Characteristics And Evaluation Of Interlayer Shale Oil Source Rocks In Chang 7 Member Of Ning 228 Well Ordos Basin

The Chang 7 member of the Ordos Basin develops a set of organic-rich shale with thin-layer sandstone lacustrine deposits,which has the characteristics of source-reservoir symbiosis.It is the key layer of shale oil exploration and development in Changqing Oilfield.Guided by the theories of sedimentology,petroleum geology and unconventional oil and gas geology,based on the core and logging interpretation data of the whole well section of Chang 7 member of Yanchang Formation in Ning 228 well,combined with the supporting petrological and geochemical analysis and test results,this paper studies the lithologic characteristics,geochemical characteristics and effective source rock development characteristics of Chang 7 member source rocks,simulates and restores the burial history,thermal history and hydrocarbon generation history of source rocks,and finally comprehensively evaluates the source rocks of each sub-member of Chang 7member,so as to effectively explore and develop shale oil in Chang 7 member.Based on the detailed observation and description of the core of Chang 7 section of Ning 228 well,combined with the analysis and test of cast thin section and X-ray diffraction,the results show that the Chang 7 section of Ning 228 well develops fine sandstone,siltstone,argillaceous siltstone,silty mudstone,dark mudstone,black shale and tuff.The main lithology of source rock is silty mudstone,dark mudstone and black shale.The thickness of single sand body in Chang 7 member of Ning 228 well is counted and the sand ratio is calculated.The results show that the thickness of single sand body in Chang 7 member is less than 5m and the sand ratio is less than 30%,which is a typical interlayer shale oil.Based on the experimental analysis results of the source rock samples of the Chang7 member of Ning 228 well,the geochemical characteristics of the source rocks were studied.It was found that the source rocks of each sub-member were in the mature stage.The organic matter abundance of the Chang 7₁ and Chang 7₂ sub-members was evaluated as“medium-good”source rocks,and the organic matter types were II₁ and II₂.The organic matter abundance of Chang 7₃ submember is evaluated as“good-very good”source rock,and the organic matter types are type I and type II₁.Based on the relationship between total organic carbon content（TOC）and pyrolysis residual hydrocarbon content（S₁）,the lower limit value of TOC of effective source rocks in Chang 7 member is identified to be 1.33%,and the natural gamma energy spectrum method is determined as the best TOC prediction model.Then,the thickness of effective source rocks in each sub-member is counted,and the distribution characteristics of effective source rocks are analyzed.It is found that the lateral continuity of effective source rocks in Chang 7₁ and Chang 7₂ sub-members is poor.The burial history,thermal history and hydrocarbon generation history of Ning 228well were restored by basin simulation technology,and the hydrocarbon generation period and accumulation period of Chang 7 source rock were determined.Based on the above research results,the comprehensive evaluation of source rocks shows that the source rocks of Chang 7₃ are“high quality”source rocks,Chang 7₁ and Chang 7₂ are“good”source rocks,and Chang 7 shale oil has good exploration and development prospects.