Spatial distribution of porosity and minerals in clay rocks from the Callovo-Oxfordian formation (Meuse/Haute-Marne, Eastern France)—implications on ionic species diffusion and rock sorption capability

Imaging techniques, adapted to clay rocks, combined with mathematical correlations of petrophysical parameters bring a new petrographic description of this clay rock integrating multi-scale data. Rock heterogeneities (mineral and porosity distributions) are studied at the mineral pore, the core and the borehole scale for interpreting diffusion tests. Although the infra-micrometric pores are associated with the clay particles and larger pores are associated with bioclasts and detrital minerals, especially tectosilicates, relationships between minerals and pores are quite complex at the borehole scale. They are mainly controlled by mineral spatial arrangements and contents, since micro- to macrostructures of centimetric size are encountered in each lithofacies. Effects of surface interactions and pore-constriction electrochemical controls were underscored for a large proportion of the effective porosity. Although the effective porosity is physically interconnected in the rock volume, diffusion and sorption are discussed in terms of structure and accessibility to sorption sites. These data could be the basis for the construction of a quantitative transport model by diffussion taking into account the sample heterogeneities from the mineral pore size to the centimetric core size ranges. The conceptualisation of the organization of the porosity in such low permeability clayey medium, considered together with the analysis of interface-derived effects constitutes an important step forward in understanding the mechanisms affecting the migration of single ionic species. This understanding should help in extrapolating transport parameters obtained at the centimetric scale from laboratory experiments to the geological formation scale.