Abstract: The article discusses the accumulative conditions of the Middle Ordovician oil and gas in Lunnan area, Tarim basin,by studying the strata and the sedimentary facies and in combination with synthetical interpretation of seismic data. It points out that the conditions of oil source reservoirs and cap rocks are suitable for the carbonate rock traps, the formation times of traps coincides with those of source rocks which drained hydrocarbons, and the growth of reservoir is controlled by sedimentary facies. It is possible for this conditions to form lithological oil trap.