Failure mechanism of secondary fractures associated with strike slip fault and its stress field implication: a case study of Miaoxinan Uplift, Bohai Bay Basin

Formation of secondary tensile fractures can be attributed to stress perturbations that developed around the preexisting main strike slip fault. These secondary fractures are generally oriented oblique to the pre-existing flaw, and their orientations tend to follow a striking similar pattern. To further study the common features of the secondary faults, fracture mechanics were used to interpret stress field conditions responsible for secondary fractures that adorn the major strike-slip fault. In our study, a tentative geomechanical model is conceived, by evaluating the correlation between their geometric information and local stress state. Ideally, homogeneous displacement is considered along the main part of large-scale strike-slip faults. As for the end zone, appropriate assumption has been made to avoid the presence of stress singularity, that displacement is absorbed by surrounding rock mass, which is referred to viscoelastic behavior of the rock. Hence LEFM (linear elastic fracture mechanism) and CEZ (cohesive end zone) model deriving from fracture mechanics were respectively chosen in our research when dealing with different part of strike-slip fault. Results show that the angle between the main strike-slip fault and secondary fracture is related to the ratio of the shear stress responsible for the fracture to the normal stress responsible for the opening of the fracture. Apply these research achievements to the Miaoxinan uplift, although the strike-slip displacement and fault throw at northern high area of Penglai 20-2 are relatively small, the characteristics of the associated fracture angles indicate compressional stress state in the area. Additionally, the region is situated in an overlapping zone of a dextral left-stepping strike-slip fault system, which not only controls the formation of structural traps, but also ensures effective sealing conditions in the area. This technique provides quantitative interpretation of stress states responsible for the development of secondary fractures and has favorable application prospect in Miaoxinan uplift henceforth.