Abstract: The Well Mashen-1 is a key wildcat well of the Exploration Branch of Sinopec located at the high position of the Malubei Structure in the Tongnanba tectonic zone, northeast Sichuan. Completed at a depth of 8418 m, the well was designed to explore the Longwang Temple Formation of Lower Cambrian Series. The fourth interval of the well was drilled to 6 225.4~7 699 m, penetrating the Longmaxi Formation consisting of thick shales, which caused severe borehole instability. Problems also encountered in this interval included high formation pressure coefficient, high formation temperature, and contamination to the drilling fluid by carbonate ions, rendering high risk to the drilling operation. Difficulties in running the drilling fluid in the fourth interval were rheology control of high density mud at elevated temperatures, borehole stabilization and carbonate/bicarbonate ions contamination to the drilling fluid. To deal with these difficulties and problems, a series of laboratory experiments were done to select proper additives and then to formulate a drilling fluid suitable for use in drilling the Well Mashen-1. Based on laboratory experiments, a high temperature polyamine BCG-7 was selected as shale inhibitor at a concentration of 0.4% in the drilling fluid. SMP-3, a quality high temperature filter loss reducer that did not viscosify the drilling fluid, was used at a concentration between 5% and 6%. High performance polymer filter loss reducer PFL-L and HPL-3 were added at concentrations of 2% and 1.5%, respectively, they did not viscosify the drilling fluid either. HR-300 and SMS-19, two thinners, were to be used based on the actual situation. An anti-oxidant was used to try to improve the high temperature stability of the drilling fluid. The final basic formulation was as follows:3%NV-1+0.3%KOH+5%KCl+1.5%HPL-3+1%AOP-1+3% SCL+3%FT+5%SMP-3+3%LF-1+0.4%BCG-7+3%QS-2. The actual concentration of each additive was adjusted based on the actual situation if filed application. In drilling the fourth interval, the drilling fluid had good high temperature rheology, strong inhibitive capacity, high plugging performance and tolerance to carbonate/bicarbonate ions contamination. This drilling fluid provided a powerful technical support for the successful drilling of the fourth interval of the Well Mashen-1, and a KCl-amine based polymer sulfonate drilling fluid finally came into being.