# Understanding the resource potential of natural hydrogen on Earth: Scientific gaps, uncertainties and recommendations

Giuseppe Etiope, Geoffrey S Ellis, Omid H Ardakani, Christopher J Boreham, Peter Klitzke, Antonio Martín-Monge, Humberto L.S. Reis, Alexis S Templeton, Hyeong Soo Kim, Eric Gaucher, et al.

► To cite this version:

Giuseppe Etiope, Geoffrey S Ellis, Omid H Ardakani, Christopher J Boreham, Peter Klitzke, et al. Understanding the resource potential of natural hydrogen on Earth: Scientific gaps, uncertainties and recommendations. Earth-Science Reviews, 2026, 275, pp.105413. 10.1016/j.earscirev.2026.105413. hal-05507708

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## Review Article

# Understanding the resource potential of natural hydrogen on Earth: Scientific gaps, uncertainties and recommendations

Giuseppe Etiope<sup>a,b,**</sup>, Geoffrey S. Ellis<sup>c</sup>, Omid H. Ardakani<sup>d,e,*</sup>, Christopher J. Boreham<sup>f</sup>, Peter Klitzke<sup>g</sup>, Antonio Martín-Monge<sup>h</sup>, Humberto L.S. Reis<sup>i,j</sup>, Alexis S. Templeton<sup>k</sup>, Hyeong Soo Kim<sup>l</sup>, Eric Gaucher<sup>m</sup>, Olivier Sissmann<sup>n</sup>

## ARTICLE INFO

### Keywords:

Natural hydrogen

Source rocks

Migration

Reservoirs

Surface exploration

Uncertainties

## ABSTRACT

A comprehensive scientific research roadmap is essential to bridge knowledge gaps and deepen the understanding of key geological, geochemical, and geophysical aspects of natural hydrogen (H₂) as a potential new energy resource. This paper reviews major scientific uncertainties on natural H₂, suggesting research priorities, as a guide for defining exploration strategies, techniques, and data interpretation. The uncertainties concern all phases of the natural H₂ cycle, from generation (source rocks) through migration (advection and diffusion) and accumulation (reservoir and cap rocks) to the application and interpretation of subsurface and surface geochemical and geophysical exploration techniques. Understanding H₂ sources and generation rates (the amount of H₂ generated by a given volume of rock over time) is crucial for determining whether a geological H₂ system operates as a short-term dynamic system with rapid H₂ production and release, or as a conventional gas system with long-term accumulations, analogous to petroleum reservoirs. Preliminary estimates for serpentinitation, radiolysis, and organic matter degradation suggest that H₂ generation is not inherently fast, especially for non-hydrothermal continental systems (crystalline basement of shields, ophiolites, peridotite massifs, sedimentary basins), and long-term accumulations, like those of fossil natural gas systems, represent the most likely scenario. The mechanisms of H₂ migration through geological formations require application of fundamental principles of fluid-flow physics, distinguishing advection and diffusion, as well as their forms (from gas-phase, bubble flows to aqueous solutions). Additional studies of H₂ accumulation and retention in subsurface reservoirs could improve understanding of mechanisms of H₂ migration by focusing on the rock fluid-bearing properties and the factors affecting H₂ preservation, such as the presence of cap rocks impermeable to H₂, pressure conditions, residence times, and microbial or abiotic consumption. Advanced techniques, including reservoir modelling, flow simulations, 3D imaging (micro-CT) of H₂-bearing rocks, and extraction and analysis of gas occluded in rocks, can provide insights into the stability and potential recoverability of H₂ accumulations. The interpretation of surface exploration techniques, including gas geochemistry, geophysics, and remote sensing, long employed in mineral and energy resource exploration, is now being adapted for natural H₂ studies,

<footnote><sup>a</sup> Istituto Nazionale di Geofisica e Vulcanologia, Roma, Italy</footnote>

<footnote><sup>b</sup> Faculty and Environmental Science and Engineering, Babes-Bolyai University, Cluj-Napoca, Romania</footnote>

<footnote><sup>c</sup> U.S. Geological Survey, Denver, CO, USA</footnote>

<footnote><sup>d</sup> Natural Resources Canada, Geological Survey of Canada, Calgary, Canada</footnote>

<footnote><sup>e</sup> Department of Earth, Energy, and Environment, University of Calgary, Calgary, Canada</footnote>

<footnote><sup>f</sup> Geoscience Australia, Canberra, Australia</footnote>

<footnote><sup>g</sup> Federal Institute for Geosciences and Natural Resources (BGR), Hannover, Germany</footnote>

<footnote><sup>h</sup> Repsol Exploración, S.A, Madrid, Spain</footnote>

<footnote><sup>i</sup> Dep. de Ciencia da Computação/Instituto de Geosciencias, Universidade Federal de Minas Gerais, Belo Horizonte, MG, Brazil</footnote>

<footnote><sup>j</sup> HsR Consulting Energy and Geosciences, Belo Horizonte, MG, Brazil</footnote>

<footnote><sup>k</sup> University of Colorado, Boulder, CO, USA</footnote>

<footnote><sup>l</sup> Department of Earth and Environmental Sciences, Korea University, Seoul, Republic of Korea</footnote>

<footnote><sup>m</sup> Lavoisier H₂ Geoconsult, Chamonix, France.</footnote>

<footnote><sup>n</sup> IFP Energies Nouvelles, Rueil-Malmaison, France.</footnote>

<footnote>* Corresponding author at: Department of Earth, Energy and Environment, University of Calgary, Canada.</footnote>

<footnote>** Corresponding author at: Istituto Nazionale di Geofisica e Vulcanologia, Roma, Italy.
E-mail addresses: giuseppe.etiope@ingv.it (G. Etiope), omid.haeriardakani@ucalgary.ca (O.H. Ardakani).</footnote>
but challenges remain in the data interpretation. Distinguishing H₂ seepage due to geological degassing from H₂ produced near the surface by modern microbial processes or artificial sources, such as hammering or drilling for soil-gas sampling, drilling into aquifers, and corrosion in boreholes, is an essential step in exploration. The simple detection of H₂ in soils, even in morphological structures like sub-circular depressions or “fairy circles”, cannot be cursorily interpreted as a signal of natural H₂ seepage from a deep source. A holistic geochemical approach, including isotopic analyses of gases associated with H₂, is recommended to distinguish among the variety of possible H₂ origins. Observations of H₂ in wells should be interrogated to rule out possible artifacts such as corrosion and drill bit metamorphism. The integration of multiple geophysical methods, including seismic, gravimetric, magnetic, and electro-magnetic surveys, is recommended to mitigate interpretation ambiguities regarding the structure of a subsurface H₂ system (source and reservoir rocks, including fluid and gas storage), due to the non-uniqueness of rock-specific physical properties.

Contents

<table><tbody><tr><td>1. Introduction and definitions</td><td>2</td></tr><tr><td>2. H<sub>2</sub> sources, generation rates and systems</td><td>3</td></tr><tr><td>2.1. H<sub>2</sub> source rocks and generation mechanisms</td><td>3</td></tr><tr><td>2.1.1. Present knowledge</td><td>3</td></tr><tr><td>2.1.2. Scientific gaps, uncertainties and recommendations</td><td>3</td></tr><tr><td>2.1.2.1. Oxidation of Fe-rich rocks (Fe-silicate serpentinisation, Fe-carbonates and Fe-oxides)</td><td>3</td></tr><tr><td>2.1.2.2. Radiolysis</td><td>4</td></tr><tr><td>2.1.2.3. OM thermal decomposition</td><td>4</td></tr><tr><td>2.1.2.4. Mechanoradical reactions</td><td>4</td></tr><tr><td>2.1.2.5. Mantle and primordial sources</td><td>4</td></tr><tr><td>2.1.2.6. General issues</td><td>4</td></tr><tr><td>2.2. H<sub>2</sub> generation rates</td><td>5</td></tr><tr><td>2.2.1. Present knowledge</td><td>5</td></tr><tr><td>2.2.2. Scientific gaps, uncertainties and recommendations</td><td>6</td></tr><tr><td>2.3. H<sub>2</sub> systems: short-term, fast generation versus long-term, slow generation</td><td>6</td></tr><tr><td>2.3.1. Present knowledge</td><td>6</td></tr><tr><td>2.3.2. Scientific gaps, uncertainties and recommendations</td><td>7</td></tr><tr><td>3. Understanding H<sub>2</sub> migration mechanisms</td><td>7</td></tr><tr><td>3.1. Present knowledge</td><td>7</td></tr><tr><td>3.1.1.1. Diffusion forms</td><td>8</td></tr><tr><td>3.1.1.2. H<sub>2</sub> consumption and retention during migration</td><td>8</td></tr><tr><td>3.2. Scientific gaps, uncertainties and recommendations</td><td>9</td></tr><tr><td>4. H<sub>2</sub> accumulation and retention in reservoir rocks</td><td>9</td></tr><tr><td>4.1. Lithology and permeability of H<sub>2</sub> reservoirs and seals</td><td>9</td></tr><tr><td>4.1.1. Present knowledge</td><td>9</td></tr><tr><td>4.1.2. Scientific gaps, uncertainties and recommendations</td><td>9</td></tr><tr><td>4.2. H<sub>2</sub> preservation in accumulations: Loss via biotic and abiotic reactions</td><td>10</td></tr><tr><td>4.2.1. Present knowledge</td><td>10</td></tr><tr><td>4.2.2. Scientific gaps, uncertainties and recommendations</td><td>10</td></tr><tr><td>4.3. Reservoir and basin modelling</td><td>10</td></tr><tr><td>4.3.1. Present knowledge</td><td>10</td></tr><tr><td>4.3.2. Scientific gaps, uncertainties and recommendations</td><td>11</td></tr><tr><td>4.4. The problem of artificial H<sub>2</sub> generated by corrosion in wells intercepting reservoirs and aquifers</td><td>11</td></tr><tr><td>5. Interpretation of surface gas geochemistry</td><td>11</td></tr><tr><td>5.1. General concepts and definitions</td><td>11</td></tr><tr><td>5.2. Non-geological H<sub>2</sub> in surface or near surface environments</td><td>11</td></tr><tr><td>5.2.1. Present knowledge</td><td>11</td></tr><tr><td>5.2.2. Scientific gaps, uncertainties and recommendations</td><td>12</td></tr><tr><td>5.3. The problem of the H<sub>2</sub> isotopic composition</td><td>12</td></tr><tr><td>5.3.1. Present knowledge</td><td>12</td></tr><tr><td>5.3.2. Scientific gaps, uncertainties and recommendations</td><td>13</td></tr><tr><td>5.4. The use of helium and radon to support H<sub>2</sub> origin's interpretation</td><td>13</td></tr><tr><td>5.4.1. Present knowledge</td><td>13</td></tr><tr><td>5.4.2. Scientific gaps, uncertainties and recommendations</td><td>13</td></tr><tr><td>5.5. Reliability of commercial portable H<sub>2</sub> sensors and sample containers</td><td>13</td></tr><tr><td>5.5.1. Present knowledge</td><td>13</td></tr><tr><td>5.5.2. Scientific and technical requirements, and recommendations</td><td>14</td></tr><tr><td>6. Geophysical methods</td><td>14</td></tr><tr><td>6.1. General concepts</td><td>14</td></tr><tr><td>6.2. Seismic methods (reflection, refraction, and passive monitoring)</td><td>14</td></tr><tr><td>6.2.1. Present knowledge</td><td>14</td></tr><tr><td>6.2.2. Scientific gaps, uncertainties and recommendations</td><td>14</td></tr><tr><td>6.3. Gravity surveys</td><td>15</td></tr><tr><td>6.3.1. Present knowledge</td><td>15</td></tr><tr><td>6.3.2. Scientific gaps, uncertainties and recommendations</td><td>15</td></tr></tbody></table>
<table><tbody><tr><td>6.4. Magnetic surveys</td><td>15</td></tr><tr><td>6.4.1. Present knowledge</td><td>15</td></tr><tr><td>6.4.2. Scientific gaps, uncertainties and recommendations</td><td>15</td></tr><tr><td>6.5. Electromagnetic surveys</td><td>16</td></tr><tr><td>6.5.1. Present knowledge</td><td>16</td></tr><tr><td>6.5.2. Scientific gaps, uncertainties and recommendations</td><td>16</td></tr><tr><td>6.6. Radiometric surveys</td><td>16</td></tr><tr><td>6.6.1. Present knowledge</td><td>16</td></tr><tr><td>6.6.2. Scientific gaps, uncertainties and recommendations</td><td>16</td></tr><tr><td>6.7. Well logging and petrophysics</td><td>16</td></tr><tr><td>6.7.1. Present knowledge</td><td>16</td></tr><tr><td>6.7.2. Scientific gaps, uncertainties and recommendations</td><td>17</td></tr><tr><td>6.8. Integrating geophysical and other data</td><td>17</td></tr><tr><td>7. Remote sensing: image analysis and atmospheric measurements</td><td>17</td></tr><tr><td>7.1. Present knowledge</td><td>17</td></tr><tr><td>7.2. Uncertainties</td><td>17</td></tr><tr><td>8. Conclusions</td><td>18</td></tr><tr><td>Funding sources</td><td>18</td></tr><tr><td>Declaration of competing interest</td><td>18</td></tr><tr><td>Acknowledgments</td><td>18</td></tr><tr><td>Data availability</td><td>18</td></tr><tr><td>References</td><td>18</td></tr></tbody></table>

# 1. Introduction and definitions

Natural hydrogen (hereinafter “natural H₂”) is currently the focus of international scientific research and geological exploration due to its potential as a low-carbon energy resource. Current scientific literature provides a considerable array of articles on natural H₂ encompassing case studies from all continents as well as global reviews. Review articles typically embody, with varying degrees of detail, the principal aspects of natural H₂ formation and distribution within the Earth's crust, along with the associated implications for its detection, exploration, and exploitation as an energy resource (e.g., Zgonnik, 2020; Milkov, 2022; Stalker et al., 2022; Patonia et al., 2024; Ballentine et al., 2025; Mao et al., 2025; The Royal Society, 2025; Liu et al., 2026).

This paper aims to move forward by identifying existing scientific gaps and uncertainties and suggesting future research actions and strategies. A scientific research roadmap is therefore proposed to fill major knowledge gaps and to comprehend essential geological, geochemical, and geophysical aspects of natural H₂. Rather than an exploration roadmap, this work offers a scientific context that suggests research priorities and serves as a guide for the development of exploration strategies, techniques, and data interpretation.

For natural H₂, we consider the definition that is widely adopted in the recent scientific literature addressing H₂ as a potential energy resource: accordingly, natural H₂ (also called “geological”, “native”, “white”, or “gold” H₂) is the hydrogen naturally produced in geological environments (Earth's crust and mantle) by geochemical processes involving naturally occurring inorganic or organic reactants. It is important to clarify that in this sense natural H₂ does not include the H₂ naturally produced by modern microbial activity (e.g., fermentation) or oxidation of metal-rich minerals in the soil or shallow aquifers. This modern H₂, being quantitatively limited and dispersed, is not an exploration target, but its recognition is important in surface geochemical exploration of natural H₂. The terminology referring to the types of H₂ detectable during exploration is summarised in Fig. 1. It includes H₂ that is artificially generated in boreholes (by drilling or corrosion of metal casing) or during soil-gas prospections (by probe drilling or hammering). This work does not address H₂ that can be artificially enhanced or stimulated in crustal rocks (e.g., “orange” H₂, Osselin et al., 2022b).

