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Exploration for Natural Hydrogen

List of Contents:

1. Overview
2. Introduction
3. Occurrence & Potential Resource Endowment
4. Natural Hydrogen Systems
5. Remote Sensing, Indicators & limitations
6. Geochemical and Geophysical Surveys
7. Exploration Drilling
8. Development Opportunities & Challenges with LP Gas
9. Exploration, Access and Regulations
10. Research and Development in Support of Natural H2 Exploration
11. References
12. Additional Reading and Recommended Resources
13. See Also (links to other related pages in PetroWiki)
14. External Links (to third party website with training videos, etc.)  

Overview

Natural Hydrogen (H₂) is sometimes called Native hydrogen, geological hydrogen or "white or gold" hydrogen. Hydrogen (H2) is a naturally occurring, odorless, colorless and tasteless gas. The assigned colours are just a construct to designate the source of the gas.

Emissions of Natural Hydrogen occur all over the world and have been know since Roman times. Samples from such emissions may contain almost pure hydrogen or be a mixture of gases, for example methane, hydrogen, helium and/or carbon dioxide.

Interest in Natural Hydrogen exploration was spurred by the development of a shallow, natural hydrogen discovery in Mali, near the village of Bourakébougou, 60km from Bamako, in Block 25. In 2011, the Bougou -1 discovery well was re-entered and tested. Sampling confirmed the gas as 98% hydrogen, with 1% nitrogen and 1% methane. This accumulation of natural hydrogen occurs within multiple host rocks reservoirs at depths varying from 30 to 1500m (100 to 4,920ft) and it performance over the first 5 years or so suggested a dynamic flow of natural hydrogen. This project became the world's first natural hydrogen to electricity project.

This chapter of PetroWiki, will attempt to describe the natural hydrogen accumulation systems and the relatively minor differences in the setting and exploration processes, compared to Natural Gas Exploration.

Introduction

Fig. 2.1 Chimaera Eternal Flame (courtesy G. Tari & G. Etiope.)

Emissions of Natural Hydrogen occur all over the world and have been extensively reported over an extended period. One famous example is the "The Eternal Flames" at Mount Chimaera in S.W. Turkey: a natural seepage of 10% hydrogen and 87% methane that was reported by Pliny the Elder in the first century AD, during the early Roman Empire (Tari).

Fig. 2.2 Sampling H₂ rich gas from sludge pump at Ramsay Oil Bore - 1 in 1931 (SA Dem photo N001671)

The Government of South Australia (Ref. 2.2) has reported that several early oil and gas exploration wells in that State tested high percentages of hydrogen gas:

• Robe-1: 25% H₂ at 1241m (4072 ft) in 1915.
• American Beach Oil -1: 64 - 80% H₂ at 187 to 290m (614 to 951 ft) in 1921.
• Ramsay Oil Bore - 1: 51 - 84% H2 at 241 to 508 m (790 to ft) in 1931. (This accumulation was redrilled by Gold Hydrogen in 2023, with Ramsay - 1.)

A discovery of almost pure natural hydrogen was made during a water-well drilling campaign in Mali in 1987, near the village of Bourakébougou, 60km from Bamako. Gas percolating into the Bougou-1 well was set on fire by someone lighting a cigarette (Ref. 2.3). In 2006, Petroma Inc (now Hydroma Inc) acquired an exploration permits for Block 25 in Mali and conducted a geophysical exploration campaign. In 2011, the Bougou -1 well was re-entered and tested 98% hydrogen, 1% nitrogen and 1% methane (Ref 2.4 Additional drilling and an extended well test confirmed a large accumulation of natural hydrogen within multiple host rocks reservoirs at depths varying from 30 to 1500m (100 to 4,920ft) and suggested a dynamic flow of natural hydrogen. This became the world's first natural hydrogen to electricity project and has sparked a resurgence of interest in natural hydrogen exploration.

Fig 2.3 Cross section through some of the Bougou Wells in Mali showing multiple Natural Hydrogen bearing zones (Courtesy Hydroma Ref 2.4)

Fig 2.4 Sources and Migration Path for Natural Hydrogen

Like helium, natural hydrogen has become a significant Exploration target globally, as well as an active portfolio for academics and government geo-energy policy makers. Researchers in France have investigated potential of natural hydrogen targets since around 2010. The presence of natural hydrogen has been confirmed in the Pyrenean foreland, the Jura, and Lorraine. As of December 2023, TBH2 Aquitaine was the first company to be granted a hydrogen exploration licence and there were an additional five licences under investigation (Le Monde, 2023). CSIRO kicked off a research and development program in Australia in 2018 leading to revisions to the Energy and Resources Act and Regulations in South Australia in 2021. In the United States, the first well drilled specifically to target natural hydrogen was drilled in Nebraska by Natural Hydrogen Energy LLC in 2019. The USGS began investigating natural hydrogen resources in 2021. In 2024, active exploration campaigns are ongoing in Australia, Brazil, Colombia, the United States, and various countries in Europe and Asia.

Natural hydrogen is produced through natural, chemical and/or biological processes, seemingly within an optimum temperature window for each process, as summarized in Section 4 and illustrated in Figure 2.4, including:

• Serpentinization of certain iron rich rocks (like olivine) in the presence of water (Fig 2.4 a).
• Radiolysis of water by the products of radioactive decay (Fig 2.4 b)
• Ascent of hydrogen from deep, "high-temperature, high pressure kitchens", where kerogen has been overcooked (Fig 2.4 c).
• As a by-product of igneous intrusions, metamorphic or volcanic processes (Fig 2.4 d)

Due to the small size and extremely mobile properties of hydrogen molecules, almost any porous or naturally fractured rock is a potential storage reservoir. However, the reservoir sealing mechanisms are challenging and will, likely, require dense igneous or metamorphic rocks, thick dense shales or evaporites.

