Keywords: 
• accumulation mechanisms /
• reservoir formation models /
• Excess pressure /
• Chang8 reservoirs /
• Ordos Basin

Abstract: The exploration and reservoir scale are of differences in different regions of Chang8 interval of Yanchang Formation in Ordos Basin Ordos, in order to clarify the difference reason, oil source, reservoir characteristics, the capillary force, buoyancy and the excess pressure are studied on Chang 8 reservoir. The excess pressure is much greater than the capillary force, source and reservoir pressure can overcome the corresponding reservoir capillary force and migration and accumulation. On this basis, according to the Jiyuan area, Longdong area and Shanbei area of hydrocarbon source rock and reservoir development characteristics, physical properties and migration channel characteristic, built three kinds of different reservoir forming pattern. Jiyuan area is two-way expulsion, composite reservoir model: Chang 7 source rocks of abnormal development, pressurization action of hydrocarbon generation is strong, which makes the generated hydrocarbon fluid in excess pressure driven to overlying the Chang 6 and Chang 4+5 formation and the underlying Chang 8 strata in bidirectional expulsion, in multi-layer accumulation; Longdong area is the upper source and below-storage reservoir，the lower reservoir model: Chang 7 source rocks, higher in the presence of excess pressure, underlying Chang 8 reservoir was superior to the overlying the Chang 6 reservoir, favorable hydrocarbon mass downward migration, accumulation in Chang 8; Shanbei area is the upper lateral migration, the upper reservoir model: Chang 7 hydrocarbon source rocks are not developed in this area, and overlying Chang 6 reservoir is far superior to that of the underlying Chang 8 reservoir, hydrocarbon priority in Chang 6 reservoir, due to the limited size of Chang 8 reservoir. The three accumulation models represented with Chang 7 as the main hydrocarbon source rock reservoir are the main reservoir forming mechanism; the three models have obvious difference in the distribution of oil and gas.