This paper addresses the following major research topics and critical scientific aspects, which are relevant for natural H₂ exploration and exploitation:

<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Terminology adopted in the present work for the main categories of H₂ detectable near the surface (soil, springs, shallow aquifers) and in the subsoil during natural H₂ exploration. In surface environments, natural H₂ is allochthonous (resulting from fluid seepage), while non-geological H₂ is autochthonous (or migrating from boreholes affected by corrosion). H₂ that can be artificially enhanced or stimulated in crustal rocks is not addressed in this work.</figcaption></figure>
(a) *H*₂ *sources and generation rates* (Section 2). While the types of rocks that can potentially generate H₂ are known, the appropriate conditions and capacities to act as effective H₂ source rocks are poorly defined. Knowing the H₂ generation rates, expressed in terms of amount of H₂ generated by a given volume of rock over time, is fundamental to determine whether a geological H₂ system can be considered as “a dynamic short-term system” or a “conventional” gas system. The first would refer to fast H₂ production and release (which may imply that natural H₂ can be a renewable resource at human time scales); the second refers to H₂ production and accumulation over longer timescales, analogous to petroleum systems.

(b) *H*₂ *migration mechanisms* (Section 3). Once generated, H₂ can migrate within the source rock (primary migration), from source rock to reservoirs, and then to the surface (secondary migration and seepage). The relative importance of the different migration mechanisms (advection vs. diffusion) and forms (H₂ as gas-phase, in microbubbles, or dissolved in water), the correct terminology, and the physical concepts of gas migration in porous and fractured media, as well as H₂ retention along migration pathways, need to be clarified and appropriately applied to natural H₂ exploration.

(c) *H*₂ *accumulation and retention in reservoir rocks* (Section 4). The primary target of natural H₂ exploration is reservoir rocks that, like conventional hydrocarbon fields, may contain substantial volumes of gas that can be economically and safely recovered. Understanding reservoir rocks, together with their sealing rocks, as well as H₂ pressure, residence time, and preservation, is critical to drive H₂ exploration. Here, we address the importance of identifying the lithologies that can act as reservoir rocks, their fluid-bearing properties, and fluid phase behaviour (e.g., H₂ dissolved in an aqueous phase vs. free gas). The balance between the influx of H₂ from the source rock and loss through abiotic and microbiological consumption, reservoir modelling, flow simulations, 3D image analyses of H₂ bearing rocks, and the direct analysis of the gas extracted from rocks are also important study targets.

(d) *Interpretation of data from surface exploration techniques: geochemistry, geophysics and remote sensing* (Sections 5, 6 and 7). Surface geochemical, geophysical, and remote sensing methodologies have been used in mineral and energy resource exploration for decades. These approaches are now being applied to natural H₂ studies and exploration, but many of them have significant limitations and/or offer data with ambiguous interpretations. Unambiguously recognising natural H₂ in surface or near-surface environments (e.g., soils or shallow aquifers) is critical for exploration. Multiple geochemical techniques and concepts to distinguish natural H₂ seepage from H₂ produced microbially (e.g., fermentation near the surface) or artificially (e.g., by drilling, corrosion in wells) should be considered. Geophysical methods, including seismic, gravimetric, magnetic, electromagnetic, and radiometric surveys, each offer distinct advantages and limitations. Integrating these techniques through a multidisciplinary approach is essential to reduce interpretational non-uniqueness and produce more robust, reliable subsurface models.

Each section addresses the "Present knowledge" and the "Scientific gaps, uncertainties, and recommendations" in separate sections. The "Present knowledge" does not aim to provide an exhaustive review of the state-of-the-art, but rather a concise overview of major concepts whose uncertainties are subsequently discussed. Accordingly, essential and representative references are cited. Definitions and new details are provided for certain processes that are not always appropriately considered in the natural H₂ literature (e.g., gas migration mechanisms, the concept of seepage, and H₂ generation rates).

## 2. H₂ sources, generation rates and systems

### 2.1. *H*₂ *source rocks and generation mechanisms*

#### 2.1.1. *Present knowledge*

There are dozens of known geochemical processes that can generate natural H₂ (e.g., Milkov, 2022). However, only a limited number of these are likely to be capable of forming economic H₂ accumulations. The main generation mechanisms are mostly related to water-rock reactions such as (McCollom and Bach, 2009; Sherwood Lollar et al., 2014; Warr et al., 2019; Geymond et al., 2023):

(a) serpentinisation of peridotites (oxidation of Fe-silicate, as olivine and pyroxene hydration),

(b) radiolysis of water in crystalline rocks (through the radioactive decay of potassium, thorium, and uranium-bearing minerals), and,

(c) oxidation of Fe-carbonate (siderite) and Fe-oxides (magnetite, hematite, goethite).

Accordingly, serpentinised peridotites (e.g., dunite, harzburgite), granite and similar radiogenic crystalline rocks, and sedimentary or metamorphic iron-rich rocks (e.g., banded iron formations - BIFs) represent the main potential H₂ source rocks.

The direct analyses of gas occluded in these rocks, either as free gas in microfractures, veins, or fluid inclusions, can inform about their capacity to produce and host natural H₂. A first dataset on H₂ extracted from ophiolitic rocks has shown that serpentinised peridotites contain much more H₂ (from 0.1 to 16.2 mL H₂ $kg_{rock}^{-1}$) than any other ophiolitic rock, i.e., basalts, chromitites and gabbros (Etiope, 2024a). Only chromitites in contact with serpentinised peridotites may have >0.1 mL H₂ $kg^{-1}$, as they may host H₂ produced in the adjacent peridotites. Archean to Proterozoic komatiites, which are the extrusive equivalents of peridotites, have also been suggested as H₂ productive rocks, as studied in H₂-rich hydrothermal environments and cratons (e.g., Ueda et al., 2016; Tamblyn and Hermann, 2023). However, H₂ generation rates and accumulation after such ancient serpentinisation processes are not documented.

Natural H₂ can also be generated by:

(d) thermal decomposition of organic matter (OM) in sedimentary rocks (e.g., Horsfield et al., 2022; Mahlstedt et al., 2022; Kimayim Gaduwang et al., 2024),

(e) mechanoradical reactions along active faults in the presence of water (e.g., Lefeuvre et al., 2025 and references therein),

(f) mantle-primordial sources (e.g., Larin, 1993; Vidavskiy and Rezaee, 2022 and references therein).

The possibility that all these sources may lead to exploitable H₂ deposits is highly uncertain.

#### 2.1.2. *Scientific gaps, uncertainties and recommendations*

#### 2.1.2.1. *Oxidation of Fe-rich rocks (Fe-silicate serpentinisation, Fe-carbonates and Fe-oxides)*
Improved chemo-mechanical models are needed to predict the competing role of micro-fracturing during serpentinisation (which may provide fresh surface materials for maintaining continued reaction) versus precipitation of minerals (which tends to seal the rock and stop the reaction; Osselin et al., 2022b; Lodhia et al., 2024 and references therein). The rate of serpentinisation and associated H₂ generation is likely controlled by the rate of reactive-driven cracking, which will also be sensitive to depth and compressive stresses (Malvoisin et al., 2017).

How iron partitioning among mineral products affects H₂ production rates is poorly understood. There is ample experimental evidence that
the sequestration of Fe(II) in the products of serpentinisation would tend to reduce the amount of H₂ generated (McCollom et al., 2016). However, there is also evidence that this iron, initially partitioned to the serpentinisation by-products such as brucite and serpentine, can become unstable as the reaction progresses and oxidise during later stages of alteration (Bach et al., 2006; Schwarzenbach et al., 2016; Mayhew and Ellison, 2020). Understanding the regulation of Fe distribution and speciation during serpentinisation requires knowledge of the equilibrium distribution of Fe(II) and Fe(III) among mineral phases. Knowledge of the variation in the relative thermodynamic stabilities of product minerals as a function of pressure and temperature (PT), reaction kinetics, and silica and oxygen activity (Frost and Beard, 2007; Evans, 2008; McCollom and Bach, 2009; Kim, 2026) is also quite limited.

### 2.1.2.2. Radiolysis.
Although radiolysis has been widely studied (e.g., Warr et al., 2023, and references therein), the potential consumption of generated H₂ via back reaction with co-generated oxidised species (Lin et al., 2005) or reduction of localised minerals (e.g., iron minerals where redox conditions are modified) needs to be quantified.

### 2.1.2.3. OM thermal decomposition.
H₂ produced from high-maturity OM thermal decomposition has been demonstrated in laboratory experiments under high-temperature open-system conditions (e.g., Horsfield et al., 2022; Mahlstedt et al., 2022; Kimayim Gaduwang et al., 2024), but it has been only partially verified in nature (Suzuki et al., 2017; Suzuki et al., 2024). H₂ is also generally consumed during generation of hydrocarbons from OM; the net amount of H₂ that can be released from hydrocarbon source rocks under subsurface conditions remains unknown and needs specific investigation (Horsfield et al., 2022; Mahlstedt et al., 2022; Mahlstedt et al., 2024; Moretti et al., 2024).

### 2.1.2.4. Mechanoradical reactions.
H₂ production by mechanoradical reactions is a known process, but available data from experiments and models have significant uncertainties in terms of the amount of H₂ that can be generated under natural conditions (e.g., Lefeuvre et al., 2025 and references therein). These reactions need to be modelled, taking into account the lithology, permeability and porosity distribution around the fault zone that can produce H₂ (Lefeuvre et al., 2025).

### 2.1.2.5. Mantle and primordial sources.
With regard to mantle and primordial H₂ sources, some authors proposed that infinite and renewable mantle H₂ is continuously migrating up to the surface (theory of Primordially Hydridic Earth; Larin, 1993; Vidavskiy and Rezaee, 2022). However, our current understanding indicates that the Earth's mantle is not likely to contribute substantial H₂ mass to the continental crust (e.g., Ballentine et al., 2025, and references therein). Whereas studies of noble gas geochemistry suggested the possibility of primordial H₂ derived from the solar nebula and incorporated into the Earth during early planetary accretion, the amount incorporated or retained is unknown (e.g., Mukhopadhyay and Parai, 2019 and references therein). Hydrodynamic escape is widely thought to have resulted in early loss of most, if not all, nebular volatiles, including H₂ (Mukhopadhyay and Parai, 2019).

Thermodynamic equilibrium studies suggest that a large amount of H₂ can exist at deep lower mantle PT conditions, but at depths shallower than about 90 km, hydrogen occurs primarily as H₂O (e.g., Frost and McCammon, 2008; Moine et al., 2020). Migration of H₂ from the lower mantle to the crust requires fluids to exist in a state of disequilibrium throughout the transit of the upper mantle, which would involve unusual circumstances (e.g., rapid fluid migration, incomplete mixing, etc.) and is unlikely to allow for substantial mass transfer. Moreover, the flux of ³He from the mantle to continental crust (~270 mol ³He yr⁻¹) in conjunction with the ³He/H molar ratio in the mantle (~1.14 × 10⁻¹⁰) can constrain the total flux of H from the mantle to a maximum of 2.4 ×

<table><caption>**Table 1**
Orders of magnitude of H₂ generation rates for radiolysis, serpentinisation and thermal decomposition of organic matter (OM).</caption><thead><tr><th>H₂ generation mechanism</th><th>H₂ generation rate (m³ H₂ m<sub>rock</sub><sup>-3</sup> y<sup>-1</sup>)</th><th>Reference</th></tr></thead><tbody><tr><td>Radiolysis</td><td>10<sup>-11</sup> - 10<sup>-10</sup><br>10<sup>-8</sup> (40 °C)<br>10<sup>-4</sup> (80 °C)</td><td>Warr et al. (2019); Warr et al. (2023)<br>Etiope (2024b), combining Leong et al. (2023) and Lamadrid et al. (2021)</td></tr><tr><td>Serpentinisation</td><td>10<sup>0</sup> (140 °C)<br>10<sup>0</sup> (>200 °C)<br>10<sup>-6</sup> (1 °C My<sup>-1</sup>)<br>(av.)</td><td>McCollom et al. (2020)</td></tr><tr><td>OM thermal decomposition</td><td>10<sup>-5</sup> (10 °C My<sup>-1</sup>)<br>(av.)</td><td>This study</td></tr></tbody></table>

10¹² mol yr⁻¹ (2.4 × 10⁶ t H yr⁻¹) (Ballentine et al., 2025). Of that amount only a fraction, if any, is likely to occur as H₂.

An extensive review of H₂ occurrence in volcanic gases (i.e., magmatic fluids) reported H₂ concentrations rarely exceeding 1 mol% (Aiuppa and Moussallam, 2024). The review articles of Zgonnik (2020) and Milkov (2022) report a wider range of H₂ concentration values in volcanic-magmatic and hydrothermal systems, including Mid-Ocean-Ridges (MORs). However, a cursory review of the cited sources reveals that in many cases the high H₂ concentrations in these systems derive from shallow water-rock interactions (e.g., serpentinisation of exhumed peridotites in MORs), and some data refer to artificial H₂ generated during gas sampling and storage in metal canisters (e.g., 57 vol% H₂ from Etna volcano; Zgonnik, 2020). Nonetheless, obtaining clear, unambiguous evidence for mantle-sourced H₂ is difficult and the number of sites that have been rigorously studied is small. Studies of sites with high H₂ concentrations in continental crust in the Canadian Shield, the Fenoscandian Shield, the Kaapvaal Craton South Africa and the Yilgarn craton Australia found no conclusive evidence for a substantial contribution of H₂ of mantle origin (e.g., Sherwood Lollar et al., 1993; Holland et al., 2013; Ballentine et al., 2025). Evidence for limited mantle-derived H₂ in sedimentary gas deposits and continental peridotite settings has been suggested based on the occurrence of mantle-sourced ³He (Guélard et al., 2017; Vacquand et al., 2018; Liu et al., 2025). However, noble gases and H₂ origins are not necessarily coupled, as mixed sources of gases is a commonly observed phenomenon (Etiope et al., 2011; Etiope and Sherwood Lollar, 2013; Boles et al., 2015).

The absence of substantial amounts of H₂ associated with mantle-derived fluids at the surface of the Earth does not preclude the possibility that some H₂ may have migrated from the lower mantle into the lower crust and subsequently been consumed by biologic or abiotic processes while migrating to the surface. The potential for mantle-sourced H₂ in the Earth's crust cannot be completely discounted, but the available evidence indicates that it is quite unlikely that it could form appreciable accumulations.

It would be beneficial to carefully check all literature data on H₂ that is attributed to volcanic-magmatic systems, such as those in the review papers of Zgonnik (2020) and Milkov (2022). The objective is to identify and eliminate H₂ data that do not represent magmatic (mantle) H₂, such as those related to shallow fluid-rock interactions (e.g., in MORs and continental volcanic-geothermal systems hosting or nearby ultramafic rocks) or affected by gas sampling artifacts. The remaining data would offer a more accurate representation of the potential quantities of magmatic H₂ that could migrate to the surface. Additional data should be collected targeting samples and locations best suited to specifically addressing the question of the magnitude of any potential flux of H₂ from the mantle.

### 2.1.2.6. General issues.
Concerning all natural H₂ generation mechanisms, the development of approaches for scaling laboratory
<figure><img src="image_4.png" /><figcaption>**Fig. 2.** Schematic representation of the dynamic, short-term H₂ system, with direct H₂ migration along faults from source rock to the surface or through accumulations rapidly re-charged (left), versus the long-term accumulation system analogous to natural gas systems (right).</figcaption></figure>

experimental results to temporal and spatial geological scales is critically important. Most H₂ generation experiments to date have been performed on small and powdered samples of pure minerals or relatively homogeneous rock samples. The first step will be to conduct more work in core-flood systems (up to meter scale) that better capture the high variation in water/rock ratio that exists between open fractures, microfractures, and the bulk rock, and to quantify the spatial variability in rock alteration and H₂ generation rates. Laboratory results can be incorporated into reactive transport models that produced testable predictions, which can be validated by field studies. To conduct large-scale experiments, it is also essential to refine our understanding of the evolution of Mg-Fe(II)-Fe(III) ratios in hydrous minerals formed or consumed through interactions between ferromagnesian silicates and fluids (H₂O-CO₂) under variable PT conditions during serpentinisation. As natural serpentinisation represents an open system, constraining the compositional variability of minerals and fluids with changing PT conditions during H₂ generation is also critical.

