Alain Prinzhofer

Measurements of the soil’s surface for natural hydrogen exploration may be considered dubious due to several artefacts. For instance, drill bit metamorphism and superficial biological activity can induce hydrogen generation that is irrelevant for energy exploration. Furthermore, adsorbed hydrogen can be released through drilling and pumping operations. Here, we present a model of the hydrogen signal in soil when air containing traces of hydrogen is pumped out. According to this model, observed hydrogen recharge after several minutes would indicate active hydrogen flow during the measurements, whereas its absence may indicate adsorbed hydrogen in the soil or artefacts. This model allows us to quantify the hydrogen flow responsible for concentration anomalies in soils. It is consistent with the hypothesis that hydrogen emission from soils is not constant but rather occurs in the form of gas flushes over a period of a few days.

This study presents the first comprehensive soil gas survey across southern Uruguay’s H₂ prospective terranes. A pre-field trip selection was done on the basement rock nature, as well as vegetation anomalies in subcircular depressions and fault presence. The Neoproterozoic terrane, north of Punta del Este, and the Archean Rio de la Plata Craton, north of Montevideo, as well as along the suture zones between the two, were targeted. Our findings reveal substantial H₂ concentrations, significantly outperforming many established basins worldwide. The suture zones act as critical migration conduits for H₂ coming from a deeper structural level. Slightly abnormal helium signatures confirm an active, deep-sourced fluid system, particularly within the Sierra Ballena and Cordillera shear zones. The Archean Rio de la Plata Craton appears promising but has only been partially sampled and warrants further investigation. These results underscore the high potential of Uruguay as a new frontier for natural hydrogen exploration.

Siderite (FeCO₃) is an iron carbonate present in Archean and Neoproterozoic Banded Iron Formations (BIFs) as well as in much more recent sedimentary basins. Due to its high iron content, it could be part of H₂ systems, and we have quantified its potential as a H₂-generating rock and its kinetics from samples from the Llanos Basin in Colombia and from the Solimões Basin in Brazil. The H₂ potential of natural siderite-rich Cenozoic formations was measured with a Hydrogen-Eval pyrolyzer, where the H₂ yield depends linearly on the siderite content of the rock. It reaches 3.5 mol/kg for a sample containing 50% of siderite. The siderite is known to be unstable and to decompose at relatively low temperatures, between 100 and 200 °C. These results suggest that sedimentary siderite could be an additional potential H₂-generating rock, through oxido-reduction, with an active H₂ generation window between 3 and 6 km. In sedimentary basin at these depths, reservoirs and traps are likely to be present, and the porosity could be ideal for accumulation. Strong external water supply through hydrodynamics will promote H₂ generation, but the dehydration of rocks such as clays will also enable it through local water supply.

Hydrogen gas seeping from Proterozoic basins worldwide is a potential non-carbon energy resource, and the vents are consequently receiving research attention. A curious feature of H₂ venting in the Sao Francisco Basin in Brazil is that the venting displays a very regular daily cycle. It has been shown that atmospheric pressure tides could explain this cycle, but solid earth tides might be an alternative explanation. We show here that it is unlikely that solid earth tides are a dominant control because they have two equally strong peaks per day whereas the H₂ venting has only one.