sedimentary rocks and rock-salt deposits from nearly 30 countries. It is found that the measured free hydrogen content is different and varies widely in different geological settings, in different areas of the same geological setting, and at different depths, with the minimum content being only a few digits and the maximal content reaching more than 90%.

2.2.2   Hydrogen in inclusions

It has been found that hydrogen is mainly presented as a trapped gas in the form of inclusions or adsorption in different types of rocks (Giardini AA et al., 1976; Jiang FLJ and Li GR, 1981). Like free hydrogen, hydrogen in inclusions is found in ultrabasic rocks, Precambrian rocks, igneous rocks, volcanic rocks, tubular kimberlites, ore bodies, coal basins, sedimentary or metamorphic rocks, and halite samples from more than ten countries around the world. The results show that the hydrogen content in these inclusions is not identical and varies from 0.2% to 100%.

2.2.3   Dissolved hydrogen

Natural hydrogen as a dissolved gas in groundwater has been observed in many cases. For example, a study on the occurrence of hydrogen in subsurface fluids by former Soviet Union’s researchers compiled distribution maps of hydrogen anomalous based on more than 2000 analysis results (Shcherbakov AV and Kozlova ND, 1986). It is shown that the values are higher in areas associated with deep faults and rift zones (Щербаков А, 1985). Hydrogen has also been found in water samples or water samples in oil and gas fields from more than ten countries. The results show that the hydrogen content in groundwater varies from trace to tens of percent, and increases with the sampling depth (Нечаева О, 1968).

3.   Global discovery of natural hydrogen and resource estimation

3.1   Global discovery of natural hydrogen

As early as 1888, a publication analyzed the gas composition of hydrogen samples leaking from coal seam cracks in a coal mine near the city of Makiivka, Ukraine (Менделеев Д, 1888). However, over the past more than 100 years, though there was much literature on the discovery of hydrogen in different regions, it was not until 2020 that Zgonnik published a paper that reviewed a total of 331 onshore hydrogen analysis data. It is found that natural hydrogen is widely distributed around the world. Hydrogen is found in the Americas, Europe, Asia, Africa, Oceania, and other regions, but its content varies between 1% and 100% in different regions and geological settings (Fig. 1a; Smith NJP, 2005; Zgonnik V, 2020). This indicates that the “hydrogen system” is complicated and more investigation and research are needed.

Figure  1.  Global discovery of natural hydrogen (modified from Zgonnik V, 2020; Nguyễn AĐ and Phan NT, 2021; Tian QN, 2022). a‒hydrogen content detected in various settings; b‒distribution of hydrogen detected according to different regions; c‒distribution of hydrogen detected according to different settings.

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It can be found in Fig. 1b that on a global scale, most hydrogen discoveries are in the former Soviet Union, with 223 locations; followed by North America and Europe. However, this does not mean that natural hydrogen is most abundant in these areas, but only suggests that the hydrogen exploration activities in these areas start relatively earlier and are more active. To some extent, it also suggests that the theory of the abiotic origin of oil and gas is popular in the former Soviet Union.

As is shown in Fig. 1c, free hydrogen is found in ore bodies, ophiolite, sedimentary rocks, volcanic settings, oil and gas fields, coal basins, geysers and hot springs, Precambrian rocks, igneous rocks, rifts, and kimberlites. Free hydrogen is frequently found in ore bodies, ophiolite and sedimentary rocks worldwide, with the highest hydrogen content, detected at over 90%. Natural hydrogen is also found in mining areas of iron, gold, uranium, mercury, nickel, copper, and polymetallic ores. The gas in the ophiolite complex can be divided into four types, hydrogen-rich gas, nitrogen-rich gas, nitrogen-hydrogen-methane mixed gas, and hydrogen-methane mixed gas, with the hydrogen content varying according to the generation area. Natural hydrogen is much abundant in sedimentary basins, which may be due to the extensive drilling during the exploration of oil and gas in those areas, just as in the case of the first hydrogen production well in Mali. Increased hydrogen content in soil is also found in areas where earthquakes and volcanic events occur. Hydrogen can be abundant enough to exceed the oxidizing capacity of fluids or rocks in volcanic settings and can be associated with hydrocarbon reservoirs and coal (Zgonnik V, 2020). The content of free hydrogen in the above settings varies from trace to tens of percent, with the highest value reaching more than 90%.