One critical procedural aspect of H₂ generation experiments is the assessment of the effects of accessory mineral phases usually introduced in reaction vessels for catalytic purposes. The presence of different sources of contamination, and the evolution of these sources with varying experimental conditions (e.g., increasing temperature and changes in fluid composition), is also worth examining. There is a notable lack of “controls” in the experimental data, and these would allow one to establish a baseline essential to proper data interpretation.

There is also a need to establish clear proxies of active, contemporary H₂ generation. For example, water chemistry (i.e., cations, anions, pH, Eh, alkalinity, and conductivity) should be systematically studied in future drilling projects conducted in water-saturated formations and compared against the state-of-the-art thermodynamic models for H₂ generation (e.g., Leong and Shock, 2020; Ely et al., 2023). The models should show how sensitive H₂ generation can be to the bulk geochemistry of the rock and the secondary mineral assemblages that are favoured to form, depending upon the effective water/rock ratio (e.g., Ballentine et al., 2025 and references therein). Comparing model predictions to field data of the fluid aqueous geochemistry and, when possible, the alteration mineralogy (including Fe-distribution and speciation), will provide needed insight into how far along a serpentinisation path a system has evolved, what H₂ generating reactions have occurred, and where fluid mixing is occurring (e.g., Ellison et al., 2021;

Leong et al., 2021).

The dataset of the amount of H₂ occluded in potential source rocks (Etiope, 2024a; Truche et al., 2024) should be expanded by analysing peridotites with varying degrees of serpentinisation (and the surrounding mafic-ultramafic rocks), as well as crystalline rocks with high potential for radiolysis and iron-rich rocks. Systematic fluid inclusion analyses are also recommended to evaluate whether the gas extracted is from primary or secondary inclusions, or if it is dispersed in microfractures and veins.

## 2.2. H₂ generation rates

### 2.2.1. Present knowledge

Knowledge of the rates and timing of H₂ generation is critical to understanding its geological and economic potential. The concept of fast H₂ generation (faster than fossil methane generation) as a general principle (without specifying the H₂ source) arises in several scientific articles and media reports (e.g., Prinzhofe and Cacas-Stentz, 2023; Hand, 2023). This point is, however, debated. The rate of H₂ generation is undoubtedly complex and the extent to which H₂ is rapidly generated in nature is unknown. Slow H₂ generation over extended time periods has the potential to form accumulations if H₂ is not consumed and is effectively trapped and sealed in reservoirs.

There is, in fact, limited knowledge of H₂ generation rates in natural systems. The generation rate of H₂ is here defined as the amount of H₂ (volume or mass) produced by a given amount (volume or mass) of rock over time. Convenient units are m³ H₂ m⁻³ rock y⁻¹ or nmol H₂ g⁻¹ rock day⁻¹. H₂ generation rates defined with these terms can be derived for radiolysis (e.g., Warr et al., 2023). For serpentinisation, H₂ production rates are available as H₂ amount generated in the reacted rock (e.g., 1800 g H₂ per m³ of reacted rock; Leong et al., 2023), which needs to be combined with serpentinisation rates (m³ of reacted rock per m³ of total rock per year; Lamadrid et al., 2021). An example of this data combination is described in Baciu and Etiope (2024) and a wider picture is provided in Table 1, derived from Etiope (2024b). A recent review of 30 published studies on experimental serpentinisation found that, while PT are obvious parameters determining the production rates, the role of accessory minerals and dissolved trace elements as catalysts remains unclear and poorly documented (Barbier et al., 2020). For example, Andreani et al. (2013) show that the presence of aluminium can speed
the process of olivine dissolution and consequently the production of H₂.

For OM thermal decomposition, we have examined available data and can provide preliminary estimates of H₂ generation rates. Under continued burial in a sedimentary basin, the H₂ generation rate from OM is considered to follow first-order kinetics with the rate constant increasing non-linearly with temperature. For example, we estimate that 1m³ of coal with 80% total organic carbon (TOC) content, density of 1.2 g cm⁻³ and H₂ yield of 20 mg H₂ gTOC⁻¹ can generate a total of 215 m³ H₂ at standard PT conditions. Using the Kinex software (https://zet-aware.com) and applying the kinetics for thermogenic H₂ generation (Mahlstedt et al., 2022; Boreham et al., 2023) together with a heating rate of 1 °C m.y⁻¹ and 10 °C My⁻¹ (typical range for sedimentary basins), the transformation (i.e., transformation ratio, TR = 1 for total transformation) to H₂ occurs over a period of 170 m.y⁻¹ and 17 m.y⁻¹ for 0.05 < TR < 0.95, respectively (TR covers the overmature temperature range relative to petroleum generation kinetics). Furthermore, the maximum instantaneous H₂ generation rate occurs at 232 °C and 254 °C (TR = 0.35) for a heating rate of 1 °C m.y⁻¹ and 10 °C m.y⁻¹, respectively. The estimated H₂ generation rates, in terms of order of magnitude, are indicated in Table 1.

It is interesting to note that most H₂ generation processes are not inherently fast. For comparison, the generation rate of biotic (thermogenic) fossil methane is in the order of 10⁻⁵–10⁻⁴ m³ CH₄ m⁻³ rock y⁻¹, as derivable from CH₄ production in shales (Schaefer et al., 1999; Etiope, 2024b). H₂ generation rates via radiolysis, low T serpentinisation (<100 °C), and OM thermal decomposition are all slower, from one to five orders of magnitude, than thermogenic methane generation. Only high temperature serpentinisation can be faster (Table 1).

High temperature (i.e., >200 °C) serpentinisation in continental settings may be relatively uncommon. In continental peridotite systems (ophiolites and massifs), geothermal gradients are generally around the average of continental setting, about 25–30 °C/km, which is substantially lower than geothermal and hydrothermal areas. In these settings, present-day temperatures above 200 °C can only occur at depths below 6–8 km. Due to the lower geothermal gradients in cratonic areas (as low as 18 °C/km on average; Reis et al., 2017), even deeper conditions are needed to generate natural H₂ through the serpentinisation of komatiite-rich greenstone belts. At the base of most ophiolite nappes (3–5 km) in orogenic systems, temperatures are below 150 °C (e.g., Etiope and Whiticar, 2019). Observations of elevated H₂ concentrations in these settings can be explained by low temperature serpentinisation reactions (<150 °C) and are further supported by isotopic analyses of minerals and clumped isotopic compositions of abiotic methane (e.g., Chavagnac et al., 2013; Etiope and Whiticar, 2019; Leong et al., 2023). Even in rift-inversion orogens, where higher serpentinisation temperatures have been estimated for the rifting and rift-inversion stages (Zwaan et al., 2025), present-day geothermal gradients are not higher than normal. For example, in the Western Pyrenees, borehole temperatures of 65 °C exist at about 2700 m, with a geothermal gradient of 24 °C/km (Barré et al., 2021). Therefore, the H₂ generation rate for serpentinisation at >200 °C (Table 1) essentially refers to very deep peridotite bodies, ancient serpentinisation events (as in rift-inversion orogens or Archean volcanic plateaus) or shallower peridotites in high heat flow areas (hydrothermal systems). For ancient and deep H₂ kitchens, geological time scale preservation and long-distance H₂ migration towards shallower reservoirs are required for feasible H₂ exploitation.

The expulsion rate of H₂ from source rocks is also extremely important. Once generated, H₂ can leave the source rock by diffusion (driven by a concentration gradient) or advection (driven by a pressure gradient). The relative proportion of H₂ transported by these processes depends on water availability and the permeability of the source rock, which can evolve throughout the process of H₂ formation (see Section 3 for details about gas migration mechanisms). Generally, advective flow is faster and can transfer larger volumes of gas than diffusive expulsion; thus, it dominates in permeable (e.g., microfractured) rocks, while diffusion prevails in impermeable rocks. In the case of serpentinisation,

permeability and aqueous H₂ concentrations increase until equilibrium is reached and then the reaction ceases (Klein et al., 2013), so sustained H₂ generation requires pressure-driven advective flow. For radiolysis in impermeable rocks, H₂ concentration continuously increases until saturation is exceeded and then H₂ may escape via diffusion; however, in permeable settings H₂ may be expelled via advection depending on the pressure gradient and may not reach saturation.

Finally, some H₂ generation processes are likely to be episodic. Mineral dissolution and precipitation may not be consistent over time and space as thermodynamic conditions change throughout the reaction process. Reactive rock surfaces may be passivated by precipitated minerals, thereby reducing or stopping H₂ generation. Alternatively, tectonic activity may play a critical role in creating fractures that expose fresh reactive surfaces that allow water-rock reactions to proceed. This is in stark contrast to hydrocarbon generation from organic-rich rocks, which is a unidirectional, kinetically controlled process driven by thermal maturation. Furthermore, the preservation of open fracture systems created during ancient tectonic episodes is variable and usually depends on factors such as their age, the local fluid composition, hydraulic pressure gradients, migration patterns, stress direction and mineral precipitation rates over time. The potential for non-uniform H₂ generation presents a unique challenge when trying to model these processes.

### 2.2.2. Scientific gaps, uncertainties and recommendations

The estimated generation rates of natural H₂ (Table 1) need to be confirmed with additional experiments. Specific research should be performed to better quantify the H₂ generation rates for serpentinisation and thermal decomposition of OM for a wide range of source rocks and temperatures. The study of the catalytic effect of certain metals is also recommended. There is a particular need to obtain kinetic data to assess H₂ production rates from iron-containing rocks other than peridotites (e. g., BIFs). For comparison with the data in Table 1, the H₂ generation rates from mechanoradical reactions, preliminarily evaluated in terms of amount of H₂ m⁻² yr⁻¹ (Lefeuvre et al., 2025), should be determined per volume of rock (m³).

All experiments should provide kinetic data that can be applied to a range of temperature and chemical conditions, including <200 °C, to account for continental non-hydrothermal systems, such as ophiolites, peridotite massifs, and crystalline basement. The impact of the fracture systems on H₂ generation rates and expulsion should be better evaluated through fracture stimulation activities and modelling. The monitoring of H₂-rich aquifer systems external to or hosted in peridotites, komatiites, organic-rich rocks, and other source rocks may also provide insights into the H₂ generation and expulsion rates.

## 2.3. H₂ systems: short-term, fast generation versus long-term, slow generation

### 2.3.1. Present knowledge

The rate of H₂ generation (discussed in Section 2.2) is fundamental in modelling H₂ systems. Two main categories of H₂ systems have been proposed (Fig. 2):

(a) dynamic, short-term systems, with “fresh” H₂ continuously and rapidly generated (e.g., Rigollet and Prinzhofer, 2022; Prinzhofer and Cacas-Stentz, 2023),

(b) long-term accumulation systems, where H₂ is slowly produced and accumulated in reservoirs over long (i.e., geological) timescale, analogous to petroleum (oil-gas) systems in sedimentary basins (Etiope, 2023; Truche et al., 2024; Baciu and Etiope, 2024; Aquino et al., 2025).

The two models imply distinct exploration and production approaches, and differing impacts on the economics of natural H₂ that depend on factors like the production rate per well and size and depth of the accumulation, as well as gas composition, separation, storage, and transportation costs. However, these two models are not mutually exclusive.
<figure><img src="image_5.png" /><figcaption>**Fig. 3.** Sketch of the different forms of gas advection, in dry and saturated fractures, applicable to H₂ (redrawn from Etiope, 2015).</figcaption></figure>

In the dynamic, “fresh” H₂ model, H₂ migrates to the surface directly from the source rock or from reservoirs with short residence times on the order of a century (Prinzhofer and Cacas-Stentz, 2023). It is considered that in surface seeps or underground accumulations, H₂ is continuously replenished. It implies the possibility that H₂ is a renewable resource (e. g., Prinzhofer and Cacas-Stentz, 2023; Hand, 2023). In a broad sense, a gas can be considered a renewable resource on the human time scale if its generation rate (input) is equal or higher than the emission or exploitation rate (output). This definition refers only to fluid dynamics, not accounting for the concept of “economically renewable”, which also depends on the H₂ demand and market conditions.

The notion of a “fresh” H₂ that is continuously, rapidly generated and refills reservoirs mainly stems from the observations of the Bourakébougou H₂ field in Mali (Maiga et al., 2023), where no pressure decline has been observed in the well despite gas extraction for local energy supply since 2012. The well reached a flow rate of 1500 m³/day and a pressure of 4.5 bars, which ultimately increased to 5 bars. This would imply a constant recharge of the reservoirs at rates that would be equal to or exceed the measured production rate. A geochemical model considering the typical mixture of gases found in association with H₂ (He, CH₄ and N₂) and their range of concentrations suggests that the Mali H₂ accumulation would be 500 years-old (Prinzhofer and Cacas-Stentz, 2023). Nevertheless, it is still unclear whether the H₂ recharge derives directly from the source rock (which is however unknown) or from deeper accumulations (gas-phase or H₂ exsolving from water). For instance, Maiga et al. (2023) have shown that under the PT conditions measured in the deepest well (~1800 m) drilled in the area, up to 3 m³ of H₂ at STP could be stored per cubic meter of pure underground water, more than a hundred times higher than under surface conditions. It is hypothesized that dolerite sills, which occur in the sedimentary section, can efficiently seal H₂ in a reservoir favouring the long-term production-accumulation-preservation model. This agrees with the hypotheses of Etiope (2023), Truche et al. (2024) and Baciu and Etiope (2024) based on H₂ fluxes and reservoirs in Turkey, Albania and Romania, respectively.

Model predictions of the global endowment of H₂ trapped in accumulations in the Earth's crust estimate that the annual recharge rate is approximately 5 Mt·yr⁻¹ (Ellis and Gelman, 2024). Only a small fraction of this H₂ can be expected to be recoverable, and studies on H₂ generation and residence time in continental crust suggest that natural H₂ cannot be considered a renewable energy resource (Ballentine et al.,

2025; Brunet and Malvoisin, 2025).

The long-term accumulation model is based on natural hydrocarbon gas systems. In this model, H₂ is generated slowly over long, geological time scales (according to the generation rates discussed above), migrates from the source rock, and accumulates into permeable rocks (sealed by impermeable rocks), mimicking hydrocarbon systems in sedimentary basins. This model stems from basic fluid dynamics considerations and the analogy with the systematic link between seeps and reservoirs observed in petroleum geology (Link, 1952; Macgregor, 1993; Abrams, 2005; Aminzadeh et al., 2013; Etiope, 2015). Studies have shown that continuous, long lasting, high-flux advective H₂ flows, reaching orders of tonnes of H₂ per year (such as Chimaera in Turkey, Bulqizë in Albania, or Nagsasa in the Philippines) cannot be sustained by slow H₂ generation rates within source rocks (Table 1) and necessarily must stem from pressurised pools (Etiope, 2023; Baciu and Etiope, 2024; Truche et al., 2024; Aquino et al., 2025).

### 2.3.2. *Scientific gaps, uncertainties and recommendations*

Further research should try to solve two challenges concerning the dynamic short-term system model:

(a) The fact that the minerals generating H₂ are not renewable: once altered, olivine and iron-rich minerals do not produce H₂ anymore; therefore, in order to produce the same (long-lasting) flux (in springs and seeps), the reaction front should move continuously involving new large volumes of rocks every day in a way that the H₂ migrates always along the same pathway.

