Abstract: In order to meet the requirement of wellbore stabilization and good performance of drilling fluids during the drilling of shale gas horizontal wells and realize commercial development at a low cost and with high efficiency, a stable oil-base drilling fluid system with low oil-water ratio was developed to accommodate the characteristics of shale reservoirs and the engineering requirements of horizontal wells. It is shown from experimental tests that this oil-base drilling fluid system is good in terms of thermal stability, contamination resistance, plugging capacity and emulsion stability. In addition, it has lower plastic viscosity, moderate shear and better rheological property. Therefore, a stable oil-based drilling fluid system can satisfy the unique conditions in drilling horizontal of shale gas wells. The oil-based drilling fluid system has been applied in 5 horizontal wells drilling in Fuling Shale Gas Field. Through low oil-water ratio gel maintenance, solid phase control and plugging while drilling, the oil-water ratio was kept below 70/30, 15% lower than previously used oil based drilling fluids in this gas field. Further, drillings cost were reduced significantly. In summary, drilling costs in the Fuling Shale Gas Field could be reduced effectively by using an oil-based drilling fluid system with a low oil-water ratio in horizontal shale gas wells. This system will provide strong support in the low-cost commercial development of shale gas and used as a model in the development of shale gas in other areas within China.