Abstract: Enping 21-4 oilfield in the eastern part of the South China Sea is a typical marginal oilfield. The reservoir is shallow in depth, with large unconsolidated sandstone formations and wide distribution of sandstone particle size. During drilling, there are risks and challenges such as clean hole, low pressure bearing capacity of unconsolidated sandstone and easy leakage, drilling multiple faults, large friction and torque, and casing resistance. According to the formation characteristics and technical challenges of this oilfield, the BIODRILL S synthetic base drilling fluid system was optimized. The system has good rheology, high low shear rate, excellent inhibition, sealing property, anti-pollution and excellent lubricity. The high temperature and high pressure filtration loss is 2.4 mL, and the extreme pressure lubrication coefficient is 0.03～0.06. Well A1H in Enping 21-4 Oilfield is the first self-designed and implemented ultra-deep extended reach well in China. BIODRILL S system was applied for the first time in ultra-deep extended reach well in eastern South China Sea. The results show that: The system has stable performance and the reading of low shear rate ϕ6 stays above 12, so it has good rock carrying capability. Excellent lubrication performance, the friction coefficient of ϕ311.1 mm hole drilling is 0.17～0.21, the friction coefficient of ϕ244.5 mm casing open hole is 0.40～0.45, the drilling process is smooth. The successful application of this system has a guiding significance for the popularization and application of drilling fluid technology in shallow ultra-deep extended reach wells.