Exploration and production for natural hydrogen requires many of the same skills and techniques as petroleum and mineral exploration. Hence, petroleum scientists and economists can contribute, and transfer skills, into natural hydrogen exploration.

Occurrence & Potential Resource Endowment

Fig. 3.1 Global occurrences of Natural Hydrogen, courtesy of OMV, Gabor Tari and Tam Lovett.

Natural hydrogen has been reported all over the world in many different settings and concentrations. In his presentation to the SPE Hydrogen Technical Section , Gabor Tari presented a map developed by Tam Lovett from the OMV data base showing the locations of reported occurrences at seeps, bore holes and mines in a wide variety of geological settings. In 2022, Milkov published a study on abundance, origins and the opportunity to explore for Natural Hydrogen in the subsurface that was the foundation for an SPE Paper by Hongwen Zhao et al (One of the primary authors of this PetroWiki chapter). Milkov reported that 34,500 gas samples from 100 countries and territories had reported significant H2 concentrations (averaging 3.5%). While the data set covered some 370 basins, the greatest number of reported natural hydrogen occurrences were former the former Soviet Union, North America and Europe, where the greatest number of wells have been drilled in the 20th Century.

The level of promotion by Petroma/Hydroma et al around the Appraisal and Development of the Natural Hydrogen discovery in at Bourakebougou in Block 25 in Mali was a game changer in terms of the level of attention paid to Natural Hydrogen by the New Ventures and R&D Teams of many operating companies. By 2018, some 18 wells had been drilled around the Bougou-1 discovery, ranging in depth from 345ft to 5930ft (105m - 1807.5m) together with a 5 year extended well test.

Fig 3.2 Fairy Circle in Brazil, after Moretti et al Ref 10, reproduced in multiple SPE documents

Ema Frery from CSIRO Energy FSP Hydrogen Project in Australia presented an excellent summary of how Natural Hydrogen Exploration worldwide led to an extensive desk-based study of remote sensing data during the COVID lockdown as an integral component of the Australian National Hydrogen Roadmap. Natural hydrogen vents and surface as seeps are often manifest as circular superficial depressions that have been identified by aerial imaging in many places including the Africa, Australia, Brazil and the USA. Moretti et al reported that >0.6 t/d (>0.7 S.T/d) of hydrogen was being emitted from a well known depression in Brazil (see Fig 3.2). In other places, like Namibia, elevated hydrogen concentration also reported without any surficial expression, probably controlled by faults.

The April 2023 USGS Newsletter indicated that the USGS had developed a model of global hydrogen resource potential and quoted their Lead Hydrogen Geologist, Dr. Geoffrey Ellis, as having said that: “Using a conservative range of input values, the model predicts a mean volume of hydrogen that could supply the projected global hydrogen demand for thousands of years". Although he went on to say that: “We have to be very careful in interpreting this number", as "most of this hydrogen is probably inaccessible."

In his recent submission to the US Senate Committee on Energy and Natural Resources in Feb. 2024, Dr. Ellis suggested that the probable distribution of in-place, global geologic hydrogen resources estimates was likely to have "an approximate mean value in the tens of millions of Mt." He noted that vast majority of the in-place hydrogen is likely to be in accumulations that are too deep, too far offshore, or too small to be economically recovered. "However," he added, "the remainder could constitute a significant resource."

Natural Hydrogen Systems

The recent developments of the Bougou-1 natural hydrogen discovery in Mali, have demonstrated the existence of active natural hydrogen system. Geoscientists suggest that the natural hydrogen systems are, in general, similar to those used to describe natural gas systems, including source rocks, reservoir quality rocks, traps and accumulation processes. So, in essence, the system describes the generation, migration, fill and preservation processes. Nevertheless, a comprehensive understanding of their functions was still evolving in 2024.

Source Rocks

Several biotic and abiotic mechanisms have been suggested for the generation of natural hydrogen in the subsurface. Biotic factors refer to living organisms in an ecosystem, while abiotic factors include non-living physical components such as temperature, pressure, pH, humidity, salinity, sunlight, and chemical or mineral features.

Serpentinization of Iron (or Fe)-rich Rocks

Serpentinization is a low-grade metamorphic process in which igneous or metamorphic rocks rich in magnesium and iron (ultramafic-mafic rocks, such as olivine, peridotite, dunites and kimberlites) are hydrated and changed into serpentinites at temperature below approximately 570°F (300 °C). This produces a greenish-brown, metamorphic, silicate rocks ( 3[Mg₃Si₂O₅(OH)₄]) with an appearance like a snake’s skin (hence, the name serpentinization); and abundant divalent hydrogen gas:

6[(Mg_1.5Fe_0.5)SiO₄](olivine) + 7H₂O = 3[Mg₃Si₂O₅(OH)₄](serpentine) + Fe₃O₄(magnetite) + H₂

Fe-rich mafic minerals are the major components of upper mantle and oceanic crust that can occur near surface and/or be exposed as a result of tectonic uplift between plate boundaries in what the geoscientists call ophiolite belts.

Water Radiolysis

This dissociation of water into hydrogen ions and hydroxyl radicals can occur as a result of ionizing radiation by α, β and γ particles, released during radio-genetic decay of Uranium (U), Thorium (Th) and Potassium (K). Both Hydrogen (H₂) and Helium (He) gases are released during the water radiolysis processes. The H₂ and He generation rates are functions of the concentrations of U, Th and K, as well as the porosity of the water-filled host rocks.

Pyrolysis of Organic Matter

Fig. 4.1 Continuous generation model depicting hydrocarbon and hydrogen generation in the subsurface after Hanson and Hanson (Ref. 13) and H. Zhao (Ref. 7).

