Compaction and failure in high porosity carbonates: mechanical data and microstructural observations

We investigate systematically the micromechanics of compaction in two carbonates of porosity above 30%, Majella grainstone and Saint Maximin limestone. The composition, grain size and pore surface area of these rocks were determined. Hydrostatic compression experiments were performed in dry and wet conditions beyond the onset of grain crushing pore collapse. A significant weakening effect of water was observed in both rocks. Series of conventional triaxial experiments were performed in dry conditions at confining pressures ranging from 3 to 31MPa. Microstructural observations were carried out on the deformed samples. Results show that the mechanical behaviour of these high porosity carbonates is dominated by shear-enhanced compaction associated in most cases with strain hardening. Stress induced cracking and grain crushing are the dominant micromechanisms of deformation in both rocks. In Majella grainstone compactive shear bands appeared at low confinement, in qualitative agreement with the deformation bands observed in the field. At higher pressures, compaction localization was inhibited and homogeneous cataclastic flow developed. In Saint-Maximin limestone, compaction localization was observed in all deformed samples. An increasing number of compactive shear bands at various orientations appeared with increasing strain. Our new data suggest that compaction localization in an important feature of the mechanical compaction in carbonates of high porosity.