Soil gas ( 222Rn, CO 2, 4He) behaviour over a natural CO 2 accumulation, Montmiral area (Drôme, France): geographical, geological and temporal relationships

Abstract

The south east basin of France shelters deep CO 2 reservoirs often studied with the aim of better constraining geological CO 2 storage operations. Here we present new soil gas data, completing an existing dataset (CO 2, 222Rn, 4He), together with mineralogical and physical characterisations of soil columns, in an attempt to better understand the spatial distribution of gas concentrations in the soils and to rule on the sealed character of the CO 2 reservoir at present time. Anomalous gas concentrations were found but did not appear to be clearly related to geological structures that may drain deep gases up to the surface, implying a dominant influence of near surface processes as indicated by carbon isotope ratios. Coarse grained, quartz-rich soils favoured the existence of high CO 2 concentrations. Fine grained clayey soils preferentially favoured the existence of 222Rn but not CO 2. Soil formations did not act as barriers preventing gas migrations in soils, either due to water content or due to mineralogical composition. No abundant leakage from the Montmiral reservoir can be highlighted by the measurements, even near the exploitation well. As good correlation between CO 2 and 222Rn concentrations still exist, it is suggested that 222Rn migration is also CO 2 dependent in non-leaking areas – diffusion dominated systems.