Study on the Fracture Formation Mechanisms in Reservoir Rocks of the Second Member of the Dengying Formation in the Sichuan Basin

Abstract The second member of the Dengying formation (DYF M2) in the Sichuan Basin holds substantial potential for exploration and development, with fractures serving as a pivotal factor for the efficient extraction of natural gas resources within this formation. Nonetheless, there exists a notable gap in systematic research concerning the fracture formation mechanisms within the reservoir rocks of this specific geological layer, and an adequate assessment of fractures at the core scale remains elusive. To address this, our study conducted triaxial compression tests and fracture toughness experiments on the reservoir rocks, focusing on the analysis of shear and tensile fracture characteristics and mechanisms across porosity-dominated, vuggy, and fracture-dominated core types. The findings reveal that, in porosity-dominated cores, shear fractures exhibit oblique, smooth, and continuous traits, often characterized by large angles, whereas tensile fractures initiate at the contact surface and midsection, with vertical extension observed in central fractures. Conversely, in vuggy cores, shear fractures tend to deflect, connect, or bypass dissolved pores, resulting in complex trajectories for tensile fractures influenced by these pores. In the case of fracture-dominated cores, the intersection patterns between shear fractures and natural fractures display a diverse range, with tensile fractures initiating near natural fractures and expanding under their influence. Further analysis reveals that in porosity-dominated cores, the formation of shear fractures is influenced by both the internal structure and triaxial stress, with tensile fractures initiating in areas of stress concentration. In vuggy cores, the presence of dissolved pores plays a dominant role in the formation of both shear and tensile fractures, altering the mechanical properties and stress distribution, which subsequently impacts fracture development. Lastly, in fracture-dominated cores, the formation of both types of fractures is controlled by natural fractures, which modify the stress field and influence the fracture trajectories. These insights provide a comprehensive understanding of fracture formation mechanisms in DYF M2, guiding future exploration and development efforts in the Sichuan Basin.