| The Ordos Basin is the second size basin rich in oil and gas in China mainland. The Upper Paleozoic gas reservoir in this basin has some similarities with the Alberta deep basin gas trap in Canada. The key problem for the gas pool is that the formation mechanism is not clear.In this study, more than 100 reservoir sandstone samples and 20 source rock samples were taken from this basin and investigated by the combination method of geochemistry and geology, including fluid inclusion techniques, PVTsim software, thermal simulation experiments, hydrocarbon generation kinetics and carbon isotope kinetics. The following recognition and conclusion are obtained:1. The fluid inclusions occurred in the Upper Paleozoic reservoir sandstones in Ordos Basin are abundant, with small size. Most of them are non- fluorescence, and only a small amount of blue fluorescence inclusions were found in the samples from the east area of the basin where the maturity of the source rocks is lower. The amount of gas inclusions and the ratios of liquid to gas in aqueous inclusions are decreasing from the south to the north in the basin. The present of the three-phase inclusions and boiling inclusions indicate that the fluids was not homogeneous when the inclusions formed.2. A new method was suggested to calculate the trapping temperature and pressure of inclusion using the isochore equations of both petroleum inclusion and its coeval aqueous inclusions. The trapping temperature and pressure of aqueous inclusions occurred in the Upper Paleozoic reservoir sandstones are in the range of 85-145# and 22-32Mpa, respectively. The trapping pressures of inclusions are lower than the strata water static pressure when the inclusion was trapped. The trapping temperatures and pressures of the fluid inclusions are decreasing from the south to the north of the basin.3. The Upper Paleozoic gas pool formed firstly in the Yanan area before 150Ma (Late Jurassic). With the migration of water-gas interface from the south to the north gradually, the gas-bearing area extended, and framework of the gas pool formed finally in 110Ma (the late Cretaceous). From the Late Cretaceous to present, the gas trap has been adjustedbecause of the tectonic movement. Combining all kinds of information of inclusions with the geologic background, it is recognized that the regional migration of gas was from the south to north direction.4. The results of the thermal simulation experiments and the kinetics calculation of hydrocarbon generation have shown that the source rock can generate a significant amount of gas in the process of tectonic uplifting. The gas generation of the source rocks in the basin ceased from the late Tertiary to the late Cretaceous, with the trend later in the west area and earlier in the east area.5. The results of the carbon isotope kinetic have shown that the gas carbon isotope values in the Upper Paleozoic gas reservoirs areas not consistent with the maturity of the source rocks from the gas pool areas. The formation of the gas pools belong to stage accumulation model and the main source are in the south area with higher maturity.6. There are two main types of gas trap in the Upper Paleozoic strata .One is lithological gas trap, and another is deep basin gas trap. The SuLiGe gas pool is a deep basin gas trap, and the YuLi and ShenMu gas pools are lithologyical gas traps.The results have provided some important information on the formation of Upper Paleozoic gas pools, having great significance for further research and exploration of the Ordos Basin...
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