Porewater geochemistry and modeling within Oligocene-Miocene clays of North Central Spain

The characterization of the interstitial waters of clays collected from a borehole of 650 m depth, and the vertical distribution of dissolved constituents was made in order to identify sources of waters and water-rock interaction processes. The study of the samples included mineralogical analyses of the rock, cation exchange capacity and exchanger population, chemical analyses of porewater samples and integration of results using the PHREEQC geochemical code. The porewaters of the clay samples were extracted at room temperature by squeezing at high pressure (60 MPa). The porewaters analyzed were Na-SO42-, with ionic strength ranging from 0.12 to 0.29 mol/l. The evaluation of the fluid composition, mineralogy, water-rock reaction processes and modeling, indicated that oxidation and degassing processes during collection and storage of the samples could occur. Taking this into account, the original porewater of the formation was reconstructed.