In his 1874 science fiction,
“The Mysterious Island,” Jules Verne suggested that one day, hydrogen extracted from water would replace depleting fossil fuels. Since then, hydrogen has been considered the “energy of the future.” At times – for instance, during the first oil shock of 1973 when the term “hydrogen economy” was popularized – huge and concentrated efforts were directed toward hydrogen research and development. We are in the midst of one such hydrogen boom, as this simplest element is viewed as a key to decarbonization.
Hydrogen is currently extracted largely from fossil fuels and some from water electrolysis or splitting. What is perhaps most novel in the current hydrogen boom is the prospect of using natural, geologic or white hydrogen. Unlike steam methane reforming, geologic hydrogen does not emit carbon dioxide; however, exploration of geologic hydrogen requires a deep understanding of its generation and concentration processes.
As petroleum geoscientists, we might be tempted to formulate hydrogen systems in terms of what is most familiar to us – petroleum system components and processes – but hydrogen systems require a different lens and distinct components.
The Petroleum System
Since the 1980s, the petroleum system approach has given a focused, scientific workflow for oil and gas exploration. Petroleum systems are situated in sedimentary basins and consist of five elements:
- Organic-rich and thermally mature source rocks (usually clay-rich sedimentary formations)
- Porous, permeable reservoir rocks (often sandstone and limestone)
- Migration pathways such as porous carrier beds or permeable fractures from the source to the reservoir
- Tight seal rocks capping the reservoirs
- 3-D closures or traps
These components are all necessary for a complete generation-migration-accumulation of oil and gas. Hydrogen sourcing and production cannot be explained with such a neatly defined process.
The Many Faces of Hydrogen
The diverse range of geological settings from which hydrogen comes indicates that there is no single, all-inclusive hydrogen system. Free hydrogen has been reported in various concentrations in volcanoes, kimberlite pipes, uranium-rich and/or iron-rich igneous and metamorphic terranes, Precambrian shields, faulted rift zones, subduction-zone arcs, coal basins, wet soils, geysers and hot springs, as well as in oil and gas fields.
The vast majority of reports on hydrogen generation from rocks involve mafic (magnesium-iron rich) and ultramafic rocks, such as ophiolites – ocean-crustal rocks obducted and outcropped on continents. In a process called serpentinization, ferromagnesian minerals, such as olivine and pyroxene, extract oxygen from hot water to form serpentine. As a result, free hydrogen is released. Because this geological process is very slow and diffuse, some researchers have encouraged creating and using technologies to “stimulate” ophiolites via fracturing and enhanced chemical reactions, hoping to increase free hydrogen generation and recovery.
Another process for splitting water is radiolysis, in which water is exposed to a high flux of ionizing radiation from radioactive elements present in rocks. This process might occur in highly radioactive granitic rocks (as fluid inclusion studies demonstrate) or may be achieved in a nuclear reactor, producing so-called nuclear hydrogen. In all these cases, whether natural or stimulated, the source of hydrogen lies in water, not in rocks or minerals, which simply act as a medium to release hydrogen from water.
Mantle Degassing
Other forms of hydrogen might be released from the mantle. A leader in the space with and author of the most-comprehensive article on the occurrence of natural hydrogen published in Earth-Science Reviews, Viacheslav Zgonnik is among scientists who emphasize a vast repository of primordial hydrogen in the Earth’s mantle, continuous mantle degassing and even Earth’s mass loss over time. Volcanic eruptions are considered a tiny portion of this planetary degassing process.
Hydrogen accounts for about 92 percent of the universe; it is, therefore, conceivable that this cosmic primordial hydrogen is also present in Earth’s core and mantle. This idea is supported by hydrogen’s affinity to iron during the formation of Earth’s core as discussed by researchers Liu and Jing in the journal Communications Earth & Environment and the presence of hydrous minerals in the lower mantle as simulated by Tsuchiya and Thompson in Progress in Earth and Planetary Science. On the other hand, a recent experimental study published in Contributions to Mineralogy & Petrology by Vlasov and colleagues indicates that immiscibility between water and hydrogen-rich fluids in the upper mantle was probably responsible for rapid hydrogen loss and massive oxidation of the upper mantle during Earth’s early history.
To what extent this deep-seated source of hydrogen can accumulate in commercial volumes in near-surface and shallow reservoirs is open to debate and investigation. Nevertheless, this suggested planet-scale mantle hydrogen system is different from “organic hydrocarbon kitchens,” which are confined to sedimentary basins.
Looking at hydrogen generation through the lens of a petroleum system may still be doable with some hydrogen found in oil and gas fields, but in many other cases, aside from loosely employed terminology or apparent similarities, there are fundamental differences between the diverse, diffuse and reactive systems of natural hydrogen and the more specified petroleum systems.