Natural hydrogen migration along thrust faults in foothill basins: The North Pyrenean Frontal Thrust case study

The existence of geological fluids rich in natural hydrogen (H2) raises the question about the energy potential of this carbon-free resource. To investigate H2 migration at the fault scale, we present new geochemical and geophysical data recorded along the North Pyrenean Frontal Thrust. Based on both soil gas and electromagnetic transects, we reveal the presence of a gas-draining fault. Soil gas concentration (H2, CO2, CH4 and 222Rn) recorded at 1 m depth increases when approaching the North Pyrenean Frontal Thrust. The maximum H2, CO2 and 222Rn concentrations recorded in the fault zone are 822 ppmv, 10.3 vol% and 57 kBq.m-3, respectively - whereas their local background concentrations are by 1 to 2 orders of magnitude lower: 10 ppmv, 0.2 vol% and 0.3 kBq.m-3 respectively. Our geochemical and geophysical data support the concept of a deep-fluid migration along the detected fault plane. In addition, the study of historical well data combined with the most recent geological and geophysical surveys carried out in the region, highlights zones where H2 could accumulate at depth. The Triassic salt formations, located at 2800 m to 4000 m deep beneath the Mauléon Basin, represent the most promising trap for H2 in the northwestern Pyrenees.