Abstract: The shallow gas reservoir presently produced in the D gas feld in western South China Sea is located in the Yinggehai formation of the Tertiary System, which is a medium porosity medium permeability mudstone reservoir. This formation has complex geological characteristics, and sloughing, borehole collapsing and pipe sticking have all been encountered during drilling operation in the past. Based on the studies on the rock properties of the reservoir and the problems associated with the drilling fluids it was concluded that the plugging and inhibitive capacity of the drilling fluids should be strengthened. That being said, the drilling fluid (PRG) previously used to drill the φ311.1 mm was treated with VIS to retard the invasion of mud fltrate into the formation and to minimize the erosion of drilling fluid to the borehole wall. The concentration of PLUS in the drilling fluid was reduced and PFFLOTROL increased to reduce viscosity and flter loss. In the φ215.9 mm interval, the PRF mud was treated with Greenseal to plug micro fractures and pore throats, and to retard the fltration of mud fltrate, thereby prolonging the time of borehole wall stability. Using more EZCARB and weighting the mud with KCl, the inhibitive capacity of the mud was improved. Evaluation experimental results showed that PRG and PRF muds had low flter loss (API fltration rate was 2.1 mL and and HTHP fltration rate was 4.3 mL), good rheology, strong inhibitive capacity (shale cuttings recovery on hot rolling test was ±90%) and excellent contamination resistance (salt resistance to 10% and calcium resistance to 2%). Application of the PRG and PRF drilling fluids on well P3H and D4H in D gas feld showed that downhole troubles previously encountered, such as bit balling, pipe sticking and mud losses were successfully avoided.