Abstract: 11 exploration wells have been completed in the Block Dabasong, the second and the third intervals of which were drilled with polymer sulfonate drilling fluid containing potassium and calcium salts. Tight hole, borehole wall collapse, mud losses,well flow, over-pull and sticking while tripping, and mud contamination etc. have been frequently encountered during drilling. To solve these problems, studies have been done to improve the properties of the polymer sulfonate drilling fluid used. A shale encapsulator and amine based inhibitor were used to improve the inhibitive capacity of the polymer sulfonate drilling fluid. The improved drilling fluid formulation was used to drill the well Datan-1, and was proved successful in solving the problems caused by the easy-to-hydrate-andswell Cretaceous and Jurassic gypsum-containing shales. Borehole instability and mud losses encountered in drilling the Xishanyao formation and the Badaowan formation were solved by the drilling fluid improved in inhibitive capacity and plugging performance, and by using more appropriatemud weight. Using a combination of NaCl and KCl, the percent recovery of shale cuttings was increased from 91.6%to 97.5%, and contamination tolerance of the drilling fluid was increased from 5%to 10%. Using well formulated LCM slurries and near-balanced drilling, possible mud losses can be prevented. Compared with offset wells, the well Dashen-1, which had larger hole sizes, had average ROP increased by 35.48%, and the monthly drilling rate was 1105.86 m per rig, 51.2%higher than that of the well Da-1, 28.1%higher than that of the well Da-9, and 48.12%higher than that of the well Da-10. No drilling problems were experienced during drilling, and the wireline logging and casing running were conducted with no delays.