Pore-water Evolution in the Shale-rich Aquitard Sequence at Mont Terri, Switzerland

A substantial geochemical data set has been collected in the Mont Terri Underground Rock Laboratory over 20 years of operation. Various techniques of pore-water sampling from low-permeability shales have been developed and used, augmented by geochemical studies on vein infills in faults. Apart from the chemical composition of pore water, tracers such as δ18O, δ2H, isotopes of sulphate and the 87Sr/86Sr ratio in waters and minerals were used to reconstruct the geochemical evolution. Connate marine pore water diffused out from the Jurassic shale sequence over tens of Ma when the area was subjected to continental conditions. Isotope data indicate that the sea-water component present in current pore water has been introduced into the system during the Tertiary. Veins formed during the thrusting of the Jura Mountains in the Miocene and record transient and localised fluid flow related to seismo-tectonic events, without markedly affecting the bulk pore water. Mountain building led to exhumation and flushing of embedding aquifers. This in turn initiated another stage of out-diffusion of salinity from the shale sequence. If given sufficient time, pore-water evolution of shale-rich sequences can be dynamic, adjusting to changing boundary conditions in the embedding aquifers via diffusion.