Abstract: The target zones of the Donghai gas field are the Huagang Formation and the Pinghu Formation, whose formation characteristicssuch as poorly cemented sandstone-mudstone interbeds, fractured mud stone and developed coalbeds, are unfavorable to drilling operation. Unstable borehole wall and poor hole cleaning are the main causes of downhole troubles. Wells drilled in the gas field of interest have maximum hole depth of 6,716 m, maximum horizontal displacement of 4,686 m, maximum TVD of 4,429 m, highest bottom hole temperature of 150℃ and highest temperature at the wellhead of 115℃. High well angle and high ratio of horizontal displacement to vertical depth make it hard to clean the hole, and high formation temperature makes equipment maintenance difficult. A low viscosity high gel strength oil base drilling fluid has been introduced to safely and efficiently drill the gas wells. The oil bade drilling fluid was treated with 2% plugging agent PF-MOLSF, 2% filming plugging agent PF-MOLPF and 2%-3% hydrophobic colloidal plugging agent PF-MOHCP. In field operations, the concentration of the gelling agent PF-HSV-4 was adjusted for the drilling fluid to have good carrying performance, and hence good hole cleaning. In drilling the unstable sections, more filming plugging agent was used. During the whole drilling process of 7 wells, no mud losses and other downhole troubles had ever occurred. Tight hole can be easily removed by reaming, and time spent reaming the hole was reduced by 70% compared with exploratory wells. Other operations were all done safely and smoothly. The HTHP filter loss in drilling the reservoir section was controlled to less than 3 mL, and the filtrate was almost whole oil, helpful in eliminating reservoir damage by water invasion, to name but one. Laboratory evaluation and field application showed that the oil base drilling fluid had good rheology, electric stability and lubricity. Borehole wall stabilization and reservoir protection were both achieved. With the aid of property maintenance and cuttings disposal measures, the application of the oil base mud has made a great coup in the Donghai gas field.