White or natural hydrogen

However, did you know that it is also possible to find it naturally and in its pure state trapped in geological formations?

Pure natural hydrogen is a less common and far more unknown phenomenon, unexploited to date. It is here, in its natural reserves, that hydrogen offers promising potential as a clean and sustainable energy source. Unlike industrially produced hydrogen, which acts as an energy vector by storing and transporting energy from other sources, natural hydrogen is a true energy source. It is generated autonomously through geological processes, which makes it a primary source of renewable and sustainable energy.

In this AtlantHy Academy article, we will talk about natural or white hydrogen, an unknown phenomenon that can be enormously useful in decarbonisation.

Where can natural hydrogen be found?

Through geochemical processes and unique geological conditions, natural hydrogen is formed. Until recently, it was believed that hydrogen reserves did not exist due to the rapidity with which hydrogen dissipates into the air and the ease with which it reacts, usually with oxygen, thus limiting the chances of finding it as a free gas. Moreover, the right tools were not available, even today only some of the analysers include hydrogen sensors, and still, it is not possible to accurately estimate the concentration of hydrogen present in these natural reservoirs.

Although the amount of naturally occurring hydrogen cannot be reliably estimated, geological studies show that it is distributed in areas such as oil basins, coal beds, and subduction zones where geological conditions favour its formation. Places such as ophiolites and suture zones, where tectonic plates collide, are particularly rich in this resource.

Natural hydrogen reserves

There is no reliable estimate of the amount of hydrogen that has been or will be formed naturally, but an increasing number of projects are dedicated to the detailed investigation of this resource (quantities, costs, location of potential reserves, etc.). It has been found that it is distributed among oil basins, organic-rich sediments, coal beds, fault zones, volcanic intrusions, ultramafic rocks (igneous rocks with very low silica content) with very low crystallisation and potassic sedimentary strata. Let us highlight the cases where there is a greater probability of finding natural hydrogen:

Submarine subduction zones: There is a constant supply of water and frequent exposure of water to iron due to plate movement (factors associated with hydrogen production).

Ophiolites and suture zones: These represent the complete subduction of an oceanic basin in which continents collided.

It is also possible to recognise hydrogen leaks as they form circular structures known as ‘fairy circles’. They are the product of hydrogen leaking into the atmosphere (Andrey Myagkiy, 2020). In Aragon, for example, the presence of a large hydrogen deposit has been confirmed under the Pyrenees.

Illustration 2. Deposits and fairy circles known and located today (Rubén Blay-Roger, 2024).

Natural hydrogen origin

The formation of natural hydrogen can involve several processes, which are described below:

Serpentinisation: Involves the reaction of water with ferrous materials of low silica content (Rubén Blay-Roger, 2024). Depending on conditions, the hydrogen generated may be trapped between impermeable rocks that prevent it from being released to the atmosphere, thus forming a reservoir. This process is also known as hydrothermal alteration of peridotite, and can occur at low temperatures, but at slower rates and as this process moves away from the seafloor expansion axis, the ferrous iron is fully oxidised, thus generating a larger amount of hydrogen as long as the oxygen leachate is depleted.

The general description of the process is given by:

Radiolysis: Radiation released by radioactive elements present in marine rocks such as uranium, thorium and potassium (type a, b and g radiation) excites water molecules, which produces hydrogen free radicals. This causes the H-O bond to break down, forming hydrogen radicals and hydroxyl radicals. Two hydrogen radicals then react with each other to form hydrogen:

This phenomenon occurs at temperatures and pressures at which water is stable, regardless of whether it is in solid, gaseous or hydrated salt form.

Fracture: When rocks fracture, they break chemical bonds and generate free radicals that react with water:

Mechanical stresses dissociate the Si-O covalent bond in silicate minerals, giving rise to free radicals, which are the product of homolytic cleavage and others of heterolytic cleavage. By paying a little attention, you can see that the former species has a charge-free surface, while the latter offers charged radicals, the difference in radicals characterises the surfaces which are able to recombine to form siloxane bonds or react with water, so that hydrogen is released as a by-product. This hydrogen is generated when tectonic rifts are active, so, if that is the case, the production is continuous, whereas in non-slip plates it is sporadic and limited to plate slippage. It is not restricted to the case of tectonic plates, two rocks containing silicates can generate hydrogen if they collide with each other, in the same way and given that there are other processes that also cause rock fragmentation (gelifraction, saline intrusion, thermal shrinkage, among others) it is likely that hydrogen is being generated continuously. However, the efficiency of these processes is unknown.

Volcanic gas or magma degassing: There is an equilibrium in magmatic systems between oxygen and hydrogen, this equilibrium is strongly shifted to the right at high temperatures (~1200ºC), indicating a high hydrogen content.

Crustal weathering: As seawater cools, the earth's crust changes, favouring the following reactions, leading to the natural production of hydrogen:

High-temperature alteration of basalt: The modification of the oceanic crust by seawater at high temperatures changes most of the ferrous silicates to ferrous minerals, whereas a small fraction is converted to ferrous minerals and then to hydrogen:

The depth to which hydrothermal fluids penetrate the oceanic crust is unknown, sampling indicates that the upper crust changes, while the lower crust remains the same.

Interactions between lava and sea: Pillow lava and seawater react and produce hydrogen in the following way:

Crystallisation: Water dissolved in the magma oxidises the iron, leading to the production of hydrogen:

Pyrite formation: During inorganic pyrite formation, which possibly takes place at high temperatures at the mouth of mid-ocean ridges, pyrite contained in seafloor and in these ridge deposits precipitates from hydrothermal fluids with the following chemical reactions:

Illustration 3. Graphical description of the origins of natural hydrogen, (a) Serpentinisation, (b) Radiolysis, (c) Rock fracturing, (d) Degassing of magma.

What to expect in the future?

Further research is needed to accurately assess the potential of this resource, to estimate the location of deposits, the concentration and quantity available, and the rate at which it is generated, are just some of the aspects of interest linked to that potential.

It is of great importance to ensure the durability of the reserves while developing and improving biotechnological processes. Micro-organisms capable of producing hydrogen are known to exist and some have been isolated for study to see if it is possible to exploit this capacity on a large scale. An in-depth phenotypic and genotypic analysis could provide the necessary information to find their optimal working conditions, and whether there is a way to increase their production.

The extraction of this type of hydrogen is still in its early stages, it can be done with existing technology for natural gas extraction, so natural hydrogen has the potential to become a significant source of this energy carrier.

References

Andrey Myagkiy, I. M. (2020). Space and time distribution of subsurface H2 concentration n so-called “fairy circles”: Insight from a conceptual 2-D transport model. BSGF-Earth Sciences Bulletin , 1-13.

Lu Wang, Z. J. (2023). The Origin and Occurrence of Natural Hydrogen. MDPI, 19.

Rubén Blay-Roger, W. B. (2024). Natural hydrogen in the energy transition: Fundamentals, promise, and enigmas. Renewable and Sustainable Energy Reviews , 9.