Geological stability of clay-rich rock formations: evidence based on natural tracer profiles

In comparison to many other rock types, clay-rich formations have high water contents and porosities. However, hydraulic conductivities measured on core samples and in boreholes are mostly very low, which is due to the extremely small apertures of individual pores (predominantly in the range of nanometers). By consequence, specific mineral-surface areas with which water molecules and solutes interact are very large. Knowledge of the microscopic characteristics of the pore space is the key to the understanding of several macroscopic properties, such as the commonly observed insignificance of advection and the large chemical buffering capacity. Given the generally long residence times, the chemical composition of pore waters in clay-rich formations is controlled by equilibria with mineral phases, such as carbonates and silicates, and ion-exchange reactions with clay minerals (Beaucaire et al., this volume). The generally reducing conditions are strongly buffered by pyrite, siderite and/or organic matter. Some constituents of the pore water are conservative, i.e. not buffered by minerals, and can move in the interconnected pore space. These constituents include: - the H2O molecules of the pore water;- halogens (e.g. Cl, Br); and - noble gases (e.g. He, Ne, Ar). H2O of the pore water and dissolved noble gases can move in the entire physical porosity of a rock, provided the pore space is interconnected. The movement of water molecules can be monitored by using their stable isotopic signatures, expressed as δ18O and δ2H. In clay-rich formations, halogen can only move in a fraction of the physical porosity due to anion exclusion effects. The conservative constituents can be used as tracers of migration processes in clay-rich formations, such as diffusive exchange with the embedding aquifers and advection across the formation. They are archives of migration processes that occurred in the formation over geological timescales in the past. If these processes are understood quantitatively and if the evolving geological boundary conditions can be constrained, predictions of future evolution can be made. (authors)