Abstract: Low-permeability sandstone gas reservoir is the main reservoir type in Shilijiahan zone of Hangjinqi area, Ordos Basin.The 1^st Member of Xiashihezi Formation, which is the major gas bearing reservoir, is characterized by strong heterogeneity and locally complex gas-water contact.Making clear of the occurrence, distribution characteristics and main controlling factors of gas and water is the key to explore and develop gas reservoirs in Shilijiahan zone.In order to analyze the vertical water-gas differentiation under the control of buoyancy in tight reservoir, the capillary resistance in three types of reservoirs were calculated on the basis of mercury injection experiment and gas-water relative flow experiment.The results show that there is no sign of water -gas differentiation in the 1^st Member of Xiashihezi Formation because the buoyancy of continuous gas column in the reservoir is less than the mean capillary pressure due to the strong reservoir heterogeneity and the small continuous thickness.With a gentle tectonic setting, buoyancy can hardly be the charging force of low permeability reservoir with a large area.Instead, overpressure is the main reservoir-forming dynamics.Reservoir-forming dynamics of three lithologic traps in the study area are deduced by analyzing the gas saturation and differences of capillary pressure curves of gas, gas-water and water reservoirs.Duguijiahan trap which is located in the western part of the study area is defined as a strong dynamic trap because of its strongest reservoir-forming energy, while Subuerga trap which is located in the eastern part is defined as a weak dynamic trap because of the minimum reservoir-forming dynamics.Based on the study of reservoir-forming dynamics, six modes of water and gas layer are recognized in the three traps according to the occurrence and overlapping patterns of gas and water layers.After analyzing the differences of geological conditions, trap dynamics and water-gas differentiation, it is suggested that hydrocarbon generation intensity and trap sealing condition are the main controlling factors of reservoir-forming dynamics.Trap dynamics and reservoir heterogeneity conjointly control the distribution of gas and water.

Key words: gas-water distribution, capillary resistance, reservoir-forming dynamic, lithologic trap, Hangjinqi area, Ordos Basin