Pyrolysis involves the cracking organic-rich Kerogen under pressure at very high temperatures to create oil and/or gas. However, at high temperatures, in the range of 300 - 600 °C (570 - 1100⁰F), hydrogen gas will also be released from the kerogen. Under even higher temperatures (> 600 °C; >1100⁰F), the kerogen can be further decomposed releasing free hydrogen and leaving behind solid carbon (e.g. as carbon black or graphite) (This is described as "overcooked" source rocks by Oil & Gas geoscientists).  

Other Deep-seated Sources of Hydrogen

The fluids in the deep Earth (i.e. within the mantle and upper core) are likely to be chemically reduced and are believed to be hydrogen-rich. Therefore, it is feasible that natural hydrogen could be released to the crust through a degassing process .

Reservoir Quality Rocks

As with oil and gas, any porous and/or fractured rocks can be effective reservoir rocks for natural hydrogen. In the Bourakebougou field in Mali, the reservoir rocks include karsified carbonates and sandstones. Fractured basement rocks are the reservoir rock for the Mt. Kitty-1 Natural H2 discovery in the Amadeus Basin, Australia.

Hydrogen Migration or Accumulation Processes or Reservoir Charge

Fig 4.2 Diffusion coefficient of hydrogen and other gases in air at different temperature after Zhao et al. (Ref 7).

Both diffusion and advection play significant roles in the migration of natural hydrogen through the subsurface. Hydrogen has a higher diffusion coefficient than conventional gases, such as methane and carbon dioxide, due to its small molecular size (Fig. 4.2).The diffusive migration of hydrogen in the subsurface is controlled by lithology and grain size, temperature, pressure, and the the salinity of formation water.

Natural hydrogen may also migrate along faults, discontinuities and via aquifers, as either a free gas molecule or as dissolved gas in the formation water. This mode of transportation is termed advection by geoscientists.

Seal or Trapping Mechanism

The accumulation and trapping oh Natural H2 in the subsurface requires more effective sealing mechanisms than natural gas, due to the small size of the molecules and their high diffusivity coefficient (Fig. 4.2). Dense Evaporites (like salt or anhydrite), thick tight well compacted shales, or metamorphic and igneous rocks have been reported as effective caprocks.

In the Bourakebougou field, Mali, the H₂ gas had accumulated in a four-way dipping closure, with an intrusive dolerite as its caprock . The thickness of the caprock and the density and size of any natural micro- fractures are critical for the sealing capacity.

At Mount Kitty, in Australia, salt is the cap rock.. Others have postulated that clay absorption is another possible trapping mechanism.

Conceptual Models for Natural Hydrogen Systems

Putting this all together, Zhao et al (2023) have described two conceptual models for the total Natural Hydrogen Systems in the subsurface.

Fig. 4.3 A schematic diagram showing a shallow hydrogen system model with structural and stratigraphic traps, non-sedimentary reservoir traps and absorbed hydrogen.

Model I - Shallow H₂ Sources

For either:

1) Iron-bearing Rocks and Radioactive Ancient (Precambrian Cratons(?)) Basement Rocks

Here the generation of hydrogen is driven by serpentinization and/or radiolysis at relatively low temperatures (less than100 ⁰C (212⁰F)), with water supplied by local hydraulic recharge or tidal pumping. The H₂ generation process may be further enhanced in an existing fracture network or faults.

2) Geothermal Activities, such as Magmatic Intrusions

The generated hydrogen then migrates via both advection and diffusion.

As shown in Figure 4.3, the H₂ accumulates in structural (1,2,3) or stratigraphic (4) traps in the adjacent rocks or is absorbed by clay minerals (5).

Model II – Deep H₂ Sources

Fig 4.4 A schematic diagram of the deep hydrogen system model with potential trap types at elevated temperatures.

For hydrogen generated by either:

4) Degassing of the Mantle via Deep-seated, Basement Faults

The elevated temperatures are caused by deep burial and/or thermal events, which facilitate pyrolysis of organic matter or hydrocarbons.

Such thermal conditions predominantly exist in extensional tectonic regimes, where crust thinning leads to high thermal gradients. These areas are also prone to magmatic thermal events or intrusions. Furthermore, these conditions are more likely to develop deep-seated faults, which serve as conduits for hydrogen originating from mantle degassing.

This model provides a comprehensive understanding of the potential mechanisms and pathways for natural hydrogen generation and accumulation in deep geological settings.

Comparison of Natural Hydrogen and Natural Gas Systems

The key elements of a hydrocarbon system and a hydrogen system are summarized in Table 4.1.

Table 4.1.  Comparison between hydrogen system and natural gas systems.
	Natural Hydrogen System	Hydrocarbon Gas System
Generation	1) H2O reduction (e.g. serpentinization) 2) Radiolysis 3) Pyrolysis of organic matter, including kerogen 4) Mantle degassing	Pyrolysis of kerogen
Migration	Diffusive and advective	Advective
Accumulation	(1) Structural traps (2) Stratigraphic traps (3) Absorbed by clay minerals (4) Dynamic traps Either sedimentary reservoirs, or altered metamorphic/igneous rocks.	(1) Structural traps (2) Stratigraphic traps (3) Source Rocks (Unconventional plays) Generally, in sedimentary, naturally fractured tight rocks or fractured or weathered basement.
Preservation	1) Fast seepage rates (104 m3/day/km2) 2) Strong biotic/abiotic consumption 3) Short residence times (10-100 years) 4) Seal rocks include impermeable igneous rocks, evaporites, tight carbonate, and compacted shale	1) Millions of years of migration and preservation. 2)Tight seals including mudstones, shales, well cemented sands, evaporites, and tight carbonates
Location	Sedimentary basins or basement	Sedimentary basins
Burial depth	Near surface – ultra deep (core or mantle source)	Above metamorphic zone just within or more likely above the upper basement