(b) H₂ co-exists with gases that certainly are not rapidly produced, like He, N₂, and abiotic or thermogenic CH₄ (Warr et al., 2019). A preliminary model has been developed that tries to explain a blend of “fresh” H₂ with old gas (Prinzhofer and Cacas-Stentz, 2023) yet it cannot be applied everywhere. This leaves open the question: if H₂ is renewable at the human time scale, what is its relationship with the ¹⁴C-free (fossil) abiotic CH₄ and C₂⁺ hydrocarbons that are frequently observed in gas seeps in ultra-mafic rock systems?

A model capable of combining the two points above should be developed. The H₂ generation rates, addressed in Section 2.2, are also important to clarify whether H₂ can be considered renewable at human time scales (although recent views refute this possibility; Ballentine
et al., 2025; Brunet and Malvoisin, 2025).

Regarding H₂ generation rate versus seepage, H₂ consumption (through biotic and abiotic processes discussed in Section 4) should also be considered. Accordingly, surface H₂ fluxes must be sustained by higher generation rates, because part of generated H₂ is expected to be consumed. This is particularly important where there is no predicted reservoir/accumulation, as in the case of fractured aquifers in ophiolite systems (e.g., Oman; Leong et al., 2023; Templeton et al., 2024). More experimental studies are needed to better understand the consumption rate of H₂ under various environmental conditions. For example, some experimental studies have shown that where CO₂ is present in the fluid, there is a suppression of the total amount of H₂ generated, especially once net H₂ consumption begins to exceed H₂-production (Miller et al., 2017; Osselin et al., 2022a).

The porosity properties and mineralogical proxies of H₂ occurrence (e.g., alteration of certain minerals) can be investigated by 3D high-resolution X-ray computed micro-tomography (micro-CT; Pappalardo et al., 2025). 3D imaging can map the interior of rock samples with high resolution, revealing the geometry and distribution of pores and microfractures, as well as their relevant flow properties and spatial relationships with solid phases, as studied in ophiolitic chromitites (Pappalardo et al., 2025).

# 3. Understanding H₂ migration mechanisms

## 3.1. Present knowledge

In any geological gas system with source and reservoir rocks, gas migration is typically subdivided into “primary migration”, which occurs within the source rock, and “secondary migration”, which takes place from source rock to reservoirs, between multiple reservoirs, and/or from a shallow reservoir to the surface (seepage). In both steps, two possible migration mechanisms can take place depending on the permeability of the rocks and the gas pressure: diffusion and advection (e.g., Etiope and Martinelli, 2002; Etiope, 2015; Rice, 2022). The importance and role of these two mechanisms are not always appropriately applied to natural H₂, especially with reference to gas seepage near the surface. Although advection and diffusion of H₂ have broadly been addressed in a few works (e.g., Etiope, 2023; Ladhia et al., 2024), clarifications concerning their various forms (from gas phase, micro-bubble flows, to transport by water, depending on the gas-water interaction) are needed, as discussed in this section.

Briefly, diffusion is driven by concentration gradients (Fick's Law), is typically radial, and is important in low permeability porous media. Advection is driven by pressure gradients (Darcy's Law) and is important in permeable and fractured media, which typically determine preferential migration pathways and channelling (e.g., Etiope and Martinelli, 2002 and references therein; Rice, 2022). In petroleum systems, diffusion is therefore dominant in the source rock (e.g., shales; Stainforth and Reinders, 1990) and through the impermeable seals of the reservoirs. Advection is dominant along permeable strata and tectonic discontinuities (faults and fracture systems), between reservoirs, and towards the surface. In H₂ systems, advection can dominate also in source rocks if they are micro-fractured, for example due to serpentinisation.

Gas seepage at the surface (more extensively discussed in Section 5), with the release of gas into the atmosphere, is fundamentally an advection process (Etiope, 2015; Rice, 2022). The exchange of gases between the soil and the atmosphere is largely diffusive for gas biologically produced within the soil, unless there are not significant barometric pressure variations and wind. Locally rapid atmospheric pressure variations, such as those induced by wind (Bernoulli effect) can, in fact, produce advective movement (pumping) of gas within the soil (e.g., de Jong, 1973; Massmann and Farrier, 1992; Rey et al., 2012; Etiope and Oehler, 2019). Diffusive movements within the soil for a gas migrating from depth may exist or may only prevail in cases of thick impermeable layers (clay, silt) or a high-water content within soil pores. However,

such a phenomenon implies the accumulation and increase of gas pressure below the impermeable layer, until the “seal” is broken, and the gas must, then, advectively reach the surface (Etiope, 2023). Nonetheless, some works (e.g., Zgonnik et al., 2019) have only considered diffusion to model the H₂ release into the atmosphere.

**Advection forms.** There are four different forms of advection, depending on the gas solubility, gas/water ratio and pressures (Fig. 3):

(a) Continuous gas-phase advection in dry fractures,

(b) gas-phase advection as slugs, bubbles and microbubbles in water-saturated fracture,

(c) gas-phase advection with water displacement in water-saturated fracture,

(d) water-phase advection of dissolved gas.

In all cases, rock permeability, especially secondary permeability related to faults and fracture systems, has a dominant control over gas transport. Water-phase advection can, however, also develop through permeable sub-horizontal strata (carrier beds) and aquifers. In water-saturated and low permeability rocks, the pressure gradient must be sufficiently large to overcome capillary forces. The partitioning of H₂ into distinct phases depends on its solubility in water. H₂ is less soluble in water than CH₄ up to around 1.5 km depth (i.e., ~55 °C at 30 °C/km and 10 °C surface temperature), thereafter H₂ is increasingly more soluble than CH₄ up to at least 10 km deep (Bazarkina et al., 2020). Therefore, H₂ tends to form an exsolved gas-phase in the shallow subsurface, while it is more likely dissolved in water at depth, as compared with CH₄.

The flow of microbubbles is an important form of gas advection, which is widely considered in natural gas systems and radon (Rn) studies (Malmqvist and Kristiansson, 1984; Price, 1986; Brown, 2000; Etiope and Martinelli, 2002; Etiope, 2015), but it is neglected in natural H₂ studies. Its importance is also related to the capacity of microbubbles to transport radionuclides, through particle flotation, binding on the gas-water interface of active elements, and aerosol transport (Etiope, 1998 and references therein), which is a critical requirement for radiometric surveys, as discussed in Section 6.6.

It is sometimes stated that H₂, being a smaller molecule than other gases (e.g., CH₄) travels faster. To be rigorous, rather than the molecule size, one should refer to the molecular weight (two molecules with similar molecular weights might have very different sizes because of differences in their shape, bond types, and arrangement of atoms). The dynamic gas viscosity (μ, kg m⁻¹ s⁻¹) and density (kg m⁻³) are the only properties of the gas influencing advection velocity (which is dominated by the rock permeability and gas pressure). H₂ viscosity (9 × 10⁻⁶ kg m⁻¹ s⁻¹ at 25 °C) is only slightly lower than that of CH₄ (1 × 10⁻⁵ kg m⁻¹ s⁻¹ at 25 °C), and the two gases may travel at similar velocities under the same pressure gradients and permeabilities.

Gas density plays a role only in case of blends of gases in gas-phase advection forms, as lighter gases move upwards faster than other denser gases (as in the case of H₂, with density of 0.08 kg m⁻³ at 25 °C, compared to CH₄, with density of 0.65 kg m⁻³ at 25 °C).

### 3.1.1.1. *Diffusion forms.*
Diffusion can occur in a gas-phase and a water-phase (Etiope and Martinelli, 2002). In dry porous media (i.e., soil, consolidated or unconsolidated rocks), gas diffusion occurs in the interstitial air (gas-phase diffusion). In saturated porous media, gas diffuses in water (water-phase diffusion) but with a velocity that is much lower than diffusion in air. Gas concentration in water, and therefore the concentration gradient, is controlled by PT (Henry's law).

### 3.1.1.2. *H₂ consumption and retention during migration.*
During migration, H₂ can be consumed through the biotic and abiotic processes
described in Section 4. Biological consumption of H₂ during migration has not yet been investigated, but it should be expected by microorganisms inhabiting faults and fractures at temperatures below 121 °C (e. g., Templeton and Caro, 2023). Abiotic consumption through the Sabatier reaction (CO₂ + 4H₂ = CH₄ + 2H₂O) can occur in deeper rocks but preferably along unsaturated migration pathways (gas-filled faults and fracture systems; Etiope and Whiticar, 2019). A more detailed discussion on H₂ consumption is reported in Section 4.2. It is clear that slower H₂ movements induce greater residence time in saturated rock, which increases the amount of H₂ that microorganisms can “attack” and consume. During migration, H₂ can also be adsorbed on OM (Raza et al., 2022); this may imply a molecular fractionation in the migrating gas blend.

## 3.2. Scientific gaps, uncertainties and recommendations

Future studies on H₂ migration and seepage should consider appropriate terminology and physical principles of gas migration, as described above. Concerning advection, although microbubble flow is considered an important gas migration mechanism, it is presently neglected in natural H₂ migration models (e.g., Lodhia et al., 2024).

It would be important to develop a diagram showing potential H₂ velocities in relation to fracture width and depth, for the several advection forms (including microbubbles), as done for a generic gas in Etiope and Martinelli (2002), as well as more experimental studies to constrain models on the influence of porosity, permeability and pore throat diameter in different rock types (Lodhia et al., 2024).

Concerning diffusion, it would be useful to develop a chart showing H₂ diffusion coefficients in the two phases (gaseous and dissolved), over a range of temperature, pressure, and salinity, distinguishing the molecular diffusion coefficient (Dm), the effective diffusion coefficient (De = Dm multiplied by the effective porosity of the medium), and the apparent or bulk diffusion coefficient, which includes the effects of porosity and tortuosity of the medium (D = Dm × porosity / tortuosity).

Abiotic and microbial H₂ consumption along gas migration pathways should be studied in more detail, comparing, for example, gas velocities and consumption rates, trying to assess which type of consumption, abiotic or microbial, is generally faster at specific environmental conditions.

# 4. H₂ accumulation and retention in reservoir rocks

## 4.1. Lithology and permeability of H₂ reservoirs and seals

### 4.1.1. Present knowledge

Although recent estimates suggest that the Earth's H₂ potential exceeds current demand and could even be capable of meeting the requirements of decarbonization scenarios (Ellis and Gelman, 2024), exploitable H₂ resource could be too scattered to be captured in a way that is economically viable. Similar to hydrocarbon (oil and gas) exploration, H₂-rich reservoir rocks are needed for economic and safe production. So far, H₂-rich gas accumulations have been found in a few localities, in different geological settings, including peridotites in ophiolites, basement crystalline or metamorphic rocks, and carbonates and sandstones in sedimentary basins (e.g., Coveney et al., 1987; Boreham et al., 2021a; Maiga et al., 2023; Baciu and Etiope, 2024; Flude et al., 2025).

Concerning ophiolites, until now, the only case of pressurised H₂-rich gas encountered in a borehole has been documented in Tisovita, Romania (Baciu and Etiope, 2024). The borehole, drilled in the 1970s for metal mining purposes, intercepted a gas accumulation (with ~29 vol% H₂) within peridotites at a depth of about 800 m, leading to a violent gas outburst followed by a fire that lasted for several days. This case shows the importance of tectonic discontinuities (faults) in developing fractured and therefore more permeable portions of peridotites (specifically, fault-related cataclastic dunite), which can act as

reservoirs; unfractured peridotites (layered dunites) and tonalite veins may seal the reservoir (Baciu and Etiope, 2024).

In sedimentary basins, H₂ accumulations have been documented both in the Archean, Proterozoic and Phanerozoic basements and in sedimentary strata (e.g., Boreham et al., 2021a; Reis and Lhote, 2025). While Archean to Paleoproterozoic basement assemblages may host H₂ in fractures and microscopic fluid inclusions (e.g., Sherwood Lollar et al., 2014; Warr et al., 2019), the overlying sedimentary rocks may store H₂-rich gases in conventional reservoirs, tight or fractured sandstones, and carbonates. The reservoirs in Mali have shown that despite its reactivity, H₂ can be stored not only in sandstone reservoirs, but also in carbonate reservoirs, and at low pressure (Maiga et al., 2023). The discovery of a H₂ accumulation in tight carbonate reservoirs is particularly interesting, as recent studies have suggested that the long-term stability of carbonate minerals in the presence of H₂ may be either low (Zeng et al., 2022) or high (Davies et al., 2025a).

In petroleum systems, seals are well known, including evaporites, undeformed clays, tabular igneous intrusions and metamorphic assemblages. The lithology acting as a seal above the H₂ accumulations discovered in Kansas have been identified as low permeability shales, located right above the Proterozoic basement (Guélard et al., 2017). The seal rocks in Mali are mainly dolerite intrusions within the sedimentary strata (Maiga et al., 2023).

A H₂-rich aquifer can be hosted in porous and permeable (fractured) H₂ source rocks or other permeable formations that receive H₂ migrating from source rocks (e.g., Dugamin et al., 2019; Lefeuvre et al., 2024). Aquifers may have the advantage of not necessitating specific sealing rocks (which are essential in conventional reservoirs), but the recovery of dissolved H₂ would require specific technologies of gas extraction from water and transfer to the wellhead.

In theory, advective and diffusive migration of H₂ from reservoirs, through seals, represent H₂ accumulation losses. However, natural gas and petroleum systems show that gas seepage from reservoirs is a common phenomenon, and even large and continuous gas escapes may have little impact on reservoir pressure and productivity (e.g., Macgregor, 1993; Etiope, 2015). The same concept is thought to apply to H₂-rich reservoirs, so that diffusive H₂ loss through seals should not represent a negative factor for reservoir productivity (Jackson et al., 2024; Hutchinson et al., 2024). The properties of the H₂-brine system in the subsurface do not preclude membrane sealing of H₂ accumulations below low-permeability intervals. In other words, what constitutes a good capillary seal for methane should be an equally good seal for a H₂ column of similar height, as shown using literature constraints on densities, interfacial forces, and wetting states for H₂-brine-rock systems (Martín-Monge and Vayssaire, 2022). Concerns voiced in relation to sealing of H₂ accumulations in the subsurface are more frequently related to diffusion. Molecular diffusion through porous rocks operates ubiquitously and perpetually, but in general has very low transport capacity. A higher diffusivity of H₂, compared to methane for example, would mean that higher levels of impermeability are likely required (e. g., Eid and Divko, 2023) to keep diffusive losses at a comparable level.

### 4.1.2. Scientific gaps, uncertainties and recommendations

Although several H₂-rich reservoirs have been identified in ophiolites, crystalline basement, and sedimentary formations, no systematic information is available regarding permeability, porosity, storage capacity, production flow rates and spatial extent.

Since permeability of igneous and metamorphic rocks is dominantly driven by fractures and faults (secondary permeability), H₂ exploration in ophiolites and komatiites should focus on the identification of deformed, fractured, and non-deformed, impermeable sections largely through geophysical methods (discussed in Section 6). Systematic fracture analysis focusing on extension, aperture, openness, and density of fracture systems should also be performed.

The permeability, porosity, flow properties, and reactivity of reservoir and sealing rocks in H₂ systems require further investigation.
Conventional petrographic analyses can reveal mineralogical and textural elements sensitive for storing the gas and controlling the reservoir quality (Paiva et al., 2025). Additional detailed information can be obtained through high-resolution 3D X-ray computed micro-tomography (micro-CT), as suggested for the source rocks (Pappalardo et al., 2025). Since specific clay minerals and coals may retain H₂ through adsorption mechanisms (e.g., Han et al., 2025), systematic petrographic studies, H₂ isothermal adsorption experiments, molecular dynamics, and gas adsorption modelling will also provide critical information.

