Abstract: Sha 3 Reservoir in Bonan Block is mainly shale with rich bedding and micro-fractures,it is difficult to maintain hole stability of horizontal well when drilling with conventional water-base drilling fluid.Diesel base drilling fluid system and low aromatic oil base drilling fluid system have been developed in laboratory,and their basic properties,fracture-blocking properties and anti-oil pollution performance were studied.Diesel base drilling fluid system developed was applied in Well Boye Ping 1,and low aromatic oil-based drilling fluid was applied at Well Boye Ping 2.The drilling of the two wells had no complex situations,smooth trips and no sloughing,the borehole enlargement rates were all less than 3%.Laboratory evaluation experiments and field application showed that strong blocking oil-base drilling fluid systems feature stable system,strong inhibition,good wellbore stability and strong lubricity,suitable for horizontal well drilling in shale formations.