Evaluating, Risking and Ranking H₂ Exploration Opportunities

There is not yet a systematic procedure to evaluate natural hydrogen terranes, plays, leads, prospects or portfolios in the public domain. A standardized process to consistently risk and rank natural hydrogen opportunities across the variety of hydrogen systems (Section 4) is required. There are similarities between some of the geological processes that generate and trap natural hydrogen and hydrocarbon gas. Therefore, the methodologies used in petroleum exploration can be adapted to provide a valuable foundation for natural hydrogen exploration. It would be wise for natural hydrogen exploration to follow the Play-Based Exploration logic, which is generally standardized and valuable for petroleum exploration. This workflow would include assessments of global terranes and basins, the quality and yield potential of source rocks, reservoir quality, seal integrity, migration pathways ("charge") and trapping mechanisms. The physiochemical properties of hydrogen and co-occurring gases should be incorporated into the risking and ranking procedures.

Natural Hydrogen Resources and Reserves

Currently, no systematic procedure exists to determine, assign, or publicly disclose natural hydrogen resources and reserves in the public domain. A standardized process is needed.

Remote Sensing Indicators of Natural Hydrogen

Fig. 5.1 A "Fairy Circle" in the Block 25 Mali with profile of the hydrogen concentrations (in ppm) (after Prinzhofer Ref. 4).

Fig 5.2 A schematic of how hydrogen emissions create a "Fairy Circles" . Courtesy Gabor Tari (Ref. 1).

The seepage of natural hydrogen to surface along the outer edge of a cone shaped shear zones in the near surface sediments is known to cause circular to sub-circular ovaloid, shallow depressions in the ground, known as "fairy circles" (Fig. 3.2 and 5.1). Such features have been documented globally and led to an extensive desk top study of satellite images of Australia by CSIRO .

The unique geometry and atypical vegetation patterns associated with "fairy circles" make them suitable targets for remote sensing surveys. Researchers have employed a combination of optical and multi-spectral satellite imagery of sub-circular depressions (SCD) as a possible indicator of natural hydrogen migration from the subsurface.

Fig 5.3 LIDAR images Fairy Circles at Arthur Road in the USA (after Zgonnik (Ref. 21)).

Fig. 5.4 Google Earth Images of Fairy Circles (after Moretti (Ref. 11)).

Researchers have employed a combination of optical and multi-spectral satellite imagery alongside LiDAR surveys to identify such features (Figs. 5.3 & 4). Data sources include Google Earth and satellite programs (e.g., LANDSAT).

Multispectral data analysis can leverage vegetation indices commonly used in agriculture, such as the Normalized Difference Vegetation Index (NDVI) and Soil Adjusted Vegetation Index (SAVI) to differentiate fairy circles from their surroundings (Fig. 5.3).

The integration of image analysis with other geophysical surveys (e.g., radiometric, gravimetric, and magnetic) holds promise for enhanced identification of Natural H2 Resources.

Limitations

Despite the promise of remote sensing, caution is warranted. The definitive link between fairy circles and hydrogen seeps remains under investigation. Not all remotely identified circular features necessarily indicate hydrogen emissions. Wisdom and gas samples are needed before haste to map such features though; Other geomorphologic processes can form circular features of similar size and similar depth, e.g. glacial kettle holes, cave roof collapses and karst sinkholes. Other geographical features, such as former water bodies and salt deposits can also appear circular in satellite imagery. Field-based gas measurements are crucial to confirm the association between these circular features and hydrogen emanations.

Geochemical & Geophysical Surveys

As with oil and gas wildcat exploration, Natural Hydrogen plays are matured into exploration prospects with relatively low-cost geochemical and geophysical processes. When working on know emissions and/or targets identified with remote imaging data, many exploration teams will try to quantify the emissions rate by collecting geochemical data at and around the site.

Geochemical surveys

The magnitude and geographic distributions of the hydrogen emissions help to identify potential hydrogen sources; understand subsurface migration processes and in predicting the depth and location of the potential accumulations. Common geochemical processes include:

Fig. 6.1 Soil Sampling Schematic (after Ref. 23)

Fig. 6.2 PARHsS Sensors, courtesy ENGIE ( G. Tari (Ref. 1)

Soil Gas Monitoring

Soil gas monitoring involves the use of portable gas sensors distributed along potential seepage lines or areas, such as "fairy circles". Typically, the sensor is connected to a composite pipe inserted into the soil. Lefeuvre presented a schematic diagram of a 1.25m (4ft) soil core and gas sampling assembly with the plastic liner connected to a multi-gas analyzer (GA-5000®), a radon monitor (Alphaguard®) and/or Swagelok® gas sampling cylinders, as shown in Figure 6.1. Of course, there were many other sensor suppliers and similar data collection systems in use in various parts of the world (see Fig. 6.2).

Fig 6.3 Sensor layout in monitoring emissions from a "Fairy Circle" in Brazil (after Ref. 20).

These systems are generally designed to track the spatial and temporal variability in the concentration of hydrogen (H₂) and other gases (methane (CH₄), carbon dioxide (CO₂), etc.). Figure 6.3 shows the layout of multiple sensors around the boundary of a large "Fairy Circle" in Brazil. Data collection over an extended period (weeks or months) will capture fluctuations in H₂ concentrations, as discussed by Pinzhofer and illustrated in Fig. 6-4).

Fig. 6.4 Variation in H2 Emissions over time in Brazil (after Ref. 20).

Remote data collection systems are generally equipped satellite or cellular communication systems.

