Effects of mineral distribution at mesoscopic scale on solute diffusion in a clay‐rich rock: Example of the Callovo‐Oxfordian mudstone (Bure, France)

Abstract

[1] The mesostructure (millimeter to micrometer scale) of clay-rich sedimentary rocks is generally characterized by a connected fine-grained clay matrix embedding coarser nonclay minerals. We use the Callovo-Oxfordian clay-rich rock formation (France) to illustrate how mesostructure influences solute transfer in clay-rich rocks at larger scales. Using micrometer resolution imaging techniques (SEM and micro-CT) major mineral phases (clay matrix, carbonates, tectosilicates, and heavy minerals) were mapped both in two dimensional (2-D) and three dimensional (3-D) at the mesoscale. Orientation and elongation distributions of carbonate and tectosilicate grains measured on mineral maps reveal an anisotropic mesostructure relative to the bedding plane, in agreement with the geological history of the sedimentary rock. Diffusion simulations were performed based on the 3-D mineral maps using a random walk method thus allowing direct computation of mesoscopic scale-related diffusion anisotropy and tortuosity. Considering an isotropic clay matrix, simulated diffusion anisotropy (1.11-1.26) was found lower than the one experimentally measured on macroscopic samples (1.5 to 2), due to the anisotropy feature of pores within the clay matrix. The effects of the mineral content variations on diffusion properties were then investigated by numerical modifications of a mineral map combined with diffusion simulations. Evolution of the tortuosity and diffusion anisotropy with the clay matrix content were successfully interpreted by the Koponen percolation/diffusion model, whereas the Archie approach fails to reproduce diffusion properties at low clay contents. A comparison of fitting parameters with those obtained experimentally indicates that diffusion coefficient variations observed at a large scale could be mainly controlled by the mesostructure.

Deformation in cemented mudrock (Callovo–Oxfordian Clay) by microcracking, granular flow and phyllosilicate plasticity: insights from triaxial deformation, broad ion beam polishing and scanning electron microscopy

“…Mineral fabric is very similar to those in the undeformed COX sample (Fig. 4; see Robinet et al, 2012). In all other BIB cross sections (Figs.…”

“…The microstructural geology community studied microstructures in deformed mudrocks to infer deformation mechanisms (Dehandschutter et al, 2004;Gratier et al, 2004;Klinkenberg et al;Renard, 2012;Robinet et al, 2012;Richard et al, 2015;Kaufhold et al, 2016), but this was limited by problems with sample preparation for highresolution electron microscopy. Conversely, the mechanical properties and related microstructures of natural and experimental high-strain fault rocks have been studied extensively (Bos and Spiers, 2001;Faulkner et al, 2003;Marone and Scholz, 1989).…”

Inputs from in situ experiments to the understanding of the unsaturated behaviour of Callovo-Oxfordian claystone

“…As a consequence, most of the porosity is located in the clay matrix [7], and there is a correlation between clay content and porosity at the scale of Callovo-Oxfordian geological layer. Performed investigations to characterize this porosity indicate that pore network is composed mainly of meso/micro pores and shows a very low connectivity for pores over 40 nm [8]. Using various imaging techniques such as electron microscopy or Xray tomography mineral maps have been obtained [9] highlighting a preferential orientation parallel to the bedding of carbonate and tectosilicates inclusions ( Figure 1).…”

Understanding fluid transport through the multiscale pore network of a natural shale

“…Currently, X-Ray micro-tomography (micro-CT) provides data of the 3D pore space down to about 700 nm. This is insufficient for clay rocks, which display no connected porosity on this scale, although the analyzed sample sizes are representative of a "macroscopic" struc-ture (up to about 180 microns) [10] (1): characterizing the sole clay matrix with FIB/SEM and (S)TEM on several samples to achieve representativeness in a statistical sense [2,5], and by (2): coupling these with micro-CT using scale change methods [8]. Where no single characterization method provides adequate representativeness for such wide pore scales as in clay rocks, the combination of micro-CT, FIB/SEM and TEM is able to provide a full representative 3D pore space [2].…”

“…In this contribution, we benefit from synthetic images derived by training image techniques from micro-CT data for the natural shale from [10]. We also combine both FIB/SEM and TEM imaging of the mesoscopic clay matrix to describe the smallest 1-100nm pores.…”