Exploring natural hydrogen in the NW Pyrenees (France)

Natural hydrogen (H2) is emerging as a promising carbon-free energy source, aligning with France’s ambition for carbon neutrality and energy sovereignty by 2050. Yet, its occurrence, distribution and long-term sustainability remain largely unexplored. In this context, the H2 and helium exploration company 45-8 Energy was granted the “Grand Rieu” exploration license in the northwestern Pyrenees (SW France), to further investigate its natural H2 system prospectivity, with the objective of drilling an exploration well in the near future.

The license covers part of the Mauleon Basin (North-Pyrenean Zone), a Cretaceous hyperextended rift basin inverted during the Tertiary Pyrenean orogeny (e.g. Saspiturry et al., 2020). This region and the adjacent Pyrenean foreland (Arzacq basin) to the north benefit from extensive historical datasets acquired since the 1950s by major academic research programs (e.g. Orogen project) and the Oil & Gas industry (e.g. historical Lacq and Meillon gas fields), including deep exploration wells, 2D/3D seismic reflection surveys and gravimetric and magnetic data.

Our current work aims to integrate and interpret these datasets to characterize each element of the H2 system and perform volumetric and risking evaluations of H2 prospectivity within the Grand Rieu license. Geophysical studies (e.g. Wang et al., 2016; Wehr et al., 2018; Lehujeur et al., 2021; Saspiturry et al., 2024) highlighted gravimetric, magnetic and velocity anomalies suggesting the existence of a large mantle body at depth (8-10 km) under ideal P-T conditions for serpentinization and H2 generation. Numerous active H2 seepages measured at the surface along the North Pyrenean Frontal Thrust system (Lefeuvre et al., 2022) suggest active serpentinization at depth and preferential migration pathways along regional faults. Proven Upper Jurassic and Lower Cretaceous carbonate reservoirs with overlying effective seals are well-known northward in the Pyrenean foreland (Lacq and Meillon gas plays). However, their presence and properties in the Mauleon Basin remain historically poorly studied and therefore needed to be further characterized to improve their predictability. Ongoing seismic interpretation, aiming to identify potential traps and H2 migration pathways at regional scale, reveals a complex structural framework directly linked to Cretaceous hyperextension and following Cenozoic Pyrenean compression. Preliminary results suggest the existence of deep-seated structures suitable for H2 accumulation.

Overall, the Mauleon Basin appears to offer a unique geological setting favorable for natural H2 generation, migration and accumulation. Further characterization of these processes through dynamic numerical modelling is necessary to better constrain the natural H2 system. In addition, volumetric and risking evaluations will guide the selection of a drilling target within the Grand Rieu license, marking a critical step toward assessing the viability of natural hydrogen as a sustainable energy resource in France’s energy transition.