Radiometric Surveys

Radiometric surveys assess hydrogen generation potential through water radiolysis. This involves measuring and calculating the zonal statistics for radiometric concentrations of uranium (U), potassium (K), and thorium (Th) ions. The potential H₂ yields can then be calculated. Geoscience Australia used radiometric technologies to measure concentration of potassium, thorium and uranium near the surface to a depth of 50 cm. By integrating radiometric data with observed hydrogen anomalies from drilling and/or surface monitoring, Chris Boreham at al reported that it was possible to map the H₂ production potential (in molecules H² kg^-1year^-1) across onshore Australia. The process identified four areas of particular interest for natural hydrogen exploration, including:

• The King Leopold and Halls Creek orogens in WA (A).
• The Pine Creek Orogen in NT (B).
• The area west of the Georgetown Inlier South of the Carpentaria Basin (C) in QLD.
• The area south of the Mt Isa Inlier under the Georgina Basin also in QLD (D).

Fig. 6.5 Radiolytic prediction of H₂ production rate (molecules H² kg^-1year^-1) over onshore Australia (after Boreham et al. Ref 24) .

Geophysical surveys

Fig. 6.6 South Nicholson Basin Geophysical Study Area, seismic line 19GA-B1 and the location of the borehole NTGS02/1 at Lake Sylvester in Northern Territories of Australia (after Boreham Ref. 24)

Fig 6.7 Multi-geophysical Array from a Regional Natural Hydrogen Exploration Survey in Northern Australia.

As with Oil and Gas exploration, leads are matured into prospects by geophysical mapping of the structures to identify potential traps.

Geophysical methods used for natural hydrogen exploration include:

·        Gravity surveys: detect anomalies induced by the presence of high density minerals, commonly found within ultramafic rocks and/or as the product of serpentinization.

·        Magnetic surveys: detect anomalies caused by large quantities of serpentine and magnetite.

·        Magnetotelluric (MT) surveys: detect anomalous conductivity in the crust, which may indicate hydrothermal flow conduits for deep sourced hydrogen-rich fluids.

·        Seismic surveys: map subsurface structures, faults, reservoirs, and seals.

Boreham et al reported on a comprehensive multi-geophysical surveys in the South Nicholson Basin, near the NT:QLD boundary in Northern Australia. This program defined the deep-seated faults as potential fluid conduits, as well as other potential hydrogen (H₂) sources. The basement depth (m) was interpreted from the seismic reflection data and overlain with their model predictions of alteration and contact zones (hematite-rich and magnetite-rich rocks). They published an image over the Sylvester Salt Lake showing the relationships between the deep structures; the subsurface data from borehole NTGS02-1 and the shallow conductivity imaged from the AusAEM1 MT line 1200003-1. Deep seismic-reflection data was used to delineate pre-existing major crustal structures and faults with the potential to act as conduits for large-scale, paleo-fluid migration and potential H₂ sources (Fig. 6.6). These findings were further confirmed by anomalously conductive structures identified through the analysis of MT data (Fig. 6-7).

Fig. 6.8 A shallow seismic survey over the discovery well in the Bourakebougou H2 play in Mali (after Briere et al., Ref. 25)

Similarly, seismic was used to image the subsurface structure and reservoir distribution in the Bourakebougou H₂ play, Mali as shown in Fig. 6-8.

Exploration Drilling

Fig. 7.1 Geological setting for a H2 Exploration Well in the Amadeus Basin, Australia, courtesy of Gabor Tari.

Well Planning

The exploration and appraisal (E&A) of deep targets is similar to any other gas prospect. The well designer will need to assess the geological prognosis, any potential drilling hazards, the predicted pressure regime and any special regulatory requirements ( Casing design#Design objective).

Depending on the deposition history, the thick, dense cap rocks may pose an increased risk of a rapid change in the pressure regime, which needs to be considered in the well design. Also, in the case of evaporites, especially salt, special drilling fluids, upgraded casing specifications and non-routine cementing techniques may be needed to minimize the potential risk of eccentric loading in the event of formation creep. At extreme depths and/or high pressures, there is a increased risk of hydrogen embrittlement and stress corrosion cracking of susceptible, high strength tubulars from any atomic hydrogen in the system (see Corrosion problems in production).

The application of Managed Pressure Drilling may be selected to minimize formation damage and lost circulation risks in naturally fractured rocks and/or carbonate targets.

Shallow targets pose a number of additional well design and equipment selection problems. Some of these shallow well design considerations are more familiar to those working in minerals or geothermal exploration than to those with an oil and gas background. These include:

Fig 7.2 Mobile minerals exploration rig (courtesy Hydroma).

Fig. 7.3 Appraisal and development drilling in Mali (courtesy Hydroma and G. Tari)

• Unfamiliar, hard, igneous or metamorphic rocks near surface.

• Difficulty in getting sufficient weight onto the bit without a push-down rig design.
• The possibility of encountering gas in the top hole section, or while drilling own-use, water supply wells.
• The need for several shallow casing strings to support a flow divertor, as in management of shallow gas in offshore wells.
• Potential of encountering a shallow gas influx while drilling top-hole sections with percussion drilling systems, using air, gas or mist as the drilling fluid.
• Lost circulation into open fractures, with the potential to damage potential pay zones.
• Unstable formation within the disturbed zone "fairy circle" were hydrogen gas and/or water is seeping to surface.
• Potential for encountering pressures close to the rock shear strength were seepages are occurring from deep sources, but have not yet sheared the rock to form a "fairy circle".
• The desire of the geoscientist to target a relatively small target on a fault plain that is sourcing seepages with limited ranging capabilities.

Fig 7.4 Hydrogen Exploration Target near "Fairy Circles" and proposed well path. Courtesy Gabor Tari, modified by Bob Pearson.

