Failure Mechanism of Secondary Fractures Associated with Strike Slip Fault and Its Stress Field Implication

Abstract

Sliding along a preexisting main strike slip fault can result in the formation of tensile fractures attributed to stress perturbations. These secondary fractures are generally oriented oblique to the preexisting flaw, and their orientations tend to follow a striking similar pattern. To further study these features of the secondary faults, fracture mechanics were used to interpret stress field conditions responsible for secondary cracks 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, in case of the presence of singularity, appropriate assumption has been made 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. We evaluated the angle at which secondary fractures initiate, using the maximum circumferential stress criterion. This relatively simple criterion is easy to compute and agrees as well with the experimental data as criteria based on energy release rate, strain energy density, or stress intensity factor. 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. Thus local stress states along strike slip faults can be further interpreted utilizing secondary fractures’ geometric information. Located at the end zone of the main strike slip fault, faults develop in north high area of Miaoxinan Uplift were analyzed using CEZ model, the geometric features suggest the existence of compressional stress load which is parallel to the strike of main strike slip, coupled with its dextral left-step position and cohesive force that develops within end zone rock mass, optimal sealing ability can be deduced. Drilling results confirm our conclusion as gas reservoir developed in relative low structure in study area. The technique provides easy interpretation and rapid evaluation of stress states responsible for the development of secondary fractures and has favorable application prospect in Bohai Sea henceforth.

AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019