Osmotic interpretation of overpressures from monovalent based triple layer model, in the Callovo-Oxfordian at the Bure site

Abstract

Overpressures in the range 0.2–0.6 MPa are measured in the Callovo-Oxfordian at the Bure site. An understanding of the origin of these pressures is needed in order to justify the hydrogeologic and transport model within this formation. A prior modeling work on this topic [Gonçalvès, J., Violette, S., Wendling, J., 2004. Analytical and numerical solutions for alternative overpressuring processes: Application to the Callovo-Oxfordian sedimentary sequence in the Paris basin, France. J. Geophy. Research 109, B02110, doi:10.1029/2002JB002278.] concluded that an osmotic process could explain at least partly these overpressures. Indeed, in these previous calculations, the presence of a maximum salinity within the clay-rock layer together with a membrane property results in a “build up” in pressure. In this paper, the pore space concentration of water activity determining solutes is analyzed taking into consideration the contribution of ions compensating the structural charge of clay minerals and the fraction of clay minerals in the total lithology according to their vertical distribution. The calculations presented here show that taking into account the fraction of the CEC compensating cations present in the diffuse layer calculated using a recent model coupling transport and a triple layer model for sorption in clayey porous media allows a reasonable representation of observed vertical over-pressure profile.