Geologic hydrogen has been at the center of much research (and speculation and new energy optimism) in recent years. The field is rapidly developing! This week, we will look at some recent studies with a focus on the science of “tracking” geologic hydrogen resources.
Rasoul Sorkhabi
Editor, Core Elements
Hydrogen Occurrence in the Continental Crust
The Pyreenees Mountains are a well-known example of a rift inversion site. Wirestock Creators/Shutterstock.com
A recent study in Nature Reviews discusses various mechanisms of geological hydrogen in the continental crust. Here are some highlights.
Generative mechanisms: The researchers emphasize the importance of two major mechanisms that generate natural hydrogen in the crust:
Water-rock interactions in which ferrous iron (Fe-II) in mafic and ultramafic rocks, such as peridotite and basalt, are oxidized to ferric iron (Fe-III), and hydrogen is released from water. This process has been suggested for many ophiolites, including those in Oman.
Radiolysis of water by radioactive elements, notably uranium, thorium, and potassium in the upper crustal rocks. In this process, ionizing alpha, beta, and gamma particles dissociate water molecules.
Different timescales: These two processes operate on different timescales:
Thousands to millions of years for water-rock interactions in highly fractured rocks
Tens to hundreds of millions of years for radiolysis in water-limited environments
No mantle hydrogen: The researchers opine that the mantle is NOT a source of hydrogen gas found in the upper crust because:
Oxygen fugacity controls the speciation of hydrogen and carbon in the mantle and that mantle-derived hydrogen is “most stable” as water at temperatures and pressures shallower than about 90 kilometers.
Many proposed sites of mantle-derived hydrogen are not supported by data.
The bottom line: The “perennial renewable” source of hydrogen from the mantle is highly speculative—a topic that deserves further investigation.
Rift-Inversion Sites for Natural Hydrogen Generation
Most studies of natural hydrogen place these resources in sedimentary basins formed by rifting and mid-oceanic spreading. However, a study published in Science Advances argues that “rift-inversion orogens” are even better sites for hydrogen generation and accumulation.
Rift-inversion orogens are geological ranges that develop from the inversion of rift structures during the collision of continental plates.
Hydrogen generation capacity: Frank Zwaan and colleagues compared hydrogen generation in rift basins and rift-inversion orogens by thermotectonic modeling.
What they found:
In rift basins, maximum serpentinization and maximum hydrogen generation capacity are 5 billion kilogram per kilometer per year and 1.5 mol per kilometer per year, respectively.
In rift-inversion orogens, maximum serpentinization and maximum hydrogen generation capacity are 100 billion kilograms per kilometer per year and 30 mol per kilometer per year, respectively.
The researchers attribute this 20x higher generation in rift-inversion orogens to the relatively cold crust in orogenic settings.
Hydrogen accumulation sites: The researchers also suggest that accumulations traps for hydrogen may be absent in rift basins but readily available in the orogenic settings. They cite the Balkans and the Pyrenees as examples of rift-inversion orogens.
Dive deeper: For more studies of geologic hydrogen in the United States and globally, readhereand here.
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Workflows for the Identification of Geologic Hydrocarbon Sites
Summit Art Creations/Shutterstock.com
A group of French scientists have developed an AI methodology to identify natural hydrogen seeps from sub-circular depressions in the crust. Let’s take a look.
Sub-circular depressions (SCDs): The researchers categorize SCDs into two groups:
Group 1 includes those rounded landforms in which hydrogen concentrations have been measured in the field. These have been mapped in Russia, the United States, Brazil, and Namibia.
Group 2 are those rounded landforms which do not emit hydrogen. These include fairy circles, fairy forts, farm circles, flooded dunes, impact craters, salt lakes, and karst caves and lakes. These occur in Group 1 countries as well as in Australia, China, and Europe.
AI modeling:
The researchers constructed a global database of 60 structures for Group 1 and 440 structures for Group 2 then classified them using a supervised machine learning model.
The model is available as an open-source software (Ultralytics YOLOv8), which uses convolutional neural network architecture for image classification.
AI model results:
By comparing images with different levels of resolution, the model achieved 90-percent-accurate, high-resolution images sourced from Google Maps. These images outperformed Sentinel-2 multispectral data.
The AI approach was tested in Brazil’s São Francisco Basin, where hydrogen has been reported from several sites. About 2,000 structures were analyzed, and 48 percent of them are likely associated with hydrogen seeps.
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A classic example of geologic hydrogen produced by serpentinization is in Bulgaria in the Rhodope Massif Mountains; Plam Petrov/Shutterstock.com
Researchers from the Colorado School of Mines have published a paper that offers a multiple geophysical methodology for exploration of geologic hydrogen generated from serpentinization.
Workflow for hydrogen source imaging: The researchers argue that electromagnetic (EM), gravity, and magnetic data routinely used for exploration of nickel deposits hosted in ultramafic rocks can also be applied to imaging hydrogen sources in these rocks.
Workflow for hydrogen reservoir accumulation:
Seismic, gravity, and electromagnetic methods can be used for hydrogen reservoir delineation and drilling de-risking.
Seismic imagery, as applied to petroleum basins, would provide information about structural traps and reservoir lithology.
The researchers acknowledge that physical and chemical properties of hydrogen are fundamentally different from hydrocarbon gases. For example, residence time of hydrogen in a reservoir may be much shorter.
To circumvent this problem, electromagnetic and gravity surveys should also be applied to detect hydrogen saturation in the reservoir.
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