Abstract: The well DS4 is an areal exploration well located in the offshore Gulf of Liaodong in the north Bohai Sea. The well DS4, a long open-hole well, penetrated formations with complex geology and many oil and gas bearing zones. Based on the studies on the inhibitive capacity and plugging capacity of the drilling fluid, as well as reservoir protection mechanism, a modified natural polymer drilling fluid was optimized. The results of the laboratory experiments have revealed that the inhibitive capacity and the plugging capacity of the drilling fluid are greatly enhanced after treatment with the strong shale inhibitor AP-1 and the plugging agent DLP. Using this modified natural polymer drilling fluid, mud losses previously occurred in drilling the highly permeable Minghuazhen formation and Guantao formation were minimized, and the borehole wall instability in drilling the lower Dongying formation (shales) was brought under control with lower mud weights, realizing safe and fast drilling. This drilling fluid has good lubricity; in the second interval, which has length of 3,131 m, horizontal displacement of 1,060 m and well angle of 34.5℃, the wireline logging was 100% successful, with no repetition. The drilling fluid has low fluorescence, which is helpful in pinpointing pay zones accurately. The drilling fluid has good reservoir protection ability; the well DS4 had good oil shows, and high production rate was obtained during well testing.