Mechanism And Pattern Of Tight Oil Accumulation Of Chang 7 Member In Ordos Basin

This paper is mainly based on the study of the characteristics of effective source rocks and tight oil reservoirs in Chang 7 member of Ordos Basin and their allocation relations,the primary and secondary migration pathways,migration force and reservoir forming factors of oil and gas are analyzed in the background of self-generating and selfstoring,and the tight oil accumulation mechanism is summarized,aiming at the problem of that people only have a hazy idea of tight oil accumulation mechanism of Chang 7 member of Yanchang formation in Ordos Basin,The results show that there are two types of source rocks in Chang 7 member of Ordos Basin,among which black shale is the main source rock with high organic matter abundance(over 8%),good types and moderate maturity.In contrast,dark mudstone is a secondary source rock with poor organic matter abundance.The reservoirs of Chang 7 are dominated by light gray fine-grained feldspar lithic quartz sandstone,with poor physical properties and complex pore throat structure.The pore types are dominated by intergranular pores,with the well developed of dissolution pores which become the space for oil and gas reservoir and percolation.Source-reservoir assemblages can be divided into four types,of which type I assemblage is the most favorable for oil and gas accumulate because of its black shale and large sand body thickness.But,type II and type III assemblages have a poor storage capacity of oil and gas because of their small sand body thickness.At the same time,type IV assemblage has a relatively good storage capacity of oil and gas and its hydrocarbon was supplied by adjacent dark mudstones,although its deficiency of the black shale.The migration force of oil and gas is mainly hydrocarbon-generating pressurization,and the residual pressure has a strong controlling effect on the distribution of tight oil.The primary migration channels are mainly mudstone microcracks,while the secondary migration channels are mainly cracks,but the connected corrosion holes can also provide channel conditions.Because of the high temperature and pressure of the Chang 7 member in the period of reservoir formation,the density,viscosity and interfacial tension of crude oil were greatly reduced.And the filling resistance was reduced too,so that it has a great vantage for the oil filling.The formation of tight oil reservoirs is mainly controlled by three factors: sustained and powerful oil filling,close quarters between sources and reservoirs,and good migration channels.