Pore network connectivity anisotropy in Jurassic argillite specimens from eastern Paris Basin (France)

Abstract

In order to test the feasibility of nuclear waste storage, Andra, the French radioactive waste management agency, gave us the opportunity to study preserved specimens of Jurassic clay-rich rocks from eastern Paris Basin. These rocks, deposited during the Callovian and beginning of the Oxfordian, are dark- to light-grey marls that consist mainly in a mixture of clay, calcite and silt. Magnetic susceptibility and remanence vary according to the clay/calcite/silt ratios and the mineral preferred orientations are characterized by the anisotropy of the magnetic susceptibility. A few test specimens, sampled from borehole-core #HTM 102, and coming from the base and top levels of the Callovo-Oxfordian argillite formation, were subjected to connected porosity measurements using the mercury injection technique. By imposing mercury to flow parallel to a given direction, we were able to determine the anisotropy of connectivity along the three principal magnetic susceptibility axes. We find that the clay-richest specimens have a large and sub-isotropic connected porosity which is mostly accessible through the smallest pore threshold diameters (<0.02 μm). By contrast, carbonate-enriched specimens have anisotropic and smaller connected porosities accessible through larger pore thresholds (∼0.08 μm). Except in a carbonate-enriched specimen where the largest connectivity axis is vertical, attributed to tension cracks normal to bedding, the pore connectivity anisotropy positively correlates with the magnetic anisotropy, hence with the mineral arrangement.