The capacity to sustain continuous H₂ production from a reservoir is still elusive. For sedimentary basins, the only flow rates reported in the literature come from the Bourakébougou field in Mali that, although continuous for more than decades, are relatively low compared to typical gas flows in conventional hydrocarbon systems (Maiga et al., 2023). However, production at this site is limited by the available off-take and the ultimate production rate achievable is currently unknown. If new reservoirs are discovered, for example through extensive drilling that is occurring in Australia and the continental United States at the present time, accurate and extended flow tests would be performed to evaluate the H₂ production capacity. Although H₂ free gas reservoirs are volumetrically likely to be most productive, the extent of adsorbed H₂ on mineral and organic matrixes needs to be better quantified (e.g., Han et al., 2025) and appropriate production procedures applied, as is the case for unconventional shale gas and coal bed gas.

Geophysical methods (discussed in Section 6) can offer important insights into the evaluation of natural H₂ reservoirs and seals, enhancing the prediction capacity, and providing information on reservoir thickness, quality, geometry, and compartmentalization (lateral continuity, lateral and vertical connections).

For H₂ dissolved in formation waters, the saturation, flow dynamics, age, and recharge can be analysed combining systematic, well-based physico-chemical and hydrogeochemical analyses, water flow monitoring, age determinations based on short-lived radionuclides, the application of tracing elements, and local- to basin-scale fluid flow modelling. These approaches may also provide further information on natural H₂ expulsion and local- to crustal-scale migration.

## 4.2. H₂ preservation in accumulations: Loss via biotic and abiotic reactions

### 4.2.1. Present knowledge

Preservation of natural H₂ accumulations is a key factor for its exploitation. As anticipated in Section 3.1.3, concerning migration pathways, H₂ can, in fact, be consumed by both microbial and abiotic processes.

In known H₂ accumulations, information on the H₂ consumption by local microbial activity is sparse. Microbes are extremely efficient at consuming H₂ (through multiple metabolisms such as methanogenesis, acetogenesis and sulphate reduction), and temperatures beyond the upper limit for life (~122 °C) are thought to be needed for long term preservation of H₂ accumulations (e.g., Thaysen et al., 2021). Biological H₂ consumption dominates where mesophilic to hyperthermophilic H₂-dependent organisms can proliferate between 30 and 90 °C. For example, microbial activity in shallower peridotite reservoirs can lead to the full consumption of H₂ supplied from deeper sources, although it is possible to find environments in the subsurface where nearly saturated H₂ fluids can persist in the presence of abundant microbial organisms (Templeton et al., 2024). Overall, it is essential to determine the physiochemical conditions under which microbial H₂ consumption is naturally inhibited and large amounts of H₂ may persist over long-time scales.

Abiotically, H₂ can react with carbon (C) compounds, especially CO₂, to form CH₄ through CO₂ hydrogenation or Sabatier reaction in the gas

phase. This gaseous reaction (catalysed by metals), expressed as CO₂ + 4H₂ = CH₄ + 2H₂O, implies that 4 mol of H₂ are consumed to form 1 mol of CH₄, which means that ~100 L of H₂ are necessary to form ~25 L of CH₄, at STP conditions. However, as known in catalytic chemistry, a large amount of H₂ does not react but it is adsorbed and dissociated on the surface of the catalyst; practically it is lost. Therefore, the total amount of H₂ consumed for the Sabatier reaction will be larger than what is predicted by stoichiometry. In laboratory experiments, it is recommended to use at least 10 mol of H₂ per mole of CO₂ (e.g., R. Thampi, personal communication). This is a major abiotic loss mechanism that deserves to be quantified.

Additional abiotic H₂ consuming reactions can occur through reactions with dissolved sulphate or ferric oxides, which are prevalent at higher temperatures (e.g., Milkov, 2022). Although abiotic reactions between H₂ and sulphate or iron oxides are kinetically inhibited at lower temperatures, enabling microbially mediated sulphate-reduction and iron reduction to prevail, they can proceed rapidly above 120 °C (e.g. thermochemical sulphate reduction).

Although H₂ is highly susceptible to consumption through chemical and microbial reactions during migration and accumulation, the occurrence of basement-derived H₂ within sedimentary successions demonstrates its capacity to persist over extended geological timescales (Flude et al., 2025). This observation supports the long-term accumulation system model, as outlined in Section 2.3.

### 4.2.2. Scientific gaps, uncertainties and recommendations

Although the microbial abundance, community composition and potential H₂-dependent metabolisms can be characterised from the fluids extracted from any H₂-system, information is lacking on how microbial H₂-consumption rates vary as a function of the aqueous geochemistry within the reservoir. The pH of the fluid and the availability of key oxidants such as nitrate, sulphate, ferric minerals, and carbon dioxide are particularly important. For now, empirical predictions can be made from the few studies of net microbial H₂ oxidation rates in rock-hosted systems (Boyd et al., 2024), until further quantitative investigation of microbial activity rates is conducted. Such predictions can be vetted against the few existing estimates of microbial growth and H₂ consumption in porous reservoirs (Thaysen et al., 2021). To move forward, microbial consumption rates should be investigated from more natural H₂ systems, using the same notation of H₂ generation rates discussed in Section 2 (m³ H₂ m⁻³rock y⁻¹), to directly validate published types of predictions (Templeton et al., 2024) and to improve reservoir-scale models.

Particular attention should also be focused on the potential inhibitors of H₂ consumption, such as H₂S, organic acids, and alcohols, which can be very effective in anoxic saturated sediments (Oremland and Taylor, 1975; Hoehler et al., 1998).

Archaea and bacteria can also exist in biofilms, whose growth in the solid matrix can be observed by micro-CT. Since temperature, mineralogy, and chemical composition of the porous medium affect the growth of the biofilms (Muller et al., 2024), micro-CT can help to quantify the growth of organisms that may contribute to H₂ bio-consumption.

Specific Sabatier (and more broadly Fischer-Tropsch type) tests are needed to more precisely estimate the total amount of H₂ consumed during the reaction, for a range of metals and minerals (simulating natural catalysts in the rocks, such as Ni, Fe, Cr, Ru) and at temperatures representing continental ultramafic rock systems (typically <150–200 °C). The detection of volatile fatty acids and other reduced compounds (alcohols, aldehydes) in aquifer and pore fluids could serve as a proxy for detecting H₂ generation at depth and subsequent consumption. However, a better characterisation of the compound's formation conditions in natural environments, and their isotopic signature, is still required to identify H₂ consumption mechanisms.
<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Sketch showing the uncertainties and potential occurrence of non-geological $H_2$ in surface and near-surface environments. Geological $H_2$ is easily identifiable only in hyperalkaline springs and gas seeps.</figcaption></figure>

## 4.3. Reservoir and basin modelling

### 4.3.1. Present knowledge

Based on information acquired through seismic and well interpretation, stratigraphic, structural, geothermic, and geochemical analyses, reservoir modelling and simulation have long been used in the oil and gas industry to provide information on the petrophysical properties (porosity, permeability) of the reservoir, the pressure conditions and gradients, its geometry, and its saturation. The purpose is to improve the estimation of reserves and support decision-making regarding the development of the fields. Furthermore, the lithology as well as the mechanical properties of the reservoir are also used to determine the maximum stress that can be applied during exploitation, before reaching mechanical failure. Seismic imaging is often used to evaluate the volume and geometry of a reservoir, identifying rock interfaces and gas pockets. Since the geometry is often complex, it is represented numerically as an assemblage of polygons comprising representative grid cells that approach the reservoir's shape. Reservoir modelling allows for simulation of production and estimates of the appropriate flow rate of the resource, as well as the behaviour of fluids (water, gas, oil) during exploitation. Additionally, it allows estimates of pressure decreases in reservoirs as a function of time. In the case of $H_2$ degassing from an underlying aquifer, it would also allow for estimates of the replenishment time of the reservoir during production (e.g., Maiga et al., 2023).

### 4.3.2. Scientific gaps, uncertainties and recommendations

Although numerous reservoir modelling software programs exist for simulating the behaviour of gaseous or liquid hydrocarbon accumulations, many challenges remain for transposing those codes to the modelling of $H_2$ reservoirs. Chief among those issues is the timescale over which $H_2$ reacts, and the time steps to consider to accurately model the migration of $H_2$ (as free gas and dissolved in water) into the basin towards a potential reservoir.

Modelling $H_2$ migration through diffusion requires precise determination of $H_2$ diffusion coefficients for different rock porosities, as well as PT conditions (Strauch et al., 2023). $H_2$ diffusion and reactivity are faster than those of hydrocarbons. Computing power, mostly that of super calculators, is needed to build accumulation models with accurate timesteps using currently available modelling tools. Since this will likely prove challenging, shortcuts still need to be developed to accurately build reservoir models rapidly. One way of doing it would be to use a constant "sink coefficient" that would consume $H_2$ as it migrates towards the reservoir to simulate mineral reactivity (and OM) at punctual temperature levels. Similarly, sink coefficients could be applied to the accumulated volume of $H_2$ for various microbial strains to represent biological consumption over time. Experimental data is still needed to apply those changes in existing software.

## 4.4. The problem of artificial $H_2$ generated by corrosion in wells intercepting reservoirs and aquifers

The geological origin of $H_2$ that is detected in boreholes intercepting conventional reservoirs and aquifers needs to be demonstrated, as non-geological $H_2$ may originate from modern microbiological activity in water-saturated sediments and, above all, corrosion within the borehole metal casing (e.g., Boreham et al., 2021a; Etiope and Orban, 2025). Up to 9 vol% $H_2$ due to corrosion was reported in geothermal wells (Feldbusch et al., 2018), 71 vol% $H_2$ in wells in Canada have been attributed to corrosion of steel casings or acidification operations (Séjourné et al., 2024) and up to 90 vol% $H_2$ was found in a well annulus on a platform offshore Norway (Thiessen et al., 2009). Therefore, the interpretation of $H_2$ detected through shallow or deep boreholes should consider the possibility that $H_2$ is not geological. Isotopic analyses of $H_2$ may not help, since $H_2$ produced by corrosion can be isotopically similar to natural $H_2$, as discussed in Section 5.3. Borehole corrosion tests and monitoring are an established technology in the oil and gas industry (e. g., Khan et al., 2020), and similar techniques, including visual inspection, ultrasonic and electrochemical probes, are strongly recommended for boreholes used in natural $H_2$ exploration.

# 5. Interpretation of surface gas geochemistry

## 5.1. General concepts and definitions

Using the basic concepts of gas migration to the surface from source and reservoir rocks, pioneered in petroleum geology and exploration, surface gas geochemistry is being extensively applied in the first steps of natural $H_2$ exploration. Surface gas geochemistry includes the analysis of surface fluid manifestations (seeps), gas in the soil, and gas dissolved in shallow (<100 m) aquifers, surface waters, and springs. Just as hydrocarbon (gas and oil) seeps and microseepage allowed petroleum reservoirs to be found (e.g., Link, 1952; Macgregor, 1993; Etiope, 2015), $H_2$-rich gas seeps and the presence of $H_2$ in the soil or shallow aquifers can provide valuable insights into the possible existence of underground $H_2$ accumulations. However, $H_2$ seeps and $H_2$ in soil-gas, shallow aquifers, or surface waters (lakes, rivers, ponds) differ fundamentally, the latter group implying more difficult interpretations.

There is little doubt that $H_2$ has a deep, geological source when it is observed in a gas seep or hyperalkaline spring, where $H_2$ may have concentrations at percentage and µM levels, and the gas flow shows an advective migration driven by pressure gradients (e.g., Etiope, 2023). $H_2$ in soil-gas, where $H_2$ concentrations can be on the order of $10^1–10^3$ ppmv, may instead originate from a variety of sources, including geological and shallow non-geological processes that are unrelated to potential $H_2$ accumulations (e.g., microbial fermentation). The several
<figure><img src="image_7.png" /><figcaption>**Fig. 5.** Concentration versus isotopic composition for H₂ in geological systems and in soils. Boundaries are derived combining data reported in Milkov (2022), Boreham et al. (2021b), Sherwood Lollar et al. (2007), Etiope (2023), Etiope et al. (2024). The isotopic signature of H₂ in “fairy circles” is unknown (concentration range from Etiope and Orban, 2025).</figcaption></figure>

A: Artificial (corrosion and drill bit metamorphism); B: Biological (marine sediments and surface ecosystems; e.g. fermentation); C(r): Crystalline rocks, with H₂ presumably related to radiolysis; M-H: Magmatic-Hydrothermal (the upper concentration boundary is uncertain since many data, e.g., as reported in Zgonnik, 2020, likely refer to artifacts during gas sampling or shallow water-rock interactions, such as serpentinisation of exhumed peridotites in MORs). S: Serpentinisation (ultramafic rock systems, seeps and springs); Lab-rad: Radiolysis obtained in laboratory (range between the vertical dashed lines); Eq: arrow indicating generic isotopic re-equilibration between H₂ and H₂O at relatively low temperatures.

possibilities and related terms were summarised in Fig. 1. Regardless, in many soil-gas investigations, including those pertaining to Sub-Circular Depressions (SCDs) (or “fairy circles” or geomorphological surface depressions), a paradigm is often accepted by which H₂ created in wet soils and aquifers is swiftly devoured by microbes and consequently should not occur in the aerated vadose zone. In these instances, soil H₂ concentrations from a few ppmv to thousands of ppmv are sometimes cursorily referred to as “seeps” or anyway representing natural H₂ (e.g., Larin et al., 2015; Zgonnik et al., 2015; Frery et al., 2021; Davies et al., 2024; Mathur et al., 2024).

There is a clear definition of seeps, or seepage in general (e.g., Etiope, 2015), traditionally used in petroleum geology: a surface manifestation of gas and/or oil stemming from an underground fluid system. Hydrocarbon seeps refer to a gas-oil system in a sedimentary basin. The word “seep” was historically used also for gas emissions that can have abiotic methane and H₂ stemming from ultramafic rock systems, such as ophiolites and peridotite massifs (e.g., Abrajano et al., 1990; Sano et al., 1993; Etiope, 2015). So, the use of the term “seep” for natural H₂ is acceptable. But H₂ concentrations in the order of 10¹⁻¹⁰ ppmv in the soil cannot be considered seeps a priori, as discussed below.

## 5.2. Non-geological H₂ in surface or near surface environments

### 5.2.1. Present knowledge

Fig. 4 summarises the terms that can be used for non-geological H₂, which includes biological and chemical-mechanical origin, either natural or anthropogenically induced. Knowing the background concentration and potential non-geological H₂ concentrations in surface or near-surface habitats is an important step in surface geochemical H₂ exploration. Nonetheless, the possible occurrence of non-geological H₂

in soil-gas and groundwater is frequently ignored in natural H₂ exploration literature.

H₂ observable in aerated soil and shallow aquifers can be produced by fermentation in wet soils or within aquifers, N₂ fixation, vegetation senescence and cellulose decomposition by termites (Conrad and Seiler, 1980; Krämer and Conrad, 1993; Sugimoto and Fujita, 2006; Meredith et al., 2017; Pal et al., 2018), by the aerobic and anaerobic oxidation or corrosion of ferrous materials (including natural minerals; e.g., Starkey and Wight, 1945; von Wolzogen Kuhr, 1961), and by the mechanical shearing of silicate-Fe minerals in soils (Dunham et al., 2021) (Fig. 4).

