Abstract: The causes for well instability in igneous formations in MTT region of the Tahe Oilfield were discussed to ensure production safety. Based on the analyses of formation lithology, geostress status, pressure profile, logging data and caliper data, combined with the collapsing features of igneous formations, it was evaluated in this paper the matching of drilling fluid property and formation lithology as well as the sealing ability of plugging materials. The causes for wellbore instability were concluded as followed. (1) Mudstones which interbedded with igneous formations changed to slurries when they met drilling fluids. (2) Micro-fractures were widespread in igneous formations, and drilling fluids could easily expand into igneous formations when their sealing ability was poor. (3) Strike-slip geostress field existed in the study area, showing strong tectonic forces, which desired for drilling fluids with high density.