Geochemistry and porewater in clay rocks and limestones

Mineralogy and geochemistry

Minerals precipitated in the rock matrix during diagenesis or hydrothermal alteration, as well as fracture infilling minerals, are archives of past fluid-flow events. Based on the textures, the chemical and isotopic compositions of rocks and individual minerals, we reconstruct the sequence of events and infer the fluid sources. Understanding the system behaviour in the geological past is a tool to constrain the spectrum of future events and processes that may affect the repository system over its lifetime.

Carbonate and sulphate minerals frequently are the primary controls on the pore-water compositions. In clay-bearing rocks, clay minerals also affect pore-water chemistry via cation-exchange equilibria. We identify and characterise these minerals as a basis for thermodynamic models that reconstruct the pore-water composition.

Porewater evolution

Pore waters in aquitards reflect the past geochemical evolution of the adjacent aquifers. Solute transport in aquitards occurs predominantly via diffusion and is therefore slow. Depending on the aquitard thickness and the diffusion coefficient, the memory of the geochemical archive may reach back tens of thousands to millions of years. While conservative solutes such as Cl-, Br-, d18O, d2H or He directly reflect the diffusive response to changing boundary conditions, SO42- and all cations are also affected by rock-water interactions, such as solubility control by carbonate or sulphate minerals or cation exchange on clay-mineral surfaces.

We use a number of methods to extract pore water from low-permeability sedimentary rocks. The spatial distribution of solute concentrations, in particular the trends towards water-conducting zones (e.g. sandstone beds in sedimentary sequences, faults in crystalline rocks), yields information on the rates of diffusive exchange and on past conditions in the water-conducting zone. We quantify the exchange process by numerical modelling, mostly targeted at conservative constituents but using the whole major-ion chemistry in more recent times.

In the context of the disposal of radioactive waste, knowledge on pore-water chemistry and its spatial distribution are used for various purposes, including the characterisation of pertinent radionuclide properties (solubility, speciation) and the identification of solute-transport mechanisms in the aquitard.

Water-conducting systems

Faults and joints are present in most aquitard units and were formed either during tectonic deformation events or as a consequence of decompaction. In aquitards rich in clay minerals, fractures self-seal efficiently and do not constitute preferential fluid pathways, except in transient stages of deformation. In clay-poor aquitards, the self-sealing capacity may be reduced, and fractures may have permeability well above that of the undeformed rock. Knowledge of the fracture geometry and of the underlying deformation mechanisms are pre-requisites for the understanding of the connectivity of fractures on larger scales and therefore for their relevance as fluid pathways.

We study water-conducting structures in drillcores and use mapping of analogous features on the surface to better understand the larger-scale geometry. We consider structures that were fluid pathways in the past (characterised by mineralisations or wallrock alterations) as well as those that conduct water today.