Modelling of the ion-exchange properties and indirect determination of the interstitial water composition of an argillaceous rock. Application to the Callovo-Oxfordian low-water-content formation

A method for indirect determination of the composition of an argilleous formation interstitial water is described. It is based on the hypothesis that cation distribution on clay minerals is a fingerprint of the solution which the mineral assemblage is in equilibrium with. Precise characterisation of the retention process of all the major cations on the clay and accurate knowledge of the cation inventories of the rock is primarily required. It was shown that the studied clay acts as a multi-site ion exchanger. Three types of sites have been characterised with respect to cation exchange capacity and selectivity coefficients for each major cation. Extraction experiments have provided estimates of the total stock of Cl (free ion), Na and K of the rock but oxidation of the core sample made it impossible to get those of SO42−, Ca and Mg. The calculated interstitial water composition, as deduced from these experiments and the proposed model, is a slightly alkaline (pH≈8.3) Na+–HCO3− type of water with high ionic strength (I≈0.4 M).