The amount of H₂ that can be biologically produced through anaerobic microbial fermentation can be substantial, if sufficient organic substrates are present and anoxic conditions are preserved within the soil or within micro-aggregates. Serendipitous detections of tens to hundreds ppmv of H₂ in the soil, in areas where natural H₂ is not expected (including urban gardens, parks, unfaulted areas, or sedimentary basins without natural gas occurrence) are common (Etiope and Orban, 2025). Particularly critical are the values in the order of 10²⁻¹⁰ ppmv, with a peak of 1 vol%, reported for H₂ in aerated soil, presumably derived by fermentation in wet layers or shallow aquifers (Etiope et al., 2024). These high H₂ concentrations were found in Alpine valleys in northern Italy and were associated with high concentrations of CH₄ and CO₂ (up to 51 and 27 vol%, respectively), which were revealed to be microbial and modern by radiocarbon (¹⁴C) analyses (Etiope et al., 2024). This case represents a new reference of the potential microbiological H₂ production near the surface and highlights the potential utility of radiocarbon analyses on CH₄ and CO₂ associated with H₂. In shallow aquifers, up to 426 nM of H₂ have been related to microbial activity, especially fermentation (Etiope and Orban, 2025 and references therein).

Concerning artificially generated H₂, it is known that drilling or hammering into the soil can frequently generate tens to hundreds of ppmv of H₂ (Boreham et al., 2024; Davies et al., 2025b). Up to 10 vol% of artificial H₂ was reported for 1 m drilling in a “fairy circle” (Halas et al., 2021) and 1.5 vol% H₂ for drilling into shallow aquifers (Bjornstad et al., 1994). Although some works assume that artificial H₂ is produced only by drill bit rotation (e.g., Lefeuvre et al., 2021), based on the experience of the authors of this work, artificial H₂ is also frequently generated (up to several hundred ppmv) by using less invasive probes operating with a sliding hammer. Corrosion within boreholes can also produce H₂ as discussed in Section 4.4, and this may also occur in wells intercepting shallow aquifers.

### 5.2.2. Scientific gaps, uncertainties and recommendations

The possibility to observe modern microbial H₂ in the soil is poorly considered in many papers dealing with soil-gas surveys for H₂ exploration. Prudence is generally recommended in presuming that ppmv concentration of H₂ in soils are a result of seepage. Although several papers and exploration strategies a priori assume that many SCDs are surface indicators of natural H₂ accumulations (e.g. Awosiji et al., 2025), the geological origin of H₂ observed in the soil of SCDs is not rigorously demonstrated. Several models have been proposed to explain the mechanical formation of ground depressions related to gas flows, due to eruption of pressurised gas (as in pockmarks, e.g., Hovland et al., 2010), mechanical compaction after flow termination (Schöpfer et al., 2025), or in relation to Fe-carbonate dissolution (Malvoisin and Brunet, 2023). However, to date, no studies have rigorously demonstrated the presence of other geologically sourced gases associated with ppmv levels of H₂ in SCDs.

Specific studies should be performed at these sites, including the analysis of ¹⁴C of CH₄ and CO₂ that can be associated with H₂. Analysis of stable C and H isotopic compositions of CH₄ and H₂ may be inconclusive (e.g., too low in concentration or non-specific), as described in Section 5.3. Research should address potential inhibitors of H₂ consumption, such as oxygen, sulphide, organic acids, and alcohols, which may promote the persistence of microbial H₂ in soils for extended time
periods. Analyses of He and Rn, which cannot be produced by biological activity, corrosion, or drilling operations, can provide potential indications of deeper gas sources (with the limitations discussed in Section 5.4). Acquiring more comprehensive soil-gas data is essential for expanding the dataset of surface non-geological H₂ generation, as a baseline for the geochemical exploration of natural H₂.

As previously noted, concentrations of H₂ in soil-gas reaching hundreds of ppmv are documented in numerous scientific publications, without considering the potential for artificial H₂ production resulting from probe hammering or drilling in the soil. Analysing the evolution of H₂ concentration readings obtained by a portable sensor immediately after the soil-gas probe is inserted into the ground enables differentiation between naturally occurring H₂ (whether microbial or geological) and artificial H₂ produced by soil hammering or drilling (Fig. 4). The signal generated by artificial H₂ generally diminishes swiftly and markedly within a few seconds. In instances where naturally occurring H₂ accumulates within soil pores and pockets, the sensor reading tends to be more stable, or the concentration may decline at a slower rate over time, depending on the quantity of H₂ present in the pores and airflow regulated by the sensor pump. In this context, single H₂ concentration data, as those derivable by gas chromatographic analyses, do not help. Detection of carbon monoxide (CO) is also recommended, as it is typically produced by drill bit metamorphism and it does not naturally occur in the soil-air (e.g., Boreham et al., 2024; de Leon et al., 2024). Monitoring of H₂ concentration in the soil gas over time can provide further insight into artificial vs natural H₂ occurrence.

## 5.3. The problem of the H₂ isotopic composition

### 5.3.1. Present knowledge

The ²H/¹H isotopic ratio of geological H₂ expressed as δ²H (vs. VSMOW standard) can vary from -100 to -1000 ‰ (Milkov, 2022). It is mostly controlled by the isotopic composition of H-feedstock (water), the equilibrium fractionation at the temperature of generation, and possible re-equilibration in shallower, colder waters. H₂ produced biologically will be close to isotopic equilibrium with the fluid temperatures due to the activity of hydrogenase enzymes that reversibly exchange H between water and H₂, whereas in the initial stage of H₂ oxidation kinetic isotopic effects are the most prevalent (Löffler et al., 2019). At low temperatures, this results in highly depleted ²H of H₂, given the equilibrium fractionation factor between water and H₂. Therefore, the δ²H value of biological H₂ within surface ecosystems generally ranges from -690 to -740 ‰ (Walter et al., 2012). However, oxidised, residual biological H₂ can have δ²H values up to -450 ‰ (Chen et al., 2015). This biological range significantly overlaps with the isotopic signature of geologically produced H₂ across a wide range of temperatures (Etiope, 2023), from continental serpentinisation systems and crystalline shields (δ²H generally < -650 ‰), to geothermal-magmatic systems (δ²H > -650 ‰; e.g., Milkov, 2022). Fig. 5 shows the overlapping δ²H-H₂ signatures from different sources.

### 5.3.2. Scientific gaps, uncertainties and recommendations

Basically, isotopic measurement of H₂ in soil, aquifers, or springs may not distinguish between geological and biological origins due to partial overlap in ²H/¹H isotope ratios (Etiope, 2023). Geological H₂ in surface environments can only be isotopically recognised if it originates from high-temperature systems, hydrothermal, or magmatic sources with δ²H > -450 ‰ and does not re-equilibrate with shallow waters. This has notably been shown in H₂-CH₄ gas mixtures from oceanic seeps, where CH₄-H₂O and H₂-H₂O isotopic fractionation yield similar temperatures, unlike shallow cold seeps on ophiolites (Vacquand et al., 2018).

Further differentiation of H₂ sources may be possible with the complementary measurement of ²H²H clumped isotopes (Mangenot et al., 2023), but the available data are very limited, and little is known about the effect of H₂ re-equilibration in water. The few natural H₂

measurements do, however, suggest rapid re-equilibration. More case studies on natural systems are needed. Any isotopic analysis of H₂ should always be accompanied by isotopic analyses of other fluids associated with H₂ (including He, Rn, radiocarbon of CH₄ and CO₂, as discussed above).

The interpretation of the δ²H values of H₂ detected in wells intercepting shallow aquifers should consider the possibility of artificial H₂ generation due to corrosion, which can be identified by traditional corrosion monitoring techniques, as discussed in Section 4.4.

## 5.4. The use of helium and radon to support H₂ origin's interpretation

### 5.4.1. Present knowledge

Radon (²²²Rn) and helium (He) have been widely used in surface geochemical exploration of seismic faults and hydrocarbons, because of their origin from radioactive decay in underground rocks (or mantle origin in the case of ³He-rich helium), enhanced migration along faults, and connections with oil-gas reservoirs (e.g., McCarthy and Reimer, 1986; Klusman, 1993; Toutain and Baubron, 1999; Fu et al., 2009). Consequently, ²²²Rn and He have been used to support the interpretation of the geological origin of H₂ detected in the ground (e.g., Lefeuvre et al., 2021; Prinzhofer et al., 2024). In order to minimise the chance of misinterpreting surface biological H₂ detection, it has been claimed that the cohabitation of crustal helium (⁴He) anomalies and H₂ in the soil is a significant feature in establishing the subsurface source of H₂ (Prinzhofer et al., 2024). Given that H₂ is primarily sourced from water reduction and that He derives mainly from radiogenic decay of U and Th containing minerals, the co-occurrence of elevated concentrations of these two species in soil-gas is not universally observed (Boreham et al., 2024).

### 5.4.2. Scientific gaps, uncertainties and recommendations

In circumstances when the sources of He and Rn, which are thorium- and uranium-rich crystalline or sedimentary rocks, are close to the surface, the interpretation of associated H₂ is not straightforward, because the gas may not come from deep sources. In particular, high concentrations of Rn can be associated with emanations from U-rich minerals near the surface (Ball et al., 1991; Åkerblom, 1993; Guerra and Etiope, 1999) and He anomalies in the soil (concentrations above the atmospheric value of 5.2 ppmv) may simply reflect groundwater circulation in fractured igneous rocks at shallow depths (e.g., Gregory and Durrance, 1987; Gascoyne et al., 1993). The theoretical maximum values of ²²²Rn produced in situ by its radium parent should be evaluated to determine whether soil-gas Rn is a tracer of gas migration from depth. This evaluation should take into account the radium (Ra) concentration in soil or near-surface rocks, the emanation power, the bulk density, and the porosity of the soil or near-surface rocks (e.g., Åkerblom, 1993; Guerra and Etiope, 1999). However, even if Rn and He are proved to be originated at depth, it does not necessarily imply that H₂ is deep originated. In fact, He (even if it has mantle signature) and major gases (CH₄, CO₂, H₂) are commonly decoupled (Etiope and Sherwood Lollar, 2013; Boles et al., 2015). Therefore, the surface occurrence of Rn and He cannot be considered as conclusive evidence for the geological origin of H₂. It is critical to show that geological "carrier" gases, like CO₂, CH₄, or N₂, exist, also because they are necessary for the long-distance transport of radon (e.g., Malmqvist and Kristiansson, 1984; Etiope and Martinelli, 2002).

## 5.5. Reliability of commercial portable H₂ sensors and sample containers

### 5.5.1. Present knowledge

Compositional analysis of soil gas and gas seepage containing H₂ has been undertaken using a variety of instruments including hand-held, single-gas or multi-gas sensors, gas chromatographs, and mass spectrometers. In this section, we deliberately avoid mentioning the several commercial brands; we only address general sensor problems and
suggest an approach to reduce analytical uncertainties.

Portable H₂ detectors from various manufactures are today available, based on electrochemical, semiconductor, or pellistor sensors capable of measuring H₂ over specific concentration ranges (typically from 1 to 2000 ppmv; some sensors can measure up to 5 vol% or even up to 100 vol%). A main issue is that manufacturers generally do not provide accurate calibrations over the whole measurement range (only one or maximum two calibration points are used). Precision and accuracy are not defined in most cases. Some sensors may suffer the disadvantage of interference from other gases (e.g., H₂S, carbon monoxide).

Concerning gas collection in the field for laboratory analysis, various types of sample containers are traditionally used in surface gas geochemistry: glass bottles, Tedlar® and multi-layer bags, copper or aluminium tubes, and stainless-steel cylinders. Sampling gas with H₂ requires a container with very low permeability.

### 5.5.2. Scientific and technical requirements, and recommendations

While there can be value in conducting broad-scale screening for H₂ in soils using low-precision methods, areas of interest identified by these studies should be further characterised using more rigorous methods. The scientific community needs portable, robust, and well calibrated analysers, capable of measuring H₂ with precision of at least 5 ppmv (preferably 1 ppmv). The instrument should be equipped with a data logger and internal pump with regulated flow rate, therefore with a sample inlet and outlet (so that the gas can be collected at the exit of the detector; tests should be conducted that H₂ concentration 'in' and 'out' are not compromised). The measuring range should be wide, preferably from 1 ppmv to 10 vol%. The sensor precision should not be affected by pressure variations at the inlet port (the reading should not change with the flow rate) or by the presence of other gases (e.g., CO₂, H₂S, CH₄) and water vapour (H₂O). Prior to field analyses, all commercial sensors should be accurately evaluated in laboratory, using certified standards, preferably covering the whole measuring range. Scientific papers should always report the measurement errors and how the sensors are calibrated or checked with calibration standards.

For gas sampling, laboratory tests (unpublished data from some authors of this work) showed that borosilicate glass bottles are acceptable if sealed with blue butyl rubber stoppers. Bags can be used if made of multi-layer aluminium foil, provided that the laboratory analysis is performed preferably within a few weeks after field sampling. A 5-layered bag involving bonded polymer/foil layers has been shown to retain 20 ppmv and 200 ppmv H₂ (and He) concentrations for at least 12 months under laboratory storage conditions (Boreham et al., 2024). Aluminium and copper tubes are prone to artificial H₂ generation in presence of saline water and sulphur compounds. Stainless steel cylinders, equipped with ball valve, remain the best container assuring "dry" H₂ storage without diffusive leakage.

# 6. Geophysical methods

## 6.1. General concepts

Geophysical surveys, including gravity, magnetic, active and passive seismic, magnetotelluric, and radiometric, have demonstrated efficacy in covering extensive areas and offer a cost-effective and scalable approach for identifying potential hydrocarbon (oil and gas) production sites, which can subsequently be examined in greater detail through geochemical surveys. They may be applied to H₂ exploration with some important differences and limitations, depending on the type of the geological setting (sedimentary basin, peridotite massifs, ophiolites, or crystalline basement). Each method exploits contrasts in specific physical properties, such as density, magnetic susceptibility, seismic velocity, or electrical conductivity. This allows one to infer the distribution of rocks involved in natural H₂ generation. Moreover, geophysical surveys help delineate faulted or fractured zones that facilitate water-rock interactions or gas migration, and assist identifying potential reservoirs,

seals as well as structural and stratigraphic traps.

## 6.2. Seismic methods (reflection, refraction, and passive monitoring)

### 6.2.1. Present knowledge

Seismic reflection methods involve the generation of acoustic waves that propagate through the subsurface and reflect off geological boundaries with different acoustic impedances. These reflected waves are then recorded by geophones or hydrophones and processed to create detailed images of subsurface structures, making this a valuable tool for resource exploration (Clement, 2021). These data are routinely used in petroleum exploration to identify critical geologic features such as reservoirs, faults, traps, and seals.

Seismic refraction methods, on the other hand, utilise the bending of seismic waves as they travel through layers of increasing velocity. Refracted waves that propagate along layer interfaces return to the surface at critical angles and are recorded to estimate subsurface velocity structures. This method is useful for determining depth to bedrock, assessing groundwater resources, and detecting geological interfaces (Lysdahl et al., 2021). Passive seismic monitoring, which does not require an active source, detects naturally occurring seismic events or ambient seismic noise to characterise subsurface properties. This method is valuable in detecting microseismic activity related to fluid migration, monitoring induced seismicity, and assessing deep geological structures without the need for controlled seismic sources (Thomas and Hoover, 2021; Chevrot et al., 2022).

