Mineralogical and geochemical OPA characterization: constraints for OPA pore water chemical modelling

The pore water chemistry in Opalinus Clay (OPA) at Mont Terri is determined by direct analyses of pore waters from boreholes, by squeezing, and by pore water chemical modelling (Fernández 2026, Wersin et al. 2026). Pore water modelling is based on anion leaching and exchangeable cations and constrained by the dissolved inorganic carbon (DIC) obtained by core degassing and mineral equilibria reactions (Pearson et al. 2011; Wersin et al. 2026). Here, the main mineral and geochemical data acquired on samples from the BPC-C 2, BHT-1 and BDB-1 boreholes at Mont Terri are presented, to determine the main parameters necessary for the pore water model: 1) the cation clay exchanger and its control (BPC-C2); 2) the partial pressure of CO₂ and its origin (BHT-1) and 3) the OPA diagenetic sequence in order to define the diagenetic minerals controlling present-day pore water chemistry (BPC-C2, BHT-1, BDB-1 and BN). Particular attention was paid to Fe-bearing minerals that are sensitive to the redox conditions.