The Lunnan area is one of the important oil and gas producing areas in the Tarim BasinStudy of reservoir characteristics of Lower Ordovician carbonate rocks is important to  oil and gas exploration The features of karst,development of fractures,sedimentary facies and filling materials in caves were describedExistence of permeable and soluble carbonate rocks, humid climate and abundant precipitation are considered to be necessary conditions for forming karst Development of structural fractures is favorable for karst developmentAccording to the property of fractures and filling degree,the fractures are divided into five types, such as calcite full\|filled extension fractures,calcite semi\|filled extension fractures,clay filled shearing compressive fractures,non\|filled micro\|fractures and stylolites,among which calcite seim\|filled extension fractures and stylolites play an important role in development of karst reservoirs.Development of karst in this area shows obvious vertical zonation,i.e. the vertical vadose zone,the horizontal phreatic zone and the transitional zone.At the bottom of vertical vadose zone and the top of the horizontal phreatic zone,the properties of reservoir are best. The two karst developing periods for the Middle and Upper Ordovician in the Lunnan area are consistent with two transgressions of the Carboniferous.The sedimentary features of filling materials in caves were also described and  the evolutional model of karst was established