Serpentinisation alters the physical properties of mantle peridotites by progressive hydration. This process significantly reduces the rock's seismic velocity from ~8.0 km s⁻¹ to ~6.0 km s⁻¹ (Christensen, 2004; Grezemeyer et al., 2007; Castro et al., 2024), as evident in seismic refraction data. The variation in velocities and densities may involve impedance contrasts that are highlighted by seismic reflection data (Gillard et al., 2019). For this reason, 2D and 3D seismic reflection and refraction data are strong tools supporting the detailed stratigraphic and structural analyses needed to interpret and predict the extent of serpentinisation, potential reservoirs and their fluid saturation, migration paths and seals. The hydration lowers the mechanical rock strength preventing brittle failure so that serpentinised zones may be indicated by reduced seismicity in passive seismic monitoring.

Furthermore, seismic reflection and refraction data can provide important information regarding potential reservoirs, traps, and seals, as well as migration pathways and fluid saturations in these settings. All of these elements are critically important for the formation of an accumulation of natural H₂.

### 6.2.2. Scientific gaps, uncertainties and recommendations

Challenges arise from the overlapping seismic properties of serpentinised and other rocks, the heterogeneous nature of serpentinisation, and the resolution limitations of seismic methods (i.e., the altered zones must be thick enough relative to the seismic waves' resolution). Serpentinisation lowers the P-wave velocity of ultramafic rocks, creating a seismic velocity contrast with unaltered rocks. However, depending on the degree of alteration and the serpentine mineral, the P-wave velocity range of serpentinised rocks (~5.0–7.0 km s⁻¹) overlaps with other lithologies, such as compacted sediments or (altered) crustal or mantle rocks, making it difficult to unambiguously identify serpentinised zones. Similar overlaps are found for S-wave velocities of partly serpentinised rocks (2.7–3.8 km s⁻¹) (Christensen, 2004). The S-wave velocity decreases significantly due to serpentinisation, as the transformation of peridotite into serpentine minerals reduces rock shear strength and increases ductility. This change is attributed to hydration and alteration of ferromagnesian minerals (e.g., olivine and pyroxene) in mafic and ultramafic rocks, resulting in the formation of serpentinite.

Additionally, seismic velocity is influenced by factors such as porosity, fluid composition and content, and PT, all of which vary spatially within a serpentinised system. As a result, serpentinisation
forms not a continuous front but may result in complex seismic re-
flections that are difficult to interpret. With increasing depth, the reso-
lution of seismic data typically decreases, making it difficult to delineate
serpentinised zones in the lower crust. In addition, serpentinisation
lowers the mechanical strength compared to unaltered rocks so that
serpentinised rocks may reduce the seismicity (e.g., Hyndman and
Peacock, 2003) and accordingly, the resolution by passive seismics.
Seismic interpretation can be improved by testing geological models
against independent geophysical variables such as the gravitational and
magnetic fields.

Since the fluid saturation may change the petrophysical parameters
of a rock's velocity, the combined analysis of S-wave and P-wave velocity
and density data can help identify serpentinised ultramafics, fluid-
saturated reservoirs, and leakage zones in the subsurface. For instance,
available conventional seismic imaging/inversion and machine
learning-based methods have demonstrated how petrophysical changes
induced by H₂ leakage can alter the seismic wave propagation properties
in the subsurface and, thus, be used to track and monitor H₂-saturated
zones (e.g., Gao et al., 2024). Coupled with other seismic inversion
methods, these techniques can be adapted and integrated with the
traditional seismic interpretation and attribute analyses for the investi-
gation of H₂ reservoirs and migration pathways.

The collection of seismic data using portable devices (e.g., Tsuji
et al., 2023), allowing relatively fast, high resolution and low-cost
monitoring applications, could also be beneficial in the interpretation
of surface H₂ occurrences.

6.3. Gravity surveys

6.3.1. *Present knowledge*

A gravimeter measures spatial variations in gravitational accelera-
tion that reflect subsurface density contrasts. Gravity data can be ac-
quired using different methods such as satellite, airborne, and ground-
based. Satellite data can be useful for regional, play fairway analyses,
whereas airborne and ground-based data provide the higher spatial
resolution required for targeted resource exploration. To isolate gravity
anomalies related to subsurface structures, the raw data are commonly
corrected for latitude, elevation, terrain effects, and tidal influences. The
resulting signal and corresponding gradients are sensitive to the density
distribution of underlying geological formations and help to identify and
differentiate geological features such as faults, sedimentary basins,
crystalline basement, and igneous intrusions.

Serpentinisation causes a significant reduction in rock density as
denser olivine and pyroxene minerals are converted to less dense
serpentine and brucite. While a pure peridotite has a density of 3.3 g/
cm³, its complete alteration produces much lighter 2.6 g cm⁻³ dense
serpentinite (e.g., Pinto et al., 2017). This density decrease generates
negative gravity anomalies. Analysis of gravity anomalies and their
gradients can be used to infer the depth and extent of serpentinised
bodies in the subsurface. Similarly, granitoids bodies, rich in U and Th,
with densities of 2.6–2.8 g cm⁻³, may be best detected where they
intruded into low-density sedimentary rocks (<2.6 g cm⁻³), producing a
local positive gravity anomaly (de Wall et al., 2019).

6.3.2. *Scientific gaps, uncertainties and recommendations*

As with the interpretation of seismic velocities, the density ranges of,
for example, granitoids and serpentinised rocks overlap with those of
compacted sedimentary, crustal, altered mantle rocks or even magmatic
rocks. Additional factors, such as crustal thickness variations or crustal
heterogeneities (e.g., metasediments), may further complicate gravity
interpretations. Consequently, gravity anomalies are inherently non-
unique, meaning multiple subsurface models may explain the same
observed signal, making it difficult to attribute anomalies directly to
rocks associated with serpentinisation and radiolysis.

The sensitivity of gravity methods is limited by the scale of the
density contrasts and depths. Serpentinisation may produce small or

gradual density variations rather than abrupt boundaries, which can
further obscure the gravity response and complicate the discrimination
of serpentinised domains from surrounding lithologies. To enhance
interpretative robustness, gravity surveys should be integrated with
magnetic and seismic studies.

6.4. Magnetic surveys

6.4.1. Present knowledge

Magnetic surveys exploit differences in the Earth's magnetic field
caused by the presence of magnetic minerals in the subsurface. The
Earth's total magnetic field originates in the Earth's outer core and local
contributions from crustal sources. Magnetic residuals from the crust
can be first obtained by a magnetometer deployed from an airplane or
helicopter at a height of several hundreds of meters above the ground, to
filter possible strongly magnetic rocks/sources at surface. Additionally,
magnetic surveys are often acquired using gradiometer setups, where
two magnetometers are spaced apart vertically or horizontally. This
setup enhances the resolution of anomalies and reduces the influence of
external noise sources.

After the removal of the main geomagnetic field produced by the
Earth's outer core and diurnal field variations due to Sun activity, the
obtained magnetic residuals represent the contribution of magnetization
produced by crustal and mantle rocks (Minelli et al., 2018).

Magnetite produced by serpentinisation of peridotites represents the
natural ferromagnetic mineral with highest magnetization per unit
volume (Bonnemains et al., 2016). Methods revealing the intensity of
crust magnetization (aeromagnetic data acquisition and analysis) are
thus pivotal to target possible sources of natural H₂ production and
accumulation.

In contrast to gravimetry, only the outer Earth shell is magnetic, as
below the so-called “Curie depth” where the 600 °C temperature is
achieved, crust and mantle rocks are virtually non-magnetic
(Wasilewski and Mayhew, 1992). Such depths may vary from 10 to
50 km depending on local geothermal gradient and can be documented
by spectral analysis of aeromagnetic data (Speranza et al., 2016). Thus,
aeromagnetic data acquisition and analysis offer the opportunity to
evaluate the crustal thermal state (heat flow and geothermal gradient)
by only remote data acquisition, which can be fundamental in cases of
remote, highly vegetated, dangerous, or offshore areas where ground
investigation is difficult or impossible. After having documented the
depth of the crustal magnetic layer, magnetic modelling may infer the
magnetization, shape, and depth of the magnetic source that should be
considered the target for H₂ production. Since basic intrusions can show
variable magnetite contents and the development of faults and other
tectonic structures can induce the precipitation of magnetic minerals,
the magnetometric data can also be useful for the identification of faults
and other types of fluid migration pathways. Obviously, not all targets
will be productive, meaning that highly magnetic sources may not al-
ways be associated with H₂ production.

6.4.2. *Scientific gaps, uncertainties and recommendations*

While serpentinisation may produce distinct magnetic anomalies,
several factors hamper their interpretation. A key challenge is that rocks
that have generated H₂ may contain highly variable amounts of
magnetite. Therefore, the magnetic signature may be an indicator of the
extent and degree of serpentinisation, but not necessarily. Lab experi-
ments have shown a decoupling between the rates of magnetite and H₂
generation, which is not well understood. In fact, magnetite formation
during H₂ generating reactions such as serpentinisation is temperature
dependent (Klein et al., 2014) and often rocks that are highly serpenti-
nised at low temperatures are dominated by secondary minerals such as
Fe(III)-bearing serpentine that is not magnetic (e.g., Ellison et al., 2021).
The amount of magnetite formed during serpentinisation depends on
factors such as the composition of the protolith (e.g., olivine-rich or
pyroxene-rich), the PT conditions, and oxygen fugacity (Cutts et al.,
2021 and references therein). As a result, the magnetic signature of
serpentinised rocks can vary significantly, even within the same
geological unit. While densities can be used to predict the degree of
serpentinisation, magnetic susceptibilities follow two different trends
with increasing serpentinisation depending on harzburgitic and dunitic
protolith (Cutts et al., 2021). Therefore, magnetic data should be
interpreted carefully to estimate the degree and spatial extent of ser-
pentinisation. In settings with present-day serpentinisation, high reso-
lution 4D gravity-magnetic surveys may offer potential to identify
reaction fronts in the subsurface source rocks, analogous to 4D seismic
monitoring in carbon capture and storage projects (e.g., Popik et al.,
2020).

6.5. Electromagnetic surveys

6.5.1. Present knowledge

Variations in the subsurface resistivity can be measured by magne-
totellurics or controlled-source electromagnetics. Magnetotelluric data
is a passive geophysical method used to exploit the electrical resistivity
structure of the Earth's subsurface (Constable, 2013). The principle is
based on the natural variations of Earth's electromagnetic fields, which
originate from solar activity and lightning discharges (Siniscalchi et al.,
2021). These electromagnetic signals induce electric and magnetic fields
within the Earth, and their interactions depend on the resistivity of the
subsurface materials. The depth of investigation is determined by the
frequency employed and, as such, magnetotellurics can be utilised for
shallow resource exploration as well as for deep mantle investigations.
Controlled-source electromagnetics is an active geophysical method that
uses an artificially generated electromagnetic field to probe subsurface
resistivity. This method is commonly used for shallow investigations,
typically up to a few hundred meters depth, providing high-resolution
imaging of conductive anomalies associated with groundwater, min-
eral deposits, and near-surface fluid flow.

Electro-magnetic methods are primarily sensitive to variations in
differences in lithology, distinguishing between high-resistivity crys-
talline rocks (e.g., granite, dry igneous rocks) and low-resistivity sedi-
mentary rocks, clays, and graphite-bearing formations (Robertson et al.,
2015; Corseri et al., 2023). In addition, it is particularly sensitive to
fluids, where the presence of water, brines, hydrocarbons, or partial
melts significantly lowers resistivity, especially if the fluids are saline. In
this sense, electro-magnetic methods are capable of identifying hydrated
and serpentinised mantle as well as possible migration routes of fluids
including natural H₂ (Reynard et al., 2011; Corseri et al., 2023).

6.5.2. Scientific gaps, uncertainties and recommendations

Cultural electromagnetic noise from power lines, railways, and in-
dustrial activities can interfere with measurements so that remote
reference techniques are used to mitigate noise. As is the case with other
geophysical methods, many geological formations exhibit similar re-
sistivity contrasts, which makes it difficult to distinguish between
different rock types. In particular, hydrothermal systems or fault zones
similarly distort conductive anomalies as observed for serpentinites.
Using joint inversion techniques that integrate electro-magnetic
methods with gravity, magnetic, and seismic data reduces the ambigu-
ity. The integration of borehole data, rock samples, and geochemical
analyses further helps constrain resistivity models and confirm geolog-
ical interpretations.

6.6. Radiometric surveys

6.6.1. Present knowledge

Radiometric techniques measure natural gamma radiation related to
radioactive elements such as potassium (⁴⁰K), thorium (²³²Th), and
uranium (²³⁵U, ²³⁸U). The detection penetration depth is quite limited
(typically <0.5 m), so they can be used to trace outcropping (not un-
derground) radioactive rocks. However, they were used for petroleum

exploration because radionuclides, which are associated with hydro-
carbon accumulations due to anoxic environments and chelation by
organic molecules, can be transported along faults towards the surface
by the waters associated to the oil deposits and precipitate (e.g., as
uranium salts) within the soil (e.g., Foote, 1969; Durrance, 1986;
Schumacher, 1996). Above an oil or gas deposit, either gamma radiation
lows or highs can form, depending on the position of the migration
pathway with respect to the oil deposit (e.g., Schumacher, 1996). Due to
the ⁴⁰K enrichment induced by fluid percolation in soils, surface gamma-
ray patterns can be applied on mapping surface and near-surface faults
and, thus, H₂ migration pathways (Reis et al., 2012).

The same radionuclides play a key role in radiolysis in crystalline
rocks; therefore, in the case of natural H₂ exploration, radiometry can be
used to trace upwelling radionuclide-bearing waters that are potentially
related to deep radiolytic H₂ source rocks. A radiometric map of
Australia has been used to assess the natural H₂ resources at the conti-
nental scale (Boreham et al., 2021a). At a more localised level, however,
Boreham et al. (2024) observed no correlation between high H₂ content
in the soil gas and high K-, Th-, and U-rich surface rocks and sediments.

6.6.2. Scientific gaps, uncertainties and recommendations

Radiometric surveys for natural H₂ exploration are conceptually
limited to radiolysis and should assume migration to the surface of ra-
dionuclides from underground H₂ source rocks, because of the limited
penetration depth (few tens of centimetres) of the radioactivity detec-
tion. Therefore, in principle they can be more effective along preferen-
tial pathways (faults) of fluid and radionuclide migration to the surface,
exactly as observed in petroleum exploration. In addition to water
transport, radionuclides can also be transported by microbubble flows
(Etiope, 1998), as discussed in Section 5.3. However, there are no
studies or case histories addressing this aspect in relation to natural H₂.
Specific studies should be performed with the aim of understanding the
generation of surface radiometric anomalies due to the transport of ra-
dionuclides along faults (by water or microbubble flow) from deep rocks
that may generate H₂ radiolytically. Similarly, systematic studies
focused on the mineral transformation in soils within H₂ seepage areas
and their impact on the background radioactivity should be also
performed.

