Transfers through argillaceous rocks over large space and time scales: Insights given by water stable isotopes

In order to characterize the large-scale transport properties of the Toarcian/Domerian formation in the Tournemire Underground Research Laboratory, the isotopic composition (δ18O and δ2H) of pore water was determined on samples from a 250 m borehole drilled from the tunnel ground down to the lower aquifer. The isotope data were obtained from core samples by diffusive vapour exchange. The comparison with previous profiles obtained from vacuum distillation shows a clear discrepancy for all the stratigraphic levels. This demonstrates that artefacts, induced by distillation and already highlighted on only one stratigraphic level, occurred all over the shale sequence. Moreover, diffusion parameters were derived from both radial diffusion experiments and petrophysical determinations. To evaluate the large-scale transport properties of the formation, we performed a series of diffusive model calculations and compared them to experimental data. In agreement with the hydrogeological history, the best simulations considered an activation of the embedding aquifers 11–13 Ma ago, corresponding to the beginning of the diffusive process through the argillaceous formations.