Physical properties variations in a shaly formation across a fault core

Abstract

Summary Faults in general, and in clay materials in particular, have complex structures that can be linked to both a polyphased tectonic history and to the anisotropic nature of the intact rock. Drilling through faults in shaly materials allows measuring properties such as the structure, mineralogical composition, stress orientation and physical properties. We combine different petrophysical measurements on core samples retrieved from a borehole drilled perpendicularly to a fault zone affecting Toarcian shales from the Tournemire underground research laboratory (France). The borehole is cross-cutting the entire fault thickness which is of the order of ten meters. We perform several types of measurements: density, porosity, saturation directly in the field, and P-wave velocities together with P-waves anisotropy on core samples taken at regular intervals. Special protocols were developed to preserve as much as possible the saturation state of the samples. From our measurements, we were able to track the increase of damage, characterized by a smooth decrease in elastic moduli from the intact zone to the fault core. We then calculated Thomsen's parameters to quantify the elastic anisotropy evolution across the fault. Our results show strong variations of the elastic anisotropy with the distance to the fault core as well as the occurrence of anisotropy reversal.