The combination of numerous laboratory analyses and calibrated data from geophysical borehole logs provides detailed, unbiased and highly resolved information on the mineralogical composition of the Opalinus Clay and its confining units.
The mineralogical composition of the Opalinus Clay shows no systematic variability laterally, i.e. the three siting regions are equivalent in this respect. Vertically, a slight upward decrease in the clay-mineral content and a slight increase in the illite/kaolinite ratio can be observed consistently throughout the study area. Overall, the mineralogy of the Opalinus Clay is homogeneous and predictable in the study area.
Large parts of the confining units are also clay-mineral-rich but show higher variability. The underlying Staffelegg Formation (Lias Group) shows little heterogeneity horizontally but consists of several members with distinct properties vertically, including subordinate limestones and silt-/sandstones. The Dogger Group above Opalinus Clay shows heterogeneity both vertically and horizontally and, apart from clay-rich lithologies, also includes limestones, silt-/sandstones and iron-rich oolites.
Diagenetic effects on the Opalinus Clay include some degree of cementation by calcite, mainly localised in silty, calcareous and bioclastic beds and lenses. Siderite occurs in cm-sized lenses or disseminated in the rock. The clay-rich matrix is very weakly cemented, which is a favourable feature from the perspective of self-sealing potential.
Diagenetic mineralisation in the rock matrix is more strongly developed in the confining units. Various diagenetic trace phases were formed, some of which are relevant as controls of the porewater composition (e.g. Ba-Sr sulphate and celestite).
The inventory of detrital and diagenetic minerals present in the Opalinus Clay does not markedly vary over the study area. The mechanisms by which the rock controls the chemical composition of the porewater are thus regionally constant and can be treated in one single porewater model.