Hydrogen in inclusions is found in igneous rocks, ore bodies, coal basins, Precambrian rocks, volcanic rocks, salt rock deposits, kimberlites, sedimentary rocks, ultrabasic rock, etc., which is most frequently reported as inclusions in igneous rocks and ore body while is rarely reported in Kimberlites, sedimentary rocks, and mafic rocks worldwide. Hydrogen in ultrabasic rock inclusions is generated by the water-rock reaction and is always present in a diamond as inclusions (Smith EM et al., 2016). Hydrogen is also found in gold ores and exists as trapped gas with extremely high content in gold-bearing dikes. Similar to free hydrogen, the hydrogen in inclusions is not uniform in content but is with a large variation range.

Dissolved hydrogen in groundwater is the most frequently reported, with 54 locations. Hydrogen detection results from the former Soviet Union show that higher hydrogen content is observed in areas related to tectonic activities, which is especially significant when these areas are associated with deep faults and rift zones (Shcherbakov AV and Kozlova ND, 1986). The percentage of hydrogen in the groundwater ranges from 0.05% to 76%, increasing with the sampling depth.

3.2   Global natural hydrogen resources estimation

Since 1983, researchers have successively estimated the amount of global natural hydrogen. It has been estimated by a paper published in 2020 that natural hydrogen is (254±91)×10⁹ m³/a, atmospheric hydrogen is (459±119)×10⁹ m³/a, and biological hydrogen is (412±106)×10⁹ m³/a (Zgonnik V, 2020). The amount of natural hydrogen is relatively small, which is about half of the atmospheric hydrogen. It has been acknowledged that soil can absorb atmospheric hydrogen and becomes the main hydrogen sink, which is about an order of magnitude higher in volume than the hydrogen utilized by microbes in the deep earth, the hydrogen dissolved in water, and the hydrogen consumed by abiotic reactions (Fig. 2; Boreham CJ et al., 2021).

Figure  2.  Estimation of hydrogen generation and hydrogen sink consumption in nature (modified from Zgonnik V, 2020; Nguyễn AĐ and Phan NT, 2021).

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4.   Global exploration and development status of natural hydrogen

In addition to the accidental discovery of natural hydrogen in Mali, Africa, many countries around the world such as Australia, the United States, Brazil, and France have started to explore natural hydrogen. Their efforts include using satellite images to identify circular and elliptical extremely shallow depressions (“fairy circles”) where natural hydrogen leaks to the surface (Fig. 3), they also conducted field hydrogen measurements, geochemistry, and geophysical survey in these areas.

Figure  3.  Circular, elliptical depressions (“fairy circles”) where natural hydrogen leaks to the surface (after Larin N et al., 2015; Zgonnik V et al., 2015; Prinzhofer A et al., 2018, 2019; Frery E et al., 2021; Halas P et al., 2021). a‒Mali; b‒North Perth Basin, Western Australia; c‒Carolina, USA; d‒São Francisco Basin, Brazil; e‒South Gironde, France; f‒The center of Russia.

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4.1   Mali: Natural hydrogen for small-scale power generation

In 1987, an explosion accident during drilling for water resources in the Bourakebougou area of Mali lead to the accidental discovery of hydrogen with a purity of 98%. In 2012, a Canadian company Petroma (now renamed Hydroma) began to explore hydrogen in that area for power generation, which transformed much previous cognition. Hydroma conducted a series of explorations around the “1987 well”, observed surface features to discover “fairy circles”, and measured hydrogen emissions from those surface structures (Fig. 3a). From 2017 to 2018, the company drilled a total of 24 wells in its mining blocks, in addition to conducting extensive geological, geophysical and geochemical studies. The results show that relatively pure hydrogen reservoirs are associated with trace amounts of methane, nitrogen, and helium. The occurrence of hydrogen in the formation is related to the existence of multiple layers of dolerite beds and aquifers. The natural hydrogen production well in Mali is unique in the world, and it proves the non-fossil origin of hydrogen and presents the characteristics of energy sustainability. It has been evaluated that natural hydrogen production in Mali is 2‒10 times cheaper than hydrogen production by fossil fuels or industrial electrolysis (Prinzhofer A et al., 2018). Mali, as the first pioneer country to use natural hydrogen, has shown a good demonstrating effect.