In the initial exploration and appraisal phase, some companies have elected to use modified mobile minerals drilling rigs and/or hydraulic rigs with a push-down capability. Certain shallow targets may benefit from the use of a slant rig to reach a target that is remote from the surface location. This has the additional advantage of increasing well productivity by exposing a greater section of pay, as measured along the hole, that is greater than the interval defined by the net pay isopach.

Fig. 7.5 Testing a "Fairy Circle in Nebraska using a conventional rig.

Other Operating Companies have opted to use conventional drilling equipment, as illustrated in Figure 7.4. This well was testing a fairy circle in Nebraska.

In essence, planning an exploration well near to "Fairy Circles" is similar to top-hole drilling in an area where there is a high risk of a shallow gas influx, or planning a pressure relief well for an internal blow-out that has resulted in the formation of a surface crater.

The forces involved in shearing and disturbing surface formations and the likely size and shape of the inverted cone have been extensively studied by the geotechnical experts looking at blow-outs and gas pipeline leaks.

The proposed casing scheme and standard well control procedures have to be modified for shallow targets in recognition of the modest formation strength at the surface and intermediate casing shoe. A flow diverter with large bore vent lines may need to be used, as it may not be feasible to safely close-in a very shallow gas kick. For deeper more highly productive targets, Managed Pressure Drilling techniques may be specified.

In exploring seepages, the final connection to the gas flow path may need to be made using fracture stimulation technology, so an understanding of the stress regime is important in planning the well path. Energized or foamed stimulation fluids can be used to minimize the formation damage risks in under-pressured horizons.

Planning an Exploration Evaluation Program

Fig 7.7 Cores and Logs from the Bourakebougou Field in Mali. Courtesy Hydroma.

Fig. 7.6 Mali H2 Accumulation and Well Testing Schematic from O. Maiga (Ref. 15)

The identification and evaluation of natural hydrogen deposits is not easy, especially if small diameter wells are used in the exploration phase. The petrophysical characteristics will vary enormously depending on the nature of the host rock, the connate water, natural gas saturations, pressures and temperatures etc. At depth, most of the natural hydrogen will dissolved in the connate water, with which it can react to form hydronium ions (H₃O+). (This occurs below 800m (>2625ft)) at Bourakebougou Field in Mali, for example). Ross Crain states that petrophysical properties of Natural Hydrogen and Hydronium poorly understood. Potential hydrogen bearing zones may be difficult to distinguish from water bearing zones, except by mud-logging, as they usually lead to a positive C1 reading. In new wells, gas chromatography can be used to confirm the H₂ content in the drilling fluid returns.

In any event, it is very difficult to re-evaluate old well logs to identify potential H₂ targets, unless the H₂ is a secondary component to Natural Gas &/or Helium. Reviewing the data from the the Bourakebougou Field in Mali, Omar Maiga concluded that free gas will mainly occur at shallow depths and can be identified by a shift in the neutron response. However, this log response could also result from increasing quantities of clay. The sonic P-wave velocity is also a good indicator of free gas. The gamma, density and neutron cross plots can be used in the usual way to define the lithology.

Fig. 7.7. Bougou_6 Synthetic Log (after Ref. 15)

For new exploration wells, advanced state-of-the art mud logging services should be specified to provide properly calibrated and depth corrected gas indications, along with a detailed description of the cuttings and cores. Relatively frequent drill-stem testing (DSTs) will likely be one of the key well evaluation tools. Crain's Petrophysical Handbook (CPH) suggests that potential pay zones can be identified in the usual manner from the Gamma Ray, SP, along with a somewhat overstated density and neutron porosity. However, unless there is a significant amount of natural gas in the mixture, there may be limited resistivity response and no cross-over on the density logs. In 2023, the logging contractors were still conducting R&D work on advanced tools to identify and quantify the hydrogen content. The CPH suggests that slim-hole gamma spectroscopy logs (e.g. SLB Pulsar Spectroscopy log) and the quad-neutron developed by Roke Technologies in Canada appear to be promising in that regard.

Fig 7.8 Test Separator used by Hydroma in Mali (after Ref. 3 & 15)

For appraisal drilling, well testing is likely to be a key resource evaluation technique and should drive the well design, costing and equipment selection. In some cases, it seems probable that the hydrogen will be co-produced with water, so that some form of artificial lift or a pump may be needed to initiate flow, as with Coalbed methane (CBM/CSG) wells. The skid mounted test separator and temporary flow lines will be similar to any other well test, but will need to meet the required specifications for the primary fluid. These separator specifications may be different from those used in gas well testing, if high concentrations hydrogen are anticipated, as in Mali.

Extended Well Testing to define the Reservoir Limits and/or the hydrogen recharge rates should ideally be carried out in a way that the energy is not wasted. In Mali, Hydroma used a portable generator to convert the produced fluids to electricity . Interestingly, Maiga reported that after 11 years of production, there appeared to be a ±10% increase in reservoir pressure from 450kPa to 500kPa (65 to73psi) in the pressure of the shallow zone that was on production.

Fig. 7.9 Skid mounted generator used by Hydroma for the EWT in Mali (After Ref. 3)

Development Opportunities & Challenges with LP Gas.

Project Management Planning Stage Gate System and Activities. Courtesy IHRDC.

Finding a pathway to resource commercialization is a major challenge in getting an exploration prospect through Project Management System Stage Gates 1 & 2, as well as for maturing Contingent Resources into Reserves. There is little doubt that Hydrogen Resources Classification will be handled in a similar manner to that outlined for natural gas in the Petroleum Resources Management System.

There is an excellent description of the Gas Treatment; Project Management, and Project Execution processes in other sections of PetroWiki.

Commercial evaluation of any exploration prospect starts of with a clear understanding of the project objectives, the target energy market, and the terms of the leasing arrangements, as well as Government and third party-takes, as taxes and royalties. For hydrogen production, there are likely to be tax offsets in terms of fiscal incentives for decarbonization of the energy supply. In many countries or regions, there may also be fiscal incentives for new industrial activity and employment that need to be incorporated into the commercial evaluation and decision analysis.

