Structural modifications of Callovo-Oxfordian argillite under hydration/dehydration conditions

The clay minerals are among the first silicated minerals to be transformed physically and chemically during changes of temperature, pressure, relative humidity and fluid composition. One of the first physical processes, extremely rapid, is the hydration and/or dehydration of swelling clays minerals. These processes could generate changes of the clay structure and consequently the texture of an argillaceous rock. In order to observe in situ structural modifications on a deep argillaceous rock ‘‘argillite’’ (Parisian basin, in France) according hydration–dehydration cycles, an Environmental Scanning Electron Microscope (ESEM) was used. This instrument allows the possibility to observe geological samples in their natural state without preliminary preparation or modification. Two types of samples were prepared, parallel and perpendicular to the lithology. Then, each sample was submitted to three water condensation/evaporation cycles. The observed samples were from the Callovo-Oxfordian formations in HTM 80743, HTM 983, HTM 02618 and EST 2159 cores. ESEM observations show that the water sensitivity of these bulk samples depends directly on the clay mineral proportion in the rock, and on the clay family present. It is also obvious that the water sensitivity depends on the anisotropy of the pore structure and the particle size and total porosity. The main structural modifications observed were the cracking of the surface, particles aggregation/disaggregating and opening/closing of pores and/or cracks. D 2003 Elsevier B.V. All rights reserved.