A FIRST COMPARISON OF NATURAL AND ARTIFICIAL SHEAR BANDS IN POROUS CARBONATE GRAINSTONES

In the last few years, a new faulting mechanism was discussed in high porosity carbonate grainstones which encompasses structures that form by compaction and shear strain into narrow tabular bands, called compactive shear bands (CSB) [1, 2]. Microstructural and textural analyses documented a series of interactive deformation processes involving strain localization into bands, pressure solution and subsequent shearing and cataclasis [1, 2]. Some questions remain unsolved such as the transitions from one deformation process to another, the anisotropy resulting from these changes and possible changes in the orientation and magnitude of applied stresses. To investigate these problems a set of conventional triaxial experiments has been performed on samples of carbonate grainstones and a comparison among natural and laboratory deformed samples carried out. The experiments have been performed with pore pressure of 10 MPa and effective pressures ranging from 5 to 20 MPa. At 5 MPa of eff. press. the deformed samples display a shear-enhanced compaction behavior [3]. The CSB, formed at high-angle to the direction of the major principal stress, is characterized by grain size and pore size reduction. Detailed microstructural analysis shows diffuse stress-induced cracking and calcite mechanical twinning, grain crushing is the main responsible for the comminution observed and pore collapse is responsible for pore size reduction. The comparison between natural and laboratory obtained structures, documents strong discrepancies among the interactive failure processes responsible for the nucleation and growth of CSB. Results suggest that further laboratory experiments, taking in account the roleof temperature, fluid chemistry and of a wider range the strain rate are needed in order to constraint the deformation processes observed in natural samples.