The successful development of sustainable georesources for the energy transition is a key challenge for humankind in the 21st century. Hydrogen gas (H2) has great potential to replace current fossil fuels while simultaneously eliminating the associated emission of CO2 and other pollutants. However, a major obstacle is that H2 must be produced first. Current synthetic hydrogen production is at best based on renewable energies but it can also be polluting if fossil energy is used.
The solution may be found in nature, since various geological processes can generate hydrogen. Yet, until now it has remained unclear where we should be looking for potentially large-scale natural H2 accumulations.
A team of researchers led by Dr Frank Zwaan, a scientist in the Geodynamic Modelling section at GFZ Helmholtz Centre for Geosciences, present an answer to this question: using plate tectonic modelling, they found that mountain ranges in which originally deep mantle rocks are found near the surface represent potential natural hydrogen hotspots. Such mountain ranges may not only be ideal geological environments for large-scale natural H2 generation, but also for forming large-scale H2 accumulations that can be drilled for H2 production. The results of this research have now been published in the journal Science Advances. Also part of the team are Prof. Sascha Brune and Dr Anne Glerum of GFZ's Geodynamic Modelling section. The other team members are based at Tufts University (Dr Dylan Vasey) and New Mexico Tech (Dr John Naliboff) in the USA, as well as at the University of Strasbourg (Prof. Gianreto Manatschal) and Lavoisier H2 Geoconsult (Dr Eric. C. Gaucher) in France.
Natural H2 potential in tectonic environments
Natural hydrogen can be generated in several ways, for instance by bacterial transformation of organic material or splitting of water molecules driven by decay of radioactive elements in the Earth's continental crust. As a result, the occurrence of natural H2 is reported in many places worldwide. The general viability of natural hydrogen as an energy source has already been proven in Mali, where limited volumes of H2 originating from iron-rich sedimentary layers are produced through boreholes in the subsurface.
However, the most promising mechanism for large-scale natural hydrogen generation is a geological process in which mantle rocks react with water. The minerals in the mantle rocks change their composition and form new minerals of the so-called serpentine group, as well as H2 gas. This process is called serpentinization. Mantle rocks are normally situated at great depth, below the Earth's crust. In order for these rocks to come in contact with water and serpentinize, they must be tectonically exhumed, i.e. being brought near the Earth's surface. There are two main plate tectonic environments in which mantle rocks are exhumed and serpentinized over the course of millions of years: (1) ocean basins that open as continents break apart during rifting, allowing the mantle to rise as the overlying continental crust is thinned and eventually split (for example in the Atlantic Ocean), and (2) subsequent basin closure and mountain building as continents move back together and collide, allowing mantle rocks to be pushed up towards the surface (for example in the Pyrenees and Alps).
Numerical modelling helps constraining regions with natural H2 resources
A thorough understanding of how such tectonic environments evolve is key to properly assess their natural hydrogen potential. Using a state-of-the-art numerical plate tectonic modelling approach, calibrated with data from natural examples, the GFZ-led research team simulated the full plate tectonic evolution from initial rifting to continental break-up, followed by basin closure and mountain building. In these simulations, the researchers were able to determine for the first time where, when, and how much mantle rocks are exhumed in mountains, and when these rocks may be in contact with water at favorable temperatures, to allow for efficient serpentinization and natural hydrogen generation.
It turns out that conditions for serpentinization and thus natural H2 generation are considerably better in mountain ranges than in rift basins. Due to the comparably colder environment in mountain ranges, larger volumes of exhumed mantle rocks are found at favorable serpentinization temperatures of 200-350°C, and at the same time plenty of water circulation along large faults within the mountains can allow for their serpentinization potential to be realized. As a result, the annual hydrogen generation capacity in mountain ranges can be up to 20 times greater than in rift environments. In addition, suitable reservoir rocks (for example sandstones) required for the accumulation of economically viable natural H2 volumes are readily available in mountain ranges, but are likely absent during serpentinization and hydrogen generation in the deeper parts of rift basins.
Natural hydrogen exploration (and more) in mountain ranges
The results of this now published research provide a strong impulse to intensify the exploration for natural H2 in mountain ranges. In fact, various exploration efforts are already underway in places such as the Pyrenees, European Alps, and Balkans, where researchers have previously found indications of ongoing natural hydrogen generation.
"Crucial to the success of these efforts will be the development of novel concepts and exploration strategies. Of particular importance is how the formation of economic natural H2 accumulations is controlled by the tectonic history of a given exploration site. In particular, we will need to determine the timing of the key geological processes involved, because if H2 reservoirs are to form during mountain building, there must have been rifting, i.e. stretching, beforehand. So insights gained from plate tectonic simulations such as those performed in this study will be of great value", says Frank Zwaan, lead author of the study.
Sascha Brune, head of the Geodynamic Modelling Section at GFZ, continues: "This new research advances our understanding of suitable environments for natural hydrogen generation. Given the economic opportunities associated with natural H2, now is the time to go further and also investigate migration pathways of hydrogen and deep, hydrogen-consuming microbial ecosystems to better understand where potential H2 reservoirs can actually form."
Zwaan adds: "Overall, we may be at a turning point for natural H2 exploration. As such, we could be witnessing the birth of a new natural hydrogen industry."
