W ith methods such as ordinary m icroscopic observation，fluorescence m icroscopic observation，homoˉgeneous temperature and ice point temperaturemeasurement and LaserRoman spectroscopic analysis of composiˉtion，a systematic study is conducted on the geometry，temperature，salinity and compositions of the fluid incluˉsions in reservoirs in the eighth member of the Shihezi Formation and the firstmember of the Shanxi Formation in theUxin Ju area，Ordos Basin.The development conditions of autogeneticm inerals and the origin of secondary pores are also discussed.Three types of fluid inclusions are identified:saline water，hydrocarbon and CO 2 incluˉsions.According to the study，it is believed that the study area experienced two stages of fluids charging process.One stage was with low temperature and low salinity and the other stage was with high temperature and high salinity.Accompanying the two stages was two phases of hydrocarbon charging process:one with moderateˉlow maturity and the other onewith highmaturity.Combinedwith burialhistory，we believe thathydrocarbon charged the reservoirsmainly from the end ofMiddle Jurrassic to the end ofEarly Cretaceous.The organic acid and CO 2 generated during thematuration ofhydrocarbon are themain causes of secondary pore development in the sandˉstone reservoirs.Later tectonic thermal activities are favorable for the preservation of primary pores and the