Constraining porewater chemistry in a 250m thick argillaceous rock sequence

The geochemistry of an argillaceous rock sequence from a deep borehole in NE-Switzerland was investigated. The focus was to constrain the porewater chemistry in low permeability Jurassic rocks comprising the Liassic, the Opalinus Clay formation, the ‘Brown Dogger’ unit and the Effingen Member (Malm). A multi-method approach including mineralogical analysis, aqueous and Ni-ethylenediamine extraction, squeezing tests and pCO2 measurements as well as geochemical modelling was applied for this purpose. A consistent dataset was obtained with regard to the main solutes in the porewaters. A fairly constant anion-accessible porosity of ~50% of the total porosity was deduced for all analysed samples which displayed variable clay-mineral contents. Sulphate concentrations were shown to be constrained by a sulphate-bearing phase, presumably by celestite or a Sr–Ba sulphate. Application of a simple equilibrium model, including cation exchange reactions, calcite and celestite equilibrium showed good agreement with squeezing data, indicating the suitability of the modelling approach to simulate porewater chemistry in the studied argillaceous rocks. The modelling highlighted the importance of correct determination of the exchangeable cation population. The analysis corroborates that squeezing of the studied rocks is a viable and efficient way to sample porewater. •The porewater chemistry of a 250m thick clay-rich rock sequence with variable lithology was constrained using a multi-method approach.•Application of a simple geochemical equilibrium showed good agreement with data from squeezed core samples.•A fairly constant anion-accessible porosity of ~50% was deduced for all samples which had variable clay-mineral contents.•The analysis corroborates that squeezing of the studied rocks is a viable and efficient way to sample porewater.