Abstract: There are rich petroleum resources and a number of possible hydrocarbon-bearing traps in Southern Junggar basin. Jurassic sandstone is major reservoir rock. A study of diagenes is in the middle section of the South margin-Manas zone indicated that the degrees of evolution and development of pores,as well as their types and distribution are related not only to sedimentary facies but also to diagenesis. Generally, two stages of dissolution occour in the sandstone forming two stages of secondary porosities, but only secondary porosity that deeply buried in the later stage and related to thermal evolution of organic matter are of possibility to store hydrocarbon. Between late Jurassic and Cretaceous, the porasity of the Tou Tunhe formation evoluted into the lower stage of primary porosity (about 5%),but older formation below it (e.g the Shan Gonghe formation)entered the stage of secondary porosity up to 17%-20%. Shan Gonghe and Ba Daowan formation in the region are better reserveir and dominated by secondary porosity. It is possible for xi Shan Yao. Tou Tunhe formation in the Nouth of the region to develop secondary porosities, which is important for storing coal-formed gas.