Fig 8.2 SoluForce Spoolable Pipe

Unless the natural hydrogen is coproduced with a more valuable product, like Helium, the economics of low pressure gas commercialization will likely be challenged by the costs of delivering the energy to the end users. In Mali, Hydroma opted to generate electricity for the local market, using modified skid mounted generators. Until a hydrogen delivery infrastructure is available, other options might be to truck the H₂, as compressed gas, or to blend it into the natural gas distribution system. (The opportunity and challenges associated with marketing H2/NG blends is discussed elsewhere in PetroWiki.)

Fig. 8.3 SoluForce H2 Tight line Pipe.

For shallow low pressure discoveries, the gathering facilities will likely be similar to those used for Coalbed Methane commercialization. A small spherical or vertical separator will likely be installed at the wellsite to knock-out the free water. Separate, low pressure lines will be used to collect the gas and water. Spoolable polyethene line pipe (as defined in API RP 15LE) will likely be used to transport the produced fluids to the central processing facility. Some service providers, like SoluForce, are extending this technology to accommodate higher operating pressures. For example, SoluForce indicate that their Hydrogen Tight, SoluForce FCP has been certified for hydrogen applications for operating pressures up to 600psi (4.2MPa or 42 bar).

Higher pressure hydrogen production systems need to consider the possibility that atomic hydrogen may exist in the flow-stream. Atomic hydrogen can permeate steel and may cause stress corrosion cracking of susceptible materials. This is most likely to occur as the stresses increase, for example due to cool-down or increased pressurization. This risk is discussed in other sections of PetroWiki.

The hydrogen business is well established, so there are existing API Standards for Compressors (617, 618 & 692) and ASME Standards for Process Piping, Pipe-lines, Pressure Vessels and Composite Lines, as well as Storage Tanks. Other certifying authorities and Standards organizations also have codes for handling hydrogen (e.g. EIGA, CGA, EI, NFPA and ISO) or have published guidelines and good-practices for handling Hydrogen.

Gas-liquid separation, gas drying and the use of cryogenic distillation to separate gas mixtures are addressed in other sections of PetroWiki. While the separation of hydrogen from methane, and/or from helium, are not specifically discussed, but the principles are similar, although the catalysts and costs will differ. In the most extreme cases, there are a number of potential product and waste streams that will incur disposal costs. When the produced water is relatively fresh, there may be beneficial usage options, for example as agricultural grade water. However, saline water will need to be disposed in the conventional manner, for example, by concentration with evaporation ponds or ionic exchange equipment prior to injection into salt water disposal wells. Similarly, any acid gas components, like carbon dioxide, will need to be removed and disposed of in the same way that natural gas streams are treated to meet pipeline specifications. To claim carbon credits and/or to attract "Green Energy Investment Funds", these acid gases and other green-house gases will need to be sequestered or stored for future usage, as discussed in other sections of PetroWiki.

Exploration, Access and Regulations.

Before exploration for natural hydrogen can commence, there needs to be a legal framework defining:

• The resource ownership.
• The structure and terms of the leasing arrangements for the exploration, development and production of hydrogen as the primary product.
• The management of secondary products, such as helium, which may be viewed as a strategic resource.
• The Government's take, as taxes and royalties or in a Production Sharing Agreement (PSA).
• Any fiscal offsets in terms of incentives for decarbonization of the local energy supply, industrial activity and local employment.
• Surface access rights and the obligations to compensate and consult with those using the land and/or owning the surface rights.

In this section, we will use an example from the State of South Australia , a jurisdiction with a long history of Oil and Gas Exploration and Production. Changes had to be made to the Acts and Regulations governing gas exploration to permit and encourage hydrogen exploration. The Commonwealth of Australia (the Federal Government); the various States and the national R&D organizations, like CSIRO and Geoscience Australia, had recognized the importance of hydrogen, not only as an energy carrier but also a natural resource.

Fig. 9.1 S. Australia Hydrogen Exploration Licences in 2024, (after Ref.27)

In 2019, the Government of South Australia published a Hydrogen Action Plan . To enable the exploration for natural hydrogen as the primary target, the Petroleum and Geothermal Energy Act was amended in February 2021 to include hydrogen as a "Regulated Substance" alongside of petroleum and substances produced with petroleum (CO₂; H₂S, N₂ etc), Carbon Dioxide and Helium. To further reinforce and strengthen the regulatory framework, the governing act was amended and renamed, as the Energy Resources Act 2023. This is an objectives and risk based regulatory framework governing the Exploration and Exploitation of Petroleum & other prescribed Regulated Substances (including hydrogen) and Deep Geothermal energy projects, as well as storage reservoirs and transmission pipelines carrying regulated substances. The expanded scope incorporated not only naturally occurring hydrogen, but also hydrogen that might be manufactured (at surface or underground) or imported as a liquified substance.

This framework includes the licence application or competitive bidding processes and various rental payments for exploration, land retention, production and storage. This includes provisions for the government to designate certain areas where exploration licences would be subject to a competitive bidding process, as well as an assessment of financial and technical capacity to undertake the proposed work plans. It also addresses the Ministerial Approval requirements for significant changes of control of the licences (>20%) and gives the Crown primacy in taking control of the licences in the event of a bankruptcy.

Under this jurisdiction, there are three licences relevant to Natural Hydrogen:

• A Regulated Substance Exploration Licence (RESL) with a defined work program for up to three 5 year terms with a 33% relinquishment obligation at the end of each term.
• A Regulated Substance Retention Licence (RSRL) to address Contingent Resources, that are not currently deemed to be commercial with 5 year terms and review processes.
• A Petroleum/Regulated Substance Production Licence addressing Reserves and Contingent Resources that can be produced in the near term and extending to 24 months after production ceases.

