A profile of helium-4 concentration in pore-water for assessing the transport phenomena through an argillaceous formation (Tournemire, France)

A vertical distribution of helium-4 was obtained in pore-water from the Toarcian/Domerian argillaceous formation at Tournemire (South Aveyron, France). Most of the studied core samples were collected from two vertical air-drilled boreholes across the impervious argillaceous formation and penetrating the bounding aquifers. Accessible porosities for helium were assumed similar to those of water and calculated by weighting before and after heating the core samples. Helium-4 concentrations were obtained after out-gassing the rock samples in tight containers. The comparison of the total amount of 4He produced since the deposition of the formation with that presently measured in the rock reveals that more than 97% of 4He has been lost from the solid phase to pore-water and afterwards to the surrounding aquifers. Helium distribution in pore water shows a complex profile because of the presence of unexpectedly 4He-rich samples in the lower part of the Upper Toarcian level. In order to understand this distribution and to assess the transport of helium at the formation scale, a series of preliminary model calculations were performed using helium diffusion coefficients three times higher than those of water isotopes. The comparison between simulated and experimental data indicates that: (1) the helium profile could be similar to those of deuterium and chloride in pore-water if some unexpectedly 4He-rich samples are not considered; (2) the estimated transport parameters (at small scale) may be transposable at the formation scale with some uncertainties; (3) a steady state has most probably been reached between the in situ helium production and its loss towards the aquifers; (4) only 17–30 Ma of diffusion time would be necessary to obtain steady state; this is short compared to the formation age (180 Ma) and thus the helium profile cannot give very precise information about diffusion time in the massif, but is in agreement with the hypothesis that transport processes are dominated by diffusion.