Anna Wallentin, Jesica Murray, Damien Lemarchand, Laurent Truche, et al. « Helium exploration in European rift systems: Regional framework and new data from the Upper Rhine Graben ». Journal of Geochemical Exploration 289 (octobre 2026): 108125. https://doi.org/10.1016/j.gexplo.2026.108125
Sampling at Römerquelle in Badenweiler.
Anna Wallentin
From hydrogen to helium
Your thesis was not originally about helium. How did you end up there?
At first, I was mainly working on hydrogen, with a whole experimental component on hydrogen in granites and a field component to see whether this gas could be detected at the surface. It was while carrying out this fieldwork that we saw helium appearing almost everywhere. That led us to try to better understand this system.
Why helium, why now
How is helium produced today, and why look for other ways to produce it?
Helium is extracted from the same deposits as natural gas, where it accumulates in the same reservoirs. Its source, however, is always the same: the radioactive decay of uranium and thorium present in granites, which releases alpha particles — in other words, helium. Today it is exploited as a by-product of gas, simply because that is where it becomes concentrated. But there are other systems, without hydrocarbons, known as “nitrogen-helium” systems (N₂–He). The prime example is the Rukwa basin in Tanzania: the system’s potential was identified by researchers in recent years, and more recently, in 2024, a company drilled a well there that delivers, continuously at the surface, a gas containing around 5% helium. For a long time natural gas was enough, so the question was raised less often. Today, the European stakes change the picture: Europe produces almost no helium and therefore imports nearly all of it. Helium is, moreover, on the European Union’s list of critical raw materials.
Map of Europe showing He concentrations in free gas from thermal fluids (bubbles or degassed).
Anna Wallentin
Compiling to see where to look
Your article proposes a European map of helium. What does it show, and how did you build it?
The idea was to bring together, on a single basis, all the already-published helium data from the free gases of thermal and geothermal waters, in order to obtain an overall picture of Europe. Until now, these results were scattered, sometimes in publications from the 1970s-1980s where helium was not even the primary objective. By bringing them together, despite very different methods, clear patterns emerge: where helium concentrations are high, we find at once the Variscan basement — hence granite, the source —, geothermal provinces (a temperature anomaly) and the proximity of rift systems. This provides a guideline for the areas to explore as a priority. It is above all a compilation effort, since most of these data points had not been acquired for helium. We wanted to create a common platform, an initial basis on which local teams can build.
Sampling at Chaudefontaine in Vecoux.
Anna Wallentin
The Upper Rhine Graben, a natural laboratory
Why did you choose the Upper Rhine Graben as your study area?
Because it brings together all the ingredients and is exceptionally well documented: an enormous amount of data, seismic surveys, decades of research. It was the ideal place to understand the general conditions, the factors that determine whether there is more or less helium. And it is only one rift system among others: very similar settings are found all along the West European Rift. What we learn there can probably be extended to the rest of that system, and to other rifts in Europe and worldwide.
How did you choose and find your 25 measurement sites?
It was a mix. There were natural springs — those were easy, we simply went there — and, above all, thermal spa facilities, which are private sites requiring authorisations. We had a list of all the facilities and springs we could find, and the map took shape according to the authorisations we obtained. People were very open, and we sampled on site with the technicians. Many measurements were made on the German side, in the Black Forest, where thermal facilities are numerous: we treated the Upper Rhine Graben as a whole, regardless of the border. I also had excellent advice from Ingrid Stober (University of Freiburg), who has worked extensively on these springs and pointed me to the right sampling points on the German side. As for the springs, they were already mapped, notably thanks to the maps of the BRGM (the French geological survey): it is a well-known territory, and what has been mapped probably already represents most of what there is to find.
Helium concentration in thermal waters of the Upper Rhine Grabben region.
Anna Wallentin
Your map shows a clear north-south gradient. How do you explain it?
In the field, the contrast is striking: in the north, the gases are almost entirely nitrogen, with good helium concentrations, up to 2.4% at Ohlsbach; in the south, helium decreases and CO₂ takes up a growing share. We cross-referenced our gas measurements with several other datasets — structural, thermal, isotopic — to understand this contrast, and a coherent picture emerges. In the north, the faults act as efficient conduits: helium rises well. This is particularly clear along the major border faults of the graben and the inherited Variscan transverse faults, as at Vittel or Bad Liebenzell. In the south, the signal is weaker, and several factors combine. First the crust itself: it thins southward, down to about 24 km, with a smaller volume of granitic basement and therefore a lower uranium-thorium stock. Then temperature: between 180 and 300 °C must be reached to release the helium trapped in the granite minerals, and this temperature is perhaps not always reached in the south, which can limit primary mobilisation. Finally, helium isotopes show a mantle contribution in the south. This mantle-derived fluid mixes with the crustal fluid: it dilutes the helium produced in the crust and adds mantle CO₂. The most promising areas are therefore those rich in nitrogen, dominated by the crustal fluid, without this mantle contribution.
He flow rates in Upper Rhine Graben thermal waters, calculated as dissolved He concentration multiplied by spring discharge.
Anna Wallentin
From the flow to the resource
What is the major challenge that now opens up?
Identifying the traps capable of concentrating these weak but continuous fluxes. The sedimentary cover of the Upper Rhine Graben is already well characterised, so we start from a good base. The real problem is knowing what it takes to retain helium. For now, we reason somewhat as for hydrogen: salt layers or thick clays can act as a seal. There are already gas reservoirs in the graben, rather to the north, near Rastatt — the Keuper and Oligocene sandstones, with Dogger claystones as the seal. For future helium targets, we are also considering other possible reservoirs such as the Buntsandstein sandstone or the fractured basement, beneath a Keuper clay-rich seal. Several reservoir-seal combinations could work; is that enough? I don’t know yet. And everything has to line up in the right place: salt, for instance, is in the south, so it won’t be salt there. To identify these structures, the reference tool remains 3D seismic, but it is very expensive. One thing not to forget is that what we have mapped are leaks, not accumulations. It is a map of leaks, not a map of reserves.
So how do you move from these leaks to an exploitable resource?
You follow the path in reverse. The source is known, it’s the granite. For helium to be released, there must be a thermal anomaly strong enough to reach the closure temperature of the minerals: that is primary mobilisation. Then comes migration, driven notably by the circulation of water in the graben along the faults. These three elements, we have. What will decide viability is what comes next: how quickly helium reaches a reservoir, the quality of that reservoir, and its capacity to retain the gas long enough for it to accumulate. Concretely, where there is a lot of leakage is not the right place to look: you have to go along the continuation of these fertile structures, where nothing reaches the surface. On the economic side, profitability thresholds vary from country to country, but they are defined for giant gas fields where the enormous volume compensates for a low grade. So what matters most is not so much the grade as the accumulated volume: a modest grade in a huge pocket is often worth more than a high grade in a small volume. Hence the interest of the co-generation route: at an already-operating geothermal site, one could potentially consider extracting helium alongside the heat, as is beginning to be done for lithium.
How does research position itself relative to the companies already exploring?
Some companies hold exploration permits and a pilot in France. The difference is that they move to the operational stage: they drill where they think there is a reservoir, relying in part on the science we publish and on their own campaigns, notably seismic. This is the pattern seen at Rukwa: first the research on thermal waters, then private players who drill and take the risk. Both worlds are needed — industry funds the work and bears the operational risk.
What next
What comes after the thesis?
It’s not decided yet, but the topic is very timely: Europe will need to understand its subsurface for these critical resources. I hope to keep contributing, on hydrogen or on helium.