6.7. Well logging and petrophysics

6.7.1. Present knowledge

Well logs provide direct and indirect information of rocks within
boreholes and have been extensively applied in hydrogeological in-
vestigations, as well as in hydrocarbon and mineral exploration over the
last decades. They comprise fundamental elements supporting petro-
physical analyses, which aim to evaluate the physical and chemical
characteristics of rock formations and their fluid contents. Currently,
well log data are classified into two types: i) basic and ii) special. Basic
well curves include gamma-ray, resistivity, sonic, neutron porosity and
density logs, which can provide detailed geophysical information in
depths ranging from centimetres to a few meters and are based on
physical principles such as the natural radioactive decay and gamma-ray
emissions, electric conductivity, acoustic wave travel time, nuclear ra-
diation, and adsorption principles (e.g., Tiab and Donaldson, 2016).
Special wellbore measurements are acquired through more advanced
techniques and concepts and include spectral gamma-ray logs, acoustic
and resistive borehole images, nuclear magnetic resonance, photoelec-
tric factor, side and whole well coring, and gas chromatography. In
hydrocarbon and mineral exploration, well log and petrophysical ana-
lyses are typically applied in structural and stratigraphic analyses, li-
thology interpretation, geomechanical studies, reservoir and seal
evaluation, and identification of ore deposits. Due to the overall lack of
wells drilled and designed for detecting H₂, systematic well log and
petrophysical analyses focused on the identification and characteriza-
tion of subsurface H₂ accumulations are scarce. The best case known so
<figure><img src="image_8.png" /><figcaption>Fig. 6. Schematic summary of the major scientific gaps and uncertainties related to natural H₂ and its exploration.</figcaption></figure>

far is that of H₂ in Mali (Maiga et al., 2023). Besides demonstrating the efficiency of well log data on supporting the identification and characterization of reservoirs and potential seals, it was shown that H₂-rich reservoirs may induce coupled changes in sonic and neutron porosity logs. Gas filled pores result in a velocity decrease of acoustic waves, and the free H₂ content may result in high neutron porosities. Neutron porosity tools are designed to measure the interaction between neutrons artificially emitted and hydrogen atoms present in minerals and fluid phases of any rock volume. Thus, the overestimation of neutron porosity curves is thought to result from higher H₂ contents in the studied reservoirs. Artificial neural networks are one approach to interpret and model well log characteristics, providing mineral composition, porosity and permeability, and estimating gas content (Wang et al., 2023; Xili et al., 2025).

### 6.7.2. Scientific gaps, uncertainties and recommendations

Designed as an alternative way for detecting natural subsurface information in the absence of direct rock samples, the well logging provides, in most cases, indirect geophysical information resulting from the combination of composition, texture, and pore-filling fluids in any rock volume. Thus, different rock and pore-filling fluid combinations may result in similar geophysical characteristics. These factors may lead to inconclusive well-log-based interpretations, which are also limited by the survey resolution and innumerable variables governing physical and chemical properties of rocks (e.g., Tiab and Donaldson, 2016). Furthermore, most of the available tools were developed and optimised for detecting hydrocarbon accumulations and petroleum system elements. For reliable application of well log and petrophysical methods to natural H₂ exploration, specific wellbore tools and sensors are needed that are designed and calibrated with an understanding of the effects of higher H₂ saturations on the geophysical properties of different types of reservoirs. This can be achieved by different strategies including: (a) calibration with basic and advanced laboratory petrophysical and petrographic well core analyses (e.g., Paiva et al., 2025) and (b) integration with other geophysical and geological methods. Since most subsurface reservoirs known so far are tight sandstones and carbonates (e.g., Maiga et al., 2023), the integration with advanced techniques such as focused ion beam-scanning electron microscope (FIB-SEM) and data derived from extended well tests may also improve the well log-based

interpretations by providing critical information on reservoir quality parameters (e.g., microporosity, pore throat, transmissibility) (Zou, 2013).

Although gas content can also be estimated from the interpreted porosity and gas saturation, direct gas analysis of the well logs is necessary to confirm the presence of H₂; however, with due consideration for the possibility that significant amounts of H₂ may be derived artificially from drilling and corrosion, as discussed in Sections 4 and 5.

### 6.8. Integrating geophysical and other data

As discussed above, the major challenge with geophysical methods is the non-uniqueness of rock-specific physical properties. Serpentinites often overlap in their properties (density, magnetic susceptibility, velocity) with other sedimentary, crustal, magmatic, and altered mantle rocks, leading to ambiguity in subsurface models. For example, unaltered ultramafics should have a low magnetic (low magnetite content) and high gravity (high density) signature. A completely serpentinised rock should have a high magnetic (high magnetite content) and low gravity (low density) signature. However, most H₂ source rocks will likely be partially serpentinised, having intermediate magnetic and gravity signatures, which can be difficult to distinguish from other lithologies. This means that geophysical data should always be interpreted carefully in a geological context. In addition, as depth increases, geophysical anomalies become broader and less distinct, making it difficult to resolve small-scale or deep-seated features. Further, integration and inversion of gravity and magnetic data can be used to model magnetite- and hematite-rich alteration with possible implications for natural H₂ generation (Boreham et al., 2021a). To overcome these challenges, the acquisition of airborne or ground-based high-resolution data may be necessary, applying, when possible, processing methodologies that more effectively preserve the signal with depth (e.g., Reardon, 2023). Smaller areas could be (re)mapped with a higher resolution, for example with the help of drones.

Only the integration of different geophysical information (seismic, gravimetric, electro-magnetic), satellite imagery, and geological maps, may reveal in more detail, and with less uncertainty, the basin structure and stratigraphy, possible fluid reservoirs, seals and gas flow systems. In general, the combined interpretation of different geophysical datasets
<table><caption>**Table 2**
Summary of research priorities and recommended approach.</caption><thead><tr><th></th><th>Research priority</th><th>Task/Approach</th></tr></thead><tbody><tr><td rowspan="7">H<sub>2</sub> system (source, reservoir, migration)</td><td>(a) Improving the knowledge of H<sub>2</sub> generation rates for all genetic mechanisms, with special emphasis to temperatures below 200 °C.</td><td>Laboratory experiments and modelling of generation rates expressed as m<sup>3</sup> H<sub>2</sub> m<sub>rock</sub><sup>-3</sup> y<sup>-1</sup> or nmol H<sub>2</sub> g<sub>rock</sub><sup>-1</sup> day<sup>-1</sup></td></tr><tr><td>(b) Extending the dataset on H<sub>2</sub> occluded in potential source and reservoir rocks (peridotites, crystalline rocks, BIF's).</td><td>Extraction of gas from rocks via milling, and analyses by sensors and gas chromatography (determining the background H<sub>2</sub> artificially produced by mechanical friction during milling).</td></tr><tr><td>(c) Studying H<sub>2</sub> variability and origin in underground water systems (aquifers) and the link with potential source rocks.</td><td>Monitoring H<sub>2</sub>-rich aquifers (periodic water sampling and analysis) integrated with geophysical images of aquifers and potential source rocks.</td></tr><tr><td>(d) Understanding potential H<sub>2</sub> velocities in relation to fracture width, depth and continuity, and petrophysics, for the several advection forms (especially microbubble flow).</td><td>Developing diagrams showing H<sub>2</sub> advection using Darcy's and bubble flow equations (e.g., Brown, 2000; Etiope, 2015) for realistic permeabilities and fracture apertures in peridotites, crystalline and sedimentary rocks.</td></tr><tr><td>(e) Understanding recoverable amounts of H<sub>2</sub> detected in wells</td><td>Long-term borehole pressure and flow tests after detection of relevant concentrations of H<sub>2</sub>.</td></tr><tr><td>(f) Understanding microbial H<sub>2</sub> consumption rates.</td><td>Laboratory experiments of microbial H<sub>2</sub> consumption under a variety of boundary conditions (to be expressed with the same notation of H<sub>2</sub> generation rates).</td></tr><tr><td>(g) Evaluating artificial H<sub>2</sub> in wells intercepting reservoirs and aquifers.</td><td>Borehole corrosion tests using techniques widely adopted in oil and gas industry (e.g., visual inspection, ultrasonic and electrochemical probes).</td></tr><tr><td rowspan="3">Surface gas geochemistry</td><td>(h) Extending the dataset of H<sub>2</sub> in soil and shallow aquifers, to determine non-geological "baseline", in any environment with different ground conditions, regardless the presence of potential natural H<sub>2</sub> sources.</td><td>Extensive soil-gas surveys in different soils (variable pedological conditions, e.g., organic matter, pH, mineralogy, vadose zone depth, water content).</td></tr><tr><td>(i) Recognising H<sub>2</sub> artificially produced during soil-gas drilling or hammering.</td><td>Measuring soil-gas H<sub>2</sub> dissipation patterns through portable sensors; detection of carbon monoxide (preferably avoiding cross-sensitive H<sub>2</sub>-CO sensors).</td></tr><tr><td>(j) Interpreting H<sub>2</sub> occurrence near the surface (soil, shallow aquifers) based on the isotopic and <sup>14</sup>C investigations of CH<sub>4</sub> and CO<sub>2</sub> and existence of other potential geological gases.</td><td>Analysis of <sup>13</sup>C-CH<sub>4</sub>, <sup>18</sup>O-CH<sub>4</sub>, <sup>13</sup>C-CO<sub>2</sub>, <sup>14</sup>C-CH<sub>4</sub> and <sup>14</sup>C-CO<sub>2</sub>.</td></tr><tr><td rowspan="2">Geophysics</td><td>(k) Reducing the problem of non-uniqueness of rock-specific physical properties.</td><td>Determine occurrence of C<sub>2+</sub> hydrocarbons and helium (evaluate, however, the existence of potential He sources near the surface).</td></tr><tr><td></td><td>Combine seismic, gravity, magnetic, electro-magnetic, and radiometric data (e.g. integrated inversion techniques).</td></tr><tr><td></td><td></td><td>High resolution 4D gravity-</td></tr></tbody></table>

<table><caption>**Table 2 (continued)**</caption><thead><tr><th>Research priority</th><th>Task/Approach</th></tr></thead><tbody><tr><td>(l) Understanding the generation of surface radiometric anomalies.</td><td>magnetic surveys to identify reaction fronts.<br>Study of surface radioactivity anomalies related to radionuclide transport, along faults, from deep rocks that may generate H<sub>2</sub> radiolytically.</td></tr><tr><td>(m) Interpreting geophysical images related to H<sub>2</sub> reservoir and source rocks.</td><td>Combine geophysical data with petrophysical (e.g., density, seismic velocity, magnetic susceptibility), mineralogical, and geochemical measurements (e.g. combine geophysical images of fluid flows with direct analyses of the fluid-bearing rocks, including occluded gas).</td></tr></tbody></table>

would further reduce the non-uniqueness of results. Nevertheless, discrepancies in combined interpretation of seismic reflection, seismic refraction, and potential field data are still possible (e.g., Peron-Pinvidic et al., 2016).

Future studies should also consider combining geophysical data with petrophysical (e.g., density, seismic velocity, magnetic susceptibility), mineralogical, and geochemical measurements (e.g., Cutts et al., 2021). For example, geophysical images suggesting fluid flows should be integrated with direct analyses of the fluid-bearing rocks (from drill-holes or in outcrops) and the gas contained in those rocks, from surface geochemical measurements (as discussed in Section 5) and analysis of gas extracted from rock samples; Section 2.1). This integration would further reduce ambiguity, increase the robustness of interpretations, in particular supporting inverse modelling to predict geophysical responses in the subsurface.

# 7. Remote sensing: image analysis and atmospheric measurements

## 7.1. Present knowledge

Remote sensing includes satellite or aircraft image analysis of surface features (e.g., mineralogy, landforms) that may be associated with H<sub>2</sub> seepage and atmospheric detection of gas assumed to stem from the ground surface.

LIDAR (Laser Imaging Detection and Ranging) imaging has been used to analyse morphological features (depth, geometry) and spatial distribution of SCDs that are considered to be formed by seepage of natural H<sub>2</sub> (e.g., Moretti et al., 2021). The spatial distribution and size of the surface features have been considered as a guide to the SCD development history and potentially of the extent of the sub-surface natural H<sub>2</sub> resource (Moretti et al., 2021; Frery et al., 2022). Although manual identification and mapping of SCDs are generally the norm, remote earth observation datasets can be automatically interrogated (e.g., Fisk et al., 2024).

There are presently no published scientific works that document the direct detection of H<sub>2</sub> from satellites or aircrafts. Communications from conferences and websites of private companies report the use of Sentinel II satellite (through visible and near-infrared - VNIR and Short-wave infrared - SWIR imaging) and Raman detectors to locate H<sub>2</sub> emissions from the ground (Pendock and Vidavskiy, 2023; Hage, 2024). The lack of detailed information on real case studies and instrument performances (detection limits, errors) prevents any further discussion.
## 7.2. Uncertainties

Published remote sensing studies have focused on SCDs, assuming they represent H₂ seeps (e.g., Moretti et al., 2021; Mosquera-Rivera et al., 2024). However, as discussed in Section 5.1, the causal mechanism by which H₂ leads to the formation of SCDs needs to be identified and the geological origin of H₂ observed in the soil of SCDs has not been conclusively demonstrated. Since many SCDs occur in wet environments, the existence of microbial H₂ (e.g., by fermentation) cannot be excluded a priori. There are cases where SCDs do not show natural H₂ (e.g., Halas et al., 2021; or in the “forest rings” in Canada; von Gunten et al., 2018). Therefore, presently, it is not possible to state that the SCDs are unambiguously an expression of H₂ seepage. Any remote sensing study addressing SCDs should consider such an uncertainty.

In the image analysis of SCDs, the depth to diameter ratio parameter has been suggested as a potential discriminative tool to verify that the depressions are not simple karstic features (Moretti et al., 2021). However, karstic features are not the only circular features that look like SCDs. In northern America, thermokarst features often have the same parameters. In Australia, arid aeolian deposits also appear very similar but are not associated to natural H₂ seepage (Getzin et al., 2021). Image data should always be supported by ground measurements, considering the parameters and processes discussed in Section 5.5.

There are no scientific publications on the detection of H₂ seepage above the ground from aircraft or satellites. Available information is limited to a few presentations at conferences and press releases. Accordingly, the methods and data need to be reviewed by the international scientific community.

## 8. Conclusions

The scientific community has considerable knowledge on natural H₂ and its general occurrence in the subsurface. Many of the exploration tools and strategies that are effectively used today in petroleum, mineral, and geothermal resource exploration can be adopted and adapted to develop optimised techniques for geological H₂ exploration. Nonetheless, important scientific gaps and uncertainties still exist, preventing unambiguous interpretation of geological, geophysical, and geochemical data. In addition, the present scientific literature shows some limits in theoretical, conceptual, methodological and interpretative approaches.

Uncertainties (summarised in Fig. 6) concern all major aspects of natural H₂ research: from H₂ generation in source rocks (e.g., the velocity of H₂ generation, the content of H₂ in source rocks), migration (differentiating the several forms of advection and diffusion, H₂ migration velocities and consumption along the pathways), accumulation in reservoir rocks (dimensions, permeability, fluid flow properties, H₂ residence times), sealing of the reservoirs (impermeable cap rocks), and the application and interpretation of surface exploration techniques, including gas geochemistry, geophysics and remote sensing.

The main research priorities, recommended tasks and approaches are summarised in Table 2. The proposed research actions can significantly narrow the several major uncertainties, thus representing a new reference for future studies and exploration of natural H₂.

## Funding sources

G.E. acknowledges support from the ENGAGE Laboratory Project funded by European Union - NextGenerationEU and the Romanian Government (PNRR-C9-I8-CF 14/11.11.2022). G.S.E. acknowledges support from the U.S. Geological Survey Energy Resources Program. O. H.A. acknowledges support from the Geological Survey of Canada GeoEnergy Program.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgments

This work is related to the report “Roadmap for scientific research” developed in the framework of Task 49 “Natural hydrogen” of the IEA (International Energy Agency) Hydrogen TCP (Technology Collaboration Programme). The IEA Communication Staff contributed to figure design and format. Helpful technical reviews were provided by Jane Hearon, Sarah Gelman, Andrew Feitz, and Daniel Coutts. The text was greatly improved by editorial comments from Brian Shaffer and Ofori Pearson. Anonymous reviewers provided constructive comments. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. C.J.B. publishes with the permission of the CEO, Geoscience Australia.

## Data availability

No data was used for the research described in the article.

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