Using Fluid Inclusion Thermodynamic Modeling to Study Paleo-Fluid Potential and Natural Gas Migration and Accumulation of the No.2 Member of Shanxi Formation of the Upper Paleozoic in Yi-Shaan Slope, Ordos Basin

Reconstruction of paleo-fluid potential during each geological period after hydrocarbon generation may be helpful to clearly recognize the hydrocarbon reservoir distribution. In this research, the samples including reservoir fluid from Taiyuan and Shanxi formations of the Upper Paleozoic in Yi-Shaan slope, Ordos basin were obtained, and the systematic analysis of the sample was used to measure the homogenization temperature for all orders of fluid inclusions mainly in trails, salinity for aqueous inclusions and the fluorescence colors for hydrocarbon inclusions. The result indicates that there are six period times of hydrocarbon charging in the sandstone reservoirs, and the gas charging is statistically dominating. By integrating the average homogenization temperatures with burial historical curves, the six charging times are determined. By employing the fluid inclusion pVT thermodynamic modeling, the paleo-formation pressure of these six times of hydrocarbon charging is obtained, and the six periods of paleo-gas potential of No.2 member of Shanxi Formation of the Upper Paleozoic in Yi-Shaan slope are calculated, and the paleo-gas potential distribution and evolution are depicted, and the evolution of regional tectonics and geothermal history are found out to be the main controlling factors. According to the regional geological conditions, the law of the nature gas migration and accumulation of No.2 member of Shanxi Formation of the Upper Paleozoic in Yi-Shaan slope is discussed. From the middle Late Triassic to the late Middle Jurassic, the natural gas mainly migrated from southwest to north, northeast, because the paleo-gas potential in the southwest zone was higher than that in the north and northeast zones; from the late Middle Jurassic to the late Early Cretaceous, the paleo-gas potential in the west zone was higher than that in the east zone and the natural gas migrated eastward and then northeastward; after that, the natural gas accumulation is in equilibrium with dissipation, which results in today's distribution features. The Yulin zone and the southern zone of Yulin have the most important significance to the exploration of the No.2 member of Shanxi Formation of the Upper Paleozoic in Yi-Shaan slope, and secondly to the Shenmu-Mizhi zone.