4.2   Australia: Conducted comprehensive assessment and issued the licence for natural hydrogen exploration

About 1000 natural gas samples from 470 wells were studied by Geoscience Australia in 2021 for compositional and isotopic analyzes of hydrogen. Geoscience Australia also estimated the amount of hydrogen produced by water radiolysis and serpentinization, concluding that hydrogen resources at a depth of less than 1 km are about 1.6×10⁶ m³/a to 58×10⁶ m³/a. Moreover, the Department for Energy and Mines (DEM) of South Australia has begun deploying projects to find natural hydrogen in South Australia and has issued permits for hydrogen exploration. The Golden Hydrogen company established in 2021 was granted a license by the DEM for the exploration, assessment, and development of natural hydrogen in the permitted areas after the company provided evidence of natural hydrogen up to 90% purity found on Kangaroo Island and southern York Peninsula. In addition, surface features reflecting subsurface hydrogen occurrences such as circular and elliptical depressions were recognized through satellite images in Pingrup, Western Australia, southern Western Australia, and southern South Australia (Fig. 3b), indicating that hydrogen may present deep down (Boreham CJ et al., 2021). Additional geophysical surveys and interpretations have thus been carried out in these potential areas.

4.3   United States: Drilled its first natural hydrogen well

American CFA Petroleum Company drilled the Scott well in the North American Rift System in 1982 and discovered hydrogen with about 50% content (Goebel ED et al., 1983). The hydrogen content of wells in that area could still reach more than 30% in 1987 (Coveney JRM et al., 1987). In 2013, a company specializing in hydrogen exploration, Natural Hydrogen Energy LLC, was established in the United States and began to search for hydrogen leaking points in many countries. This company discovered large hydrogen streams with an estimated production reaching up to several tons per day in many locations in the United States in 2015. The first natural hydrogen well was drilled in Kansas at the end of 2019 (Moretti I and Webber ME, 2021). One well in this area was sampled from 2008 to 2011. The well drilled through about 424 m of the Paleozoic sedimentary strata and about 90 m of the underlying Precambrian basement, where the highest hydrogen content reached about 91% (Guélard J at al., 2017). In addition, a study of soil gas detection in Carolina, USA, found that a large amount of hydrogen was leaking from the surface depressions (Fig. 3c). It is inferred that the hydrogen migrates from the deep to the surface, and the rocks along the migration channels are altered to form circular or elliptical subsidence depressions on the surface (Zgonnik V et al., 2015).

4.4   Brazil: Continuous monitoring of natural hydrogen

Seven continuous gas monitoring analyzers were placed in the São Franciso Basin, Brazil, in 2018 to monitor hydrogen and to assess the hydrogen leakage from the “fairy circles” (Fig. 3d). The pulsed leakage of natural hydrogen was found to be associated with the “fairy circles” (Prinzhofer A et al., 2019; Moretti I et al., 2021). The hydrogen content monitored in this area varies from 0.004% to 0.020%. The researchers believe that there may be a natural hydrogen generation source in the deep part of the study area (Prinzhofer A et al., 2019). Simultaneous geophysical interpretations suggest the existence of deep faults, a basement containing radioactive elements and mafic and ultramafic rocks, and probably a karst reservoir. As a result, a large amount of hydrogen is released from the radiation of the abundant water in the deep, which migrates along the faults and accumulates into the reservoir (Donze FV et al., 2020).

