In the race to find the best mix of clean fuel sources, many oil companies are reinventing themselves more broadly as “energy companies” and including geothermal energy, hydropower, solar and wind farms among other sources in their projects. In this context, hydrogen has recently become very important for most energy companies worldwide and offers significant potential to enable the transition to a clean, net-zero-emissions world economy.
Hydrogen was first observed by Robert Boyle experimenting with iron and sulphuric acid, although it was Henry Cavendish in 1766 who recognized it as a distinct element. Later, in 1783 Antoine Lavoisier recognized the nature of the gas and gave it the name “hydro-gen,” meaning “water forming.”
Hydrogen is the most common and lightest element in the universe. Most of the hydrogen on Earth is found in its molecular form, such as in water and organic compounds. It is colorless, odorless, non-toxic and highly combustible. This latter property has increased human interest in the element for use in power generation. Hydrogen reacts with oxygen to form water. This means that hydrogen can be oxidized without CO2 emissions. However, this reaction is very slow at low temperatures, but when accelerated by a catalyst it can be explosive. Hydrogen-specific energy (energy per unit mass) is superior to hydrocarbons; but, its energy density (energy per unit volume) is much lower than hydrocarbons, this means that, to use it in an efficient way, it needs to be liquified.
Narrowing the Hydrogen Rainbow
All the hydrogen consumed worldwide is manufactured by different industrial processes and, depending on the way it is produced and treated, a color has been assigned, resulting in a wide variety of different kinds of hydrogen: grey, brown, blue, pink, turquoise, green, gold and, most recently, orange. But in all these cases, hydrogen is an energy carrier, meaning energy is expended to create hydrogen, which is used later to produce energy. Thus, it might be useful to store the excess energy, if any, from renewables (green hydrogen). But this process has a negative energy balance: more energy is expended than what will be obtained later.
It has historically been understood that hydrogen cannot exist in nature in its pure form because of its high reactivity. Nevertheless, there are more than 300 global examples of natural hydrogen seeps in a broad array of geological settings, including oceanic spreading centers, passive and convergent margins and intraplate regions – examples of which are known as “native hydrogen.”
One of the benefits of native hydrogen is that it could be an energy resource, as energy is not expended to generate it.
This raises the possibility: can native hydrogen be part of the energy transition?
There are, as yet, too many uncertainties to answer this question conclusively.
However, it is already known and accepted that there are multiple geological sources producing hydrogen. One of the sources that seems to be responsible for major hydrogen fluxes is the water reduction during the oxidation of ferrous to ferric iron: 2 FeO + H2O = Fe2O3 + H2(aq)
Rocks abundant in ferrous iron-bearing minerals, such as peridotites, are prone to produce hydrogen, and some of the highest hydrogen concentrations are found in these rocks. Another significant source of native hydrogen is natural radiolysis, where the decay of radioactive minerals, such as potassium, thorium and uranium, generates radiation that can break apart the hydrogen-oxygen bonds of water to produce hydrogen radicals (H) and hydroxyl radicals (OH). The subsequent reaction of two hydrogen radicals generates H2. Other possible source is related to volcanic degassing, as important emissions of H2 are also observed during, or immediately after a volcanic eruption.
From an exploration point of view, many geological domains can be possible targets:
- Mid-ocean ridges where hydrogen rich fluxes were discovered initially, although these are very remote and in international waters.
- Under the continental crust or in subduction zones, reaching the mantle. But the presence of water to generate the reaction might be compromised. In any case, it will be also too deep to be considered for exploration.
- Passive margins, where the presence of exhumed mantle and active serpentinization is well known.
- Mountain ranges like the Pyrenees or the Alps among many others, with mantle rocks exhumed or close to the surface.
- Cratonic areas with an iron-rich basement.
The last three seem to be the more accessible and so should be where most of the efforts should be focused.
Scientific and Economic Questions
There are many unknowns and questions to be solved if we want to think on hydrogen exploration. When dealing with petroleum systems, we know the main elements needed to study; from source rock, expulsion and migration, to reservoir, seal and trap, and the right timing between all of them.
However, in a hydrogen system, although we know the reactions, there are plenty of other questions to consider. What really is the source rock? How can we characterize a good source rock from another? Is it a way to know how “mature” it is? How does the hydrogen migrate and how are the reactions during this process?
Although recent studies have demonstrated that some lithologies can act as an effective seal (such as Martín-Monge and Vayssaire, 2022), more are needed.
There are also many questions to be answered for economic purposes. What can be the average size of a natural hydrogen field? And what would be the cost of exploration, development and production? And very important these days: what is the carbon footprint of exploring and producing hydrogen?
Many of these questions lead to still more questions than answers right now, and they need to be solved through a close collaboration between academia and industry.
The early hydrocarbon explorers, back in 19th century, had the same unknowns and questions. However, nothing stopped them from exploring.
So, the question persists: can native hydrogen be part of the energy transition? The only answer I have right now is that it is in our hands and in the hands of future geoscientists. As Nigel Smith already stated in 2002, “It´s time for explorationists to take hydrogen more seriously.”
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