Research on the Opening Mechanism of Early-Term Inactive Faults: A Case Study of the Junggar Basin

Abstract: Early-term inactive faults can be reopened as hydrocarbon migration channels under overpressure. Clarifying their opening mechanism during the hydrocarbon accumulation period is the foundation of hydrocarbon exploration. In view of the problem that early-term inactive faults in the Junggar Basin have the potential for cross-layer migration but the overpressure-driven opening mechanism is unclear, this study constructs a mechanical model coupled with strong and weak layer combinations and pore fluid pressure. Combined with the Mohr-Coulomb fracture criterion, the mechanical response characteristics of the faults under two boundary conditions of lateral confinement and unconfined confinement are analyzed, thereby revealing the opening mechanism of early-term inactive faults. The study shows that: (1) Under lateral confinement conditions, the overpressure evolution is in two stages: "Mohr circle translation - radius enlargement". Due to stress distribution, the Mohr circle is preferentially tangent to the original fracture line, which promotes the local opening of the fault. (2) Under lateral unconfined confinement conditions, the effective stress change is concentrated on the fracture surface, driving the Mohr circle to the left and reducing the critical stress value for opening early-term inactive faults. (3) Both laterally restricted and laterally unrestricted states exhibit the characteristics of "priority of overpressure zone, dominance of lower fault, and initial opening of overpressure segment", and can further extend along the original fault. (4) The study area as a whole shows that the critical opening pressure of the western fault is higher than that of the eastern fault, and this pressure has shown an evolution trend of "increasing-decreasing-increasing" since the end of the Triassic. Areas closer to the overpressure center of the source rock and with a smaller angle between the fault strike and the maximum principal stress have better opening properties. In the late hydrocarbon accumulation stage, some early-decaying faults open due to the overpressure reaching the critical condition, and can serve as effective hydrocarbon migration channels.