Key points:

• The Opalinus Clay is characterised by average clay-mineral contents between 55 and 60 wt.-%. There is no relevant difference between the siting regions.
• In the study area, the Opalinus Clay is characterised by comparably minor lithological and mineralogical variations in the vertical dimension. Characteristic depth trends in the different boreholes can be correlated at regional scale, that is, the composition is predictable.
• The composition of the clay-mineral fraction in the Opalinus Clay varies only slightly in the vertical dimension and is also consistent laterally. No relevant spatial variability must be expected regarding swelling or sorption behaviour.
• The similar mineral composition in the Opalinus Clay across the study areas allows the use of the same set of equilibrium phases in the porewater model for all siting regions.
• The confining units are mostly clay-mineral-rich, but show a larger variability compared to the Opalinus Clay. They include layers with low clay-mineral contents and thus reduced self-sealing potential.