Natural hydrogen exploration in the Lubna area (CZ)

Due to the increasing importance of natural hydrogen (H2) as low-carbon energy source and fossil fuel alternative in terms of the global energy transition, hydrogen exploration by means of soil gas analysis has gained increasing attention. The study area selected for this purpose, located in the Western Carpathians, in the Lubna area in the Moravia region of the Czech Republic, is known for the hydrocarbon fields ¿Kostelany Zapad¿ and ¿Kostelany Vychod¿. During multiple campaigns in 2024 (summer and autumn) and 2025 (winter), a total of 638 on-site soil gas measurements were conducted. These on-site measurements were supplemented by laboratory gas chromatographic and stable isotope, as well as soil compositional and microbiological analyses. The main target was the determination of the viability of soil gas measurements for exploration purposes to adequately ascertain the presence of deep geogenic hydrogen, determine possible migration pathways as well as a potential microbiological hydrogen input. On-site measured soil gas H2 concentrations range from 0 to > 1000 ppmv. The majority of elevated values (> 385 ppmv H2) are located within the Ra¿a Unit of the Carpathian Flysch Belt, the lithology of which is predominantly characterized by sandstone and claystone. A general correlation between elevated hydrogen levels with faults and nappe thrusts could not be confirmed. Rather, the elevated hydrogen levels appear to correlate with stream routes and depressions that occasionally coincide with faults. In addition, a seasonal decrease of measured hydrogen levels from summer to autumn and winter was observed. Soil gas trends from on-site measurements and gas chromatographic analyses primarily correspond to a biological respiration trend, with only subordinate influence of exogenous CO2 addition, CO2 dissolution, atmospheric mixing or methane oxidation. Results of stable isotope analysis reveal that the gases are located within the ¿primary microbial¿, in particular the ¿methyl-type fermentation¿ genetic field. A total organic carbon (TOC) content ranging from 0.42 to 1.96 wt.% that, as with hydrogen, decreases with depth, was determined using soil composition analysis. For the southern part of the study area, dominated by woodland, a median pH value of 4.74 along with the majority of significantly elevated hydrogen levels was identified. For sand samples, the soil microbiological analysis indicates an oxic environment, with a prevailing presence of aerobes and a lack of predicted hydrogen producers, thus rendering an excessive microbial production of H2 unlikely. In contrast, the detected elevated H2 concentrations in clay samples could be explained by the observed predominance of predicted and documented hydrogen producers in the anoxic environment. The results obtained are indicative of a predominantly microbiological rather than a deep geogenic source for the detected elevated hydrogen concentrations. Furthermore, as the hydrogen ¿hot spots¿ do not show a clear correlation with outcropping fault systems, their significance for the confirmation of a potential hydrogen flux from the deeper subsurface is questionable. The results of this study highlight that the application of soil gas measurements for the exploration of deep geogenic hydrogen should generally be treated with caution.