4.5   Europe: Search for natural hydrogen has been carried out

(i) France: The CVA group identified hydrogen leakage on the western edge of the Bresse Graben in France in January 2020 (CVA 2022). The results obtained by 45-8 Energy in these areas in July 2020 show a clear hydrogen anomaly and a possible active leakage, thus proving the presence of natural hydrogen in France. 45-8 Energy is now looking for helium and hydrogen, which normally coexist in the subsurface. The company has identified various areas of high potential for natural hydrogen throughout Europe and has applied for exploration licenses (45-8 Energy, 2022). In 2020, French researchers conducted a natural hydrogen survey in the western foothills of the Pyrenees to explore the migration mechanism of natural hydrogen in an area with no record of surface gas leakage yet. This work identified several key areas with high gas content along the northern part of the Mauléon Basin. The results suggest that natural hydrogen may originate from serpentinization of mantle rocks, followed by hydrogen migration along major thrust fault zones (Lefeuvre1 N et al., 2021). Hydrogen measurements are also carried out near the depression in the Gironde province (Fig. 3e).

(ii) Russia: About 562 near-circular surface depressions with hydrogen leakage were monitored in Russia between 2005 and 2011. Near-circular surface depressions vary in diameter from 100 m to several km. Typically, these depressions are surrounded by white soil and abnormal plant growth. The core of these depressions is often a wetland marsh or a lake. According to an estimation of the composition of near-surface soil gas, the amount of hydrogen leaking from these depressions to the surface is about 24000 m³ per day (Fig. 3f; Larin N et al., 2015).

(iii) Spain: Ascent Funds Management LLC in the United States signed an agreement with Helios Aragon, Spanish hydrogen and helium exploration company, to develop “gold hydrogen”, that is subsurface natural hydrogen, aiming to open the door to Spain’s “hydrogen center”. Helios Aragon owns a gas exploration license covering an area of 89000 hm² in northern Spain, planning to explore natural hydrogen through wells originally drilled for hydrocarbon exploration. One well found significant amounts of hydrogen 3680 m below the surface. Helios Aragon collected more data to confirm this (Fuelcellsworks, 2020).

4.6   China: Natural hydrogen leakage is found in some areas

China has not conducted substantial exploration for natural hydrogen, and only found some good presence of hydrogen during a general survey of carbon dioxide gas reservoirs in the Shangdu Basin, Inner Mongolia. Based on various data incorporating geological, drilling, logging, isotopic composition analysis, etc., a comprehensive study of the origin of hydrogen and a preliminary analysis of its accumulation pattern were carried out (Li YH et al., 2007). In addition, the hydrogen content in natural gas in some oil and gas wells in the Jiyang Depression was analyzed, and the origin of hydrogen was preliminarily classified (Meng QQ et al., 2014, 2021). Other exploration activities on the hydrogen discovery are focused on the use of hydrogen as a sensitive indicator for fault movements and earthquakes, or use for theoretical research on the abiotic origin of natural gas (Shangguan ZG and Huo WG, 2001; Yu HM et al., 2005; Zhou Q et al., 2007; Shuai YH et al., 2010; Li JY et al., 2019; Kang J et al., 2020).

5.   Exploration and development potential of global natural hydrogen

Since the accidental discovery of natural hydrogen in Mali and the use of natural hydrogen for small-scale power generation, many countries have started to explore natural hydrogen as independent energy in recent years. It is worth noting that the following aspects have fully demonstrated the potential of natural hydrogen for future exploration and development.

5.1   Natural hydrogen is widely distributed

Natural hydrogen-enriched places have been found in more than 30 countries around the world. The leaked natural hydrogen from the subsurface has been detected with than 90% of hydrogen in Mali, Oman, the United States, Japan, Russia, Germany, China, and other countries. This suggests that the generation of natural hydrogen is on a global scale. At present, Australia, Brazil, France, and other countries have conducted extensive exploration work in areas with natural hydrogen leakage, to delineate the target area of natural hydrogen. Hydrogen-generating regions have also been found in China, such as in the Shangdu Basin of the Inner Mongolia Autonomous Region (Li YH et al., 2007), the Sanhu area in the Qaidam Basin (Shuai YH et al., 2010), the Tengchong Rehai geothermal area (Shangguan ZG and Huo WG, 2001), and the northern segment of the Yilan-Yitong fault (Kang J et al., 2020). Further detailed exploration in these areas may achieve promising results.

