The main objective of this study was to improve borehole wall stability in deep drilling into the Lonmenshan fault zone by developing a drilling fluid with ultra-fine inert materials. The mineral composition and microstructure of rock samples from deeper zones of the fault were analyzed by X-ray diffraction and scanning electron microscopy. It was found that the rock has a loose fabric and is microporous and highly permeable. The main clay minerals in the rock are illite and chlorite, which are prone to hydration and swelling. Under high pressures, their hydration and swelling capacity increase, which may cause instability of boreholes. Based on the obtained mineral composition and surface microstructure data and analysis of the mechanism of additive action, the following was found to be the optimum composition of the drilling fluid that helped rapid drilling into the fault zone: water + 5% sodium bentonite + 0.3% Na2CO3 + 5% sodium bentonite in the form of hollow glass microspheres (HGM) + 2% polysaccharide polymer + 3% carboxymethyl cellulose.