The effects of rock heterogeneity on compaction localization in porous carbonates

Recent field-based studies document the presence of bed-parallel compaction bands within the Oligocene-Miocene carbonates of Bolognano Formation exposed at the Majella Mountain of central Italy. These compaction bands are interpreted as burial-related structures, which accommodate volumetric strain by means of grain rotation/sliding, grain crushing, intergranular pressure solution and pore collapse. In order to constrain the pressure conditions at which these compaction bands formed, and investigate the role exerted by rock heterogeneity (grain and pore size and cement amount) on compaction localization, we carried out a suite of triaxial compression experiments, under dry conditions and room temperature on representative host rock samples of the Bolognano Formation. The experiments were performed at confining pressures that are proxy of those experienced by the rock during burial (5 to 35 MPa). Cylinders were cored out from a sample of the carbonate lithofacies most commonly affected by natural compaction bands. Natural structures were sampled and compared to the laboratory ones. During the experiments, the samples displayed shear-enhanced compaction and strain hardening associated with various patterns of strain localization. The brittle-ductile transition occurred at 12.5 MPa whereas compaction bands nucleated at 25 MPa confining pressure. A positive correlation between confining pressure and the angle formed by the deformation bands and the major principal stress axis was documented. Additional experiments were performed at 25 MPa on specimens cored oblique (parallel and at 45°) to the bedding. Detailed microstructural analyses, performed on pristine and deformed rocks by using optical microscopy, scanning electron microscopy and X-ray computed microtomography techniques, showed that grain crushing and mechanical twinning are the dominant deformation processes in the laboratory structures. Conversely, pressure solution appears to be dominant in the natural compaction bands. Experimental results highlight the strong influence exerted by bedding-parallel rock heterogeneity on both orientation and kinematics of deformation bands in the studied carbonates