Abstract. The macroscopic description of deformation and fluid flow in mudrocks can be improved by a better understanding of microphysical deformation mechanisms. Here we use a combination of scanning electron microscopy (SEM) and broad ion beam (BIB) polishing to study the evolution of microstructure in samples of triaxially deformed Callovo-Oxfordian Clay. Digital image correlation (DIC) was used to measure strain field in the samples and as a guide to select regions of interest in the sample for BIB-SEM analysis. Microstructures show evidence for dominantly cataclastic and minor crystal plastic mechanisms (intergranular, transgranular, intragranular cracking, grain rotation, clay particle bending) down to the nanometre scale. At low strain, the dilatant fabric contains individually recognisable open fractures, while at high strain the reworked clay gouge also contains broken non-clay grains and smaller pores than the undeformed material, resealing the initial fracture porosity.
The application of ion-beam milling techniques to clays allows investigation of the porosity at nm resolution using scanning electron microscopy (SEM). Imaging of pores by SEM of surfaces prepared by broad ion beam (BIB) gives both qualitative and quantitative insights into the porosity and mineral fabrics in 2D representative cross-sections. The combination of cryogenic techniques with ion-beam milling preparation (BIB and FIB, focused ion beam) allows the study of pore fluids in preserved clay-rich samples. Characterization of the pore network is achieved, first, using X-ray computed tomography to provide insights into the largest pore bodies only, which are generally not connected at the resolution achieved. Secondly, access to 3D pore connectivity is achieved by FIB-SEM tomography and the results are compared with 2D porosity analysis (BIB-SEM) and correlated with bulk porosity measurements (e.g. mercury injection porosimetry, MIP). Effective pore connectivity was investigated with an analog of MIP based on Wood's metal (WM), which is solid at room temperature and allows microstructural investigation of WM-filled pores with BIB-SEM after injection. Combination of these microstructural investigations at scales of ,1 mm with conventional stressstrain data, and strain localization characterized by strain-fields measurement (DICdigital image correlation) on the same sample offers a unique opportunity to answer the fundamental questions: (1) when, (2) where, and (3) how the sample was deformed in the laboratory. All the methods above were combined to study the microstructures in naturally and experimentally deformed argillites. Preliminary results are promising and leading toward better understanding of the deformation behavior displayed by argillites in the transition between rocks and soils.#
The application of ion-beam milling techniques to clays allows investigation of the porosity at nm resolution using scanning electron microscopy (SEM). Imaging of pores by SEM of surfaces prepared by broad ion beam (BIB) gives both qualitative and quantitative insights into the porosity and mineral fabrics in 2D representative cross-sections. The combination of cryogenic techniques with ion-beam milling preparation (BIB and FIB, focused ion beam) allows the study of pore fluids in preserved clay-rich samples. Characterization of the pore network is achieved, first, using X-ray computed tomography to provide insights into the largest pore bodies only, which are generally not connected at the resolution achieved. Secondly, access to 3D pore connectivity is achieved by FIB-SEM tomography and the results are compared with 2D porosity analysis (BIB-SEM) and correlated with bulk porosity measurements (e.g. mercury injection porosimetry, MIP). Effective pore connectivity was investigated with an analog of MIP based on Wood's metal (WM), which is solid at room temperature and allows microstructural investigation of WM-filled pores with BIB-SEM after injection. Combination of these microstructural investigations at scales of ,1 mm with conventional stressstrain data, and strain localization characterized by strain-fields measurement (DICdigital image correlation) on the same sample offers a unique opportunity to answer the fundamental questions: (1) when, (2) where, and (3) how the sample was deformed in the laboratory. All the methods above were combined to study the microstructures in naturally and experimentally deformed argillites. Preliminary results are promising and leading toward better understanding of the deformation behavior displayed by argillites in the transition between rocks and soils.#
Abstract. The macroscopic description of deformation and fluid flow in mudrocks can be improved by a better understanding of microphysical deformation mechanisms. Here we use a combination of Scanning Electron Microscopy (SEM) and Broad Ion Beam (BIB) polishing to study the evolution of micro structure in samples of Callovo-Oxfordian Clay that were previously tested in the lab. Digital Image Correlation (DIC) enabled for the measurement of strain fields in the specimens, which were used as a guide to select regions in the sample for BIB-SEM analysis. Microstructures show evidence for dominantly cataclastic mechanisms (intergranular, transgranular, intragranular cracking, grain rotation, clay particle bending) down to nm- scale. At low strain, the dilatant fabric contains individually recognizable open fractures, while at high strain the reworked clay gouge contains broken non-clay grains, with a clear change towards smaller pores than the undeformed material and corresponding resealing of initial fracture porosity. This study might provide a first step towards a micro scale basis for constitutive models of deformation and fluid flow in cemented mudstones.
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