The development of cataclastic bands in high-porosity sandstones: Insights from ring shear experiments

Investigating the deformation mechanism of cataclastic bands in high-porosity sandstone is crucial for understanding the juxtaposition sealing ability of sandstones in a bed sequence. However, deformation bands that are developed in the field and rock cores do not reflect the evolutionary history of the host rock; therefore, the continuous deformation of high-porosity sandstone cataclastic bands cannot be observed. This paper analyzes the deformation mechanism that affects a cataclastic band when high-porosity sandstone faults form. Based on the latest independently developed high-pressure/low-speed ring shear experimental apparatus, the formation and evolutionary process of cataclastic bands in high-porosity sandstone were studied through the similarities among artificial cores. During the experiments, the effective normal stresses and fault displacement were used as single variables. After the experiments, plunger samples were drilled in the lateral direction, and thin sections were prepared to observe and analyze the thickness and particle characteristics of the deformation bands, thereby characterizing the deformation process of the damage zone. The experimental results reveal that high-porosity sandstone undergoes different intensities of cataclasis during the shearing process, and the evolutionary characteristics can be divided into four primary stages. The particles in the sandstone are successively subjected to rotation, rolling, cracking, cataclasis, and other changes after forced deformation. In terms of the macrostructure, a deformation band can be divided into two layers: an inner zone and an outer zone. From the host rock and the outer zone to the inner zone, the orientational arrangement of the particles along the shear direction becomes increasingly clear, and a greater displacement indicates a stronger orientational arrangement effect of the particles. In terms of the microstructures, the particle sizes in the deformation band are 2-3 orders of magnitude smaller than those in the host rock and exhibit greater roundness. With increasing shear displacement, the thickness of the cataclastic band first increases and then remains constant. With increasing effective normal stress, the thickness first decreases slightly, then increases and finally decreases slightly. Therefore, stress and displacement are important factors controlling cataclasis. The influence of displacement on cataclasis is clear at the initial stage, and stress determines the maximum intensity of cataclasis. Additionally, the increase in displacement will be favorable for roundness when the host rock is poorly sorted. These experimental results provide a theoretical basis for future studies on the effects of cataclastic bands and fluid flow.