Helium production and transport in low permeability formations: The case of the Toarcian argillites at Tournemire (France)

Consolidated argillaceous formations are considered as potential host rocks for the disposal of radioactive wastes, because of their very low permeability and sorption properties. Diffusion is assumed to be the dominant transport phenomenon in argillites or compacted claystones like those studied in URLs at Bure (France), Mont Terri (Switzerland) and Tournemire (France). In such media, the study of natural tracers (4He, 2H, Cl-) in porewater appears as a pertinent method for understanding the water and solutes transport properties at large scales. The chemically inert behaviour of dissolved helium and its continuous in-situ production make it a suitable tracer for investigating solute transport in a subsurface rock-water system over a very long period of several million years. Twenty-one core samples, collected during two drilling campaigns performed at the IRSN’URL of Tournemire, were in-situ sealed in high-vacuum containers and analyzed for their He contents. He concentrations in the porewater range from 2.10-6 to 3.44.10-4 cm3STP/gwater. An average 4He production rate of 6.968.10-13cm3STP/grock was calculated based on the average concentration of uranium (2.74 ppm) and thorium (12.87 ppm) measured in the Toarcian/Domerian claystone. Based on the average porosity (0.11) and rock grain density (2.72 g/cm3) the average 4He accumulation rate in the porewater is 3.18.10-12 cm3 STP 4He/gwater. The comparison of the total amount of 4He produced since the deposition of the claystone (185 to 175 Ma ago) with that presently measured in this rock reveals that more than 97% of 4He has been lost from the solid phase and transferred to porewater. From there, most of the produced 4He has been lost from the low-permeability sequence to the surrounding aquifers as indicated by the concentrations of 4He present today in the porewater. The distribution of 4He was simulated using diffusion coefficients of He in porewater three times higher than those for deuterium. The results of these simulations suggest that a steady state between He production in the aquitard and its loss towards the aquifers is not reached. The profile of measured He concentrations also shows in its lower part an almost linear trend with an unexpected peak and an abrupt decrease at the top. This He profile contrasts with that obtained for 2H of porewater (Savoye et al., 2008), which shows a diffusive bell-shaped profile with an apex located in the centre of the argillaceous formation. It is therefore proposed that the shape of the He profile is linked to recent geologic events, which were not recorded by other tracers (water stable isotopes and chloride). These results imply more investigations on the recent palaeo-hydrogeological evolution of the Causse Basin, where the Tournemire formation is located. References S. Savoye, J.-L. Michelot , F. Bensenouci, J.-M. Matray, J. Cabrera, 2008: Transfers through argillaceous rocks over large space and time scales: Insights given by water stable isotopes. Physics and Chemistry of the Earth 33, S67-S74.