The exploration potential of the Permian igneous rocks in the Sichuan Basin is enormous, but the lithology and physical properties of the reservoirs in this formation are very complex. Local stratification and fracture development lead to wellbore instability incidents such as stuck pipes and wellbore collapse during drilling and completion. To investigate the impact of fracture development on wellbore stability, considering the cementation of igneous rocks, we used the discrete element method to construct wellbore models for two classic igneous rock formations, tuff and breccia. We simulated the failure characteristics of different fracture densities, fracture dip angles, and groups of fractures, studying the wellbore instability patterns and their micromechanisms. The results show that: ① Discrete element numerical experiments can effectively simulate the strength and deformation characteristics of igneous rock formations; ② With increasing fracture density, the formation failure mode shows a gradual deterioration trend. When the number of fractures reaches 200, the number of microcracks in breccia and tuff are 476 and 243, respectively, with more significant changes in the failure pattern of breccia; ③ Fractures developed at 45° to 60° have the most significant weakening effect on formation cementation, with the overall formation failure pattern developing perpendicularly to the fracture dip angle and expanding outward; ④ Groups of intersecting fractures have a dual role in the initiation and development of microcracks. Microcracks first appear at fracture intersections, where the formation matrix cementation restricts the expansion of some cracks, while the cementation failure of fractures promotes the formation of shear bands. The study results provide valuable insights into the micromechanical mechanisms of fracture development affecting wellbore instability during drilling and completion in igneous rock formations.