Helium diffusivity and fluxes from a sedimentary basin (Permo-Carboniferous trough, Northern Switzerland)

In order to estimates the rates of He migration through the PCT, we compare the produced and observed abundances within the frame of 1D diffusion model. Only two parameters, the diffusion coefficient for the whole sequence, DALL = 4.1 × 10−3 m2 year−1, and the rate of He removal from the crystalline aquifers, θCR ≈ 1.4 × 10−6 year−1, are required to obtained a good agreement between the observed and calculated He concentrations in PCT waters: the minimal square deviations are ≈(2 to 3) × 10−8 mol cm−3 H2O, comparable with the accuracy of measurements. The following parameters were derived from the modeling: the reduction factor DALL/D0 = 0.013; the He diffusion fluxes into the overlaying Muschelkalk and underlying Crystalline aquifers, FMU ≈ 1.4 × 10−7 mol m−2 year−1 and FCR = 0.24 × 10−7 mol m−2 year−1, respectively. From these values we calculated the mean helium residence times in PCT, <τPCT> ≈ 120 Ma, and in Muschelkalk aquifer, <τMU> ≈ 6 ka. The relationships between He diffusivities of the PCT and the underlying Crystalline basement are discussed. We further compare our results with other estimates of He mobility in low-permeable sediments.