Abstract: To solve the problems of wellbore instability,hole cleaning and anti-sticking in mudstone section of Shihezi formation of Block Su 53,new oil-based drilling fluids were developed.Indigenous organic clay,activators,filtrate reducer and wetting agents from different areas were used for performance comparison and to test at room temperature and after aging at 200 ℃.The all-oil drilling fluids were evaluated for their high temperature resistance,anti-contamination,inhibition,and suspending abilities,plugging ability and reservoir protection effect,and the wellsite results were examined.During the course of field testing in Well Su 53-82-50H,the all-oil drilling fluids were stable in performance and were easy to maintain,which indicated the new fluids could significantly improve ROP,and have good effect for lubricating and anti-sticking.Further,the hole enlargement rate was small,which fully met the operation requirements at the site.The results showed that oil-based drilling fluids can provide a viable way to solve the problems of drilling in long open-hole horizontal well in Block Su 53,and lessons learned could be provide technical solutions to reservoirs worldwide experiencing the same problems.