Thermal evolution and hydrogen generation potential of Paleozoic source rocks in the Aquitaine Basin (SW France)

While natural hydrogen (H 2 ) anomalies in the Pyrenean domain are often attributed to mantle serpentinization, the spatial disconnection of the Aquitaine Basin from this mantle source suggests that alternative thermogenic origins must be considered. To assess this potential, we present a new dataset of 25 Paleozoic samples from the Aquitaine Basin (SW France) and the Pyrenean domain. Organic matter maturity was quantified by Raman spectroscopy, yielding maximum burial temperatures between ∼220 °C and ∼410 °C. These results document strong spatial variability: mean peak temperatures reach 337 ± 49 °C in the South Variscan domain, 258 ± 26 °C in the Variscan foreland, and 352 ± 15 °C in the Western Pyrenees. One-dimensional basin modeling, calibrated against this new dataset and previous maturity data, identifies two distinct thermal events: (i) a Variscan high-temperature/low-pressure phase and (ii) a Cretaceous rifting phase. Both episodes likely triggered natural hydrogen generation. Critically, while the South Variscan domain and the Western Pyrenees appear largely depleted, the Variscan foreland retains a significant residual potential, with present-day conditions in the Arzacq Basin still falling within the effective H 2 generation window (∼220–330 °C; ∼3.5–5 %Ro). This integrated approach provides the first robust thermal framework for assessing hydrogen generation in the Paleozoic basement, refines the location of prospective domains beyond the North Pyrenean Zone, and offers a possible explanation for unexplained soil gas H 2 anomalies. • Thermogenic H 2 forms in overmature Paleozoic rocks rich in organic matter. • Raman data confirm peak burial temps from ∼220 °C to ∼410 °C. • 1D modeling shows two main heating phases: Variscan and Mesozoic. • Present geothermal gradients favor deep H 2 exploration zones. • Organic origin explains H 2 anomalies beyond serpentinization zones.