The S. Australia Approval Process is set out in 4 stages:

1. Licencing
2. Environmental assessment reports (EIR) and a Statement of Environmental Objectives (SEO) by the Operator.
3. Activity notifications and approvals by the Regulator.
4. Post closure liability and relinquishment obligations.

Between 1Q2021 and 2Q2024, over 40 licences had been issued to 8 Operators and Ramsay 1 and 2 wells had been drilled and tested by Gold Hydrogen, close to the abandoned 1931 Natural Hydrogen discovery in the Ramsay Oil Bore.

By 2024, Regulatory amendments to permit natural hydrogen exploration had also been implemented in Western Australia, the Northern Territories, New South Wales and Tasmania; but at the time of writing (mid-2024), there was no regulatory framework for natural hydrogen exploration drilling in the States of Queensland, Victoria or the Offshore areas administered by the Australian Federal government.

French law has officially recognised natural hydrogen as a resource, since April 2022. Companies interested in exploration can choose to apply for either an Exclusive Research Permit (PER in French) or an Exploration Permit (Concession). The permit approval process is expected to take up to 18 months. The management of natural hydrogen permits is regulated at the national level. As of December 2023, TBH2 Aquitaine was the first company to be granted a hydrogen exploration licence. The French Ministry indicated there were an additional five licences under investigation (Le Monde, 2023); and the government announced it was going to provide funding to help with the exploration for natural hydrogen in France.

R & D in support of Natural H2 Exploration

Following the announcement of the Mali extended well test results, many geo-energy companies, national geoscience agencies and R&D organizations established natural hydrogen exploration and/or R&D teams. At the time of writing (2024), notable examples in the public domain included, not only those whose companies whose staff have made presentation via the SPE Hydrogen Technical Section ( https://connect.spe.org/hydrogen/home ) and at Regional & International Conferences or published SPE or AAPG papers, but also :

• The Commonwealth Scientific and Industrial Research Organisation (CSIRO) in Australia (Natural hydrogen – HyResearch: Australian Hydrogen R&D Portal (csiro.au)),
• The USGS (The Potential for Geologic Hydrogen for Next-Generation Energy | U.S. Geological Survey (usgs.gov)).
• The Colorado school of Mines (GeoH2 - Department of Geophysics (mines.edu)).
• The Hydrogen Energy Research Center at the University of Wyoming (https://www.uwyo.edu/ser/research/centers-of-excellence/hydrogen-energy-research/index.html)

In 2024,the Advanced Research Projects Agency-Energy (ARPA-E) granted several projects on Geologic Hydrogen (arpa-e.energy.gov) aimed at exploring the potential of geologic hydrogen.

These projects are investigating the feasibility of generating hydrogen in-situ by injecting fluids into potential source rocks (in a similar fashion to harvesting heat in geothermal operations); or producing natural hydrogen from the migration paths or subsurface accumulations by drilling.

In 2024, much of the research on the Exploration and Development of Natural Hydrogen (H₂) Resources appeared to be primarily focuses on six key areas:

1. The effectiveness of the source rocks (e.g. for serpentinization).
2. Preservation of H₂ in the subsurface (e.g. seal effectiveness and residence time).
3. Characterization of potential H₂-bearing reservoirs using geophysical methods or multi-physical data (including seismic data) and forward modelling of the geophysical responses to the H2-bearing rocks in the subsurface
4. Petrophysical evaluation techniques and tools to identify high concentrations of Hydrogen and Hydronium in water.
5. Production and extraction of geological hydrogen in situ (so called "Orange" hydrogen.)
6. Local or Regional Resource Potential Assessments in support of bidding or licencing activities.

Laboratory experiments and numerical modeling are the main approaches employed in these studies.

As with most New Business Ventures or R&D initiatives, the initial step is a literature search, somewhat similar to the References cited below.

Additional Reading and Recommended Resources:

• Blay-Roger, R., et al. (2024). Natural hydrogen in the energy transition: Fundamentals, promise, and enigmas. Renewable and Sustainable Energy Reviews (Vol. 189, p. 113888). https://doi.org/10.1016/j.rser.2023.113888
• https://connect.spe.org/hydrogen/home
• Crain's Petrophysical Handbook (CPH) Chapter on Hydrogen Production - https://www.spec2000.net/petrophysics-in-the-green-economy/petrophysics-in-the-green-economy-hydrogen-production.htm

• Maiga, O., et al. Characterization of the spontaneously recharging natural hydrogen reservoirs of Bourakebougou in Mali. Sci Rep 13, 11876 (2023). https://doi.org/10.1038/s41598-023-38977-y

• USGS https://www.usgs.gov/news/featured-story/potential-geologic-hydrogen-next-generation-energy
• https://www.goodnewsnetwork.org/french-shale-gas-company-discovers-hydrogen-gas-well-that-could-start-renewable-revolution-in-coal-country/
• https://wiki.aapg.org/Main_Page

See Also (links to other related pages in PetroWiki)

• Casing design#Design objective
• Corrosion problems in production
• Project management of surface facilities
• Project execution of surface facilities
• Gas treating and processing
• CO2 sequestration

External Links (to third party website with training videos etc.)  

• https://wiki.aapg.org/Main_Page
• https://hydroma.ca/activities-natural-hydrogen/
• https://www.slb.com/products-and-services/innovating-in-oil-and-gas/reservoir-characterization/surface-and-downhole-logging/wireline-cased-hole-logging/pulsar-spectroscopy
• https://roke.ca/quad-neutron/our-technology/
• https://www.soluforce.com/product-overview/pipe-types/hydrogen-tight.html