Abstract: The well CN-H in the Block Changning is a shale gas development horizontal well located in Gong County, Yibin City, Sichuan. The target zone of this well is the Longmaxi shale formation of the lower Palaeozoic group. Shale formations in the fourth interval of the well is easy to hydrate and disperse, and collapse, causing borehole instability during drilling. Oil base drilling fluids are not allowed to use in drilling operations because of environment protection requirement. A biosynthetic base drilling fluid was then developed to replace oil base drilling fluids. The continuous phase of the biosynthetic base drilling fluid is a vegetable oil modified with a special technique. The biosynthetic base drilling fluid functions normally at 200℃, and the density of the drilling fluid is adjustable between 1.10 g/cm³ and 2.50 g/cm³. A biosynthetic base mud formulation of 2.1 g/cm³ had a rate of linear expansion of 0.8% and extreme pressure friction coefficient of 0.089 after aging at 150℃. The HTHP filtration rate of the aged mud was 1.6 mL. These data indicated that the synthetic base drilling fluid has good lubricity, plugging capacity, and inhibitive capacity, as well as good high temperature stability. Good suspension performance and cuttings carrying capacity of the drilling fluid at elevated temperatures ensured efficient and safe drilling in horizontal section of the well. Borehole wall was satisfactorily stabilized in drilling the fourth interval of the well CN-H. Average percent hole enlargement of this well was 4.9%, and the average ROP was 9.74 m/h, 2.14 m/h higher than those wells drilled with oil base drilling fluids. The biosynthetic base drilling fluid is nontoxic, satisfying the requirements of environment protection in Changning.