5.2   Natural hydrogen occurs in various geological settings

Hydrogen has been detected in a variety of geological settings and rock types. Hydrogen-rich gases are found in ophiolite, rift zones, faults, volcanoes, geysers, hot springs, and so on. Ore bodies can be the main source of hydrogen-rich gas; coal seams have a high capacity for hydrogen storage; salt rock can also store a large amount of hydrogen and can act as a good cap at the same time. Oil and gas fields usually do not contain high content of hydrogen, and hydrogen-rich gases are mainly outside the oil and gas exploration target areas in sedimentary basins. These characteristics provide a clear guide for the exploration of natural hydrogen resources in the future.

5.3   Natural hydrogen mining has just started

The total amount of hydrogen on the Earth is currently estimated to be (254±91)×10⁹ m³/a. The actual development and utilization of natural hydrogen are only in the Bougou 1 well (hydrogen content is 98%) in the Bourakebougou area of Mali. The hydrogen production rate of this well was 1.5×10³ m³/d in 2012 (Briere D et al., 2016), and the well is still in operation so far. The natural hydrogen occurrence is relatively small in oil and gas fields, so even the 20×10⁹ m³ of natural hydrogen in the world’s proven natural gas fields are fully extracted (Boreham CJ et al.,2021), it is still less than 10% of the total natural gas resources. It is worth noticing that the current estimation of natural hydrogen is not a proven reserve but an over underestimated value, though it has already indicated the great potential of natural hydrogen that can be extracted from the subsurface. Moreover, the continuous leakage of natural hydrogen can last for decades or even thousands of years, indicating that natural hydrogen is a sustainable energy.

5.4   Previous efforts on exploration, development, and utilization natural hydrogen

The current exploration strategy of natural hydrogen has many similarities with the early oil and gas industry, where oil and gas exploration in the 19^th century started from drilling at the surface leaking points until oil and gas were found. There now has been a shift from reliance on surface indicators to subsurface imaging techniques. The recent natural hydrogen exploration in Australia, France, and Brazil is to explore a set of techniques for natural hydrogen exploration, to further evaluate the production, migration, and trap system of natural hydrogen, to delineate the exploration target area, and to lay the foundation for accelerating the development of new energy. Mali has explored natural hydrogen as an independent resource and uses it to generate electricity, which demonstrates the possibility of extracting natural hydrogen directly from the subsurface and using it for power generation. This example also shows that the production of natural hydrogen is much cheaper than the production of hydrogen by other methods. At present, hydrogen energy has become a new international focus, and the infrastructure and technology for hydrogen energy have also developed rapidly. The produced natural hydrogen can utilize the existing infrastructure for storage and transportation, or it can be directly transported to the natural gas pipelines.

6.   Conclusions

(i) Hydrogen energy is considered to be important clean energy in the future, and especially under the dual-carbon goal, there are increasing application scenarios and demand for hydrogen, which makes it more important. At present, 96% of hydrogen production is from fossil energy, and the production process will generate a large number of carbon wastes; while hydrogen production by electrolysis of water is relatively expensive and is less used by industry. In this context, the future exploration and development of unconventional low-carbon or zero-carbon natural hydrogen are of great significance. By discovering potential areas with high hydrogen content and reducing the risk of its exploration, natural hydrogen may play an active role in future hydrogen economy construction.

(ii) Natural hydrogen is distributed in various geological settings, and its leakage has been found in nearly 100 locations on the seabed and land worldwide. At present, natural hydrogen exploration has been carried out in Mali, Australia, the United States, Brazil, Europe, and some European countries, and many natural hydrogen potential areas have been discovered. Mali has developed and utilized natural hydrogen for power generation in nearby villages. The Mali Natural Hydrogen Field has demonstrated for the first time the potential to drill and extract natural hydrogen directly from the subsurface, providing confidence for future exploration and development of natural hydrogen as independent energy.

(iii) Natural hydrogen is a potential low-carbon or zero-carbon energy and how to find its enrichment area becomes the key. There are significant differences in the accumulation pattern between natural hydrogen and traditional hydrocarbon energy, which requires geologists to transform their focus from the exploration and development of traditional solid and liquid mineral resources to gaseous resources, by developing new exploration workflows, adopting and improving techniques incorporating geological, remote sensing, geochemical and geophysical methods suitable for natural hydrogen surveys, and carrying out extensive exploration work. It can be expected more natural hydrogen fields like Mali can be discovered in the future.