The Distribution Characteristics Of Shale Oil In Different Lithologic Assemblages Of The Chang 7 Member Of Ordos Basin

The widely distributed black shale and dark mudstone in the 7th member of Yanchang Formation（Chang 7 member）of Upper Triassic provide a good oil source material basis for the formation of shale oil in Ordos Basin.The predecessors have carried out a lot of research on the geochemical characteristics and hydrocarbon generation potential of Chang 7 shale,but the impact of different lithologic combination characteristics on the oil bearing properties of shale intervals and the distribution of shale oil enriched sections is still unclear.In this paper,the dark mudstone and black shale in the shale series of Chang 7 member in Ordos are taken as the research objects.Based on the geochemical analysis and simulation experiments of two types of source rock samples,this paper discusses the geochemical characteristics,hydrocarbon generation potential and source rock conditions for shale oil formation of the two types of source rocks in the Chang 7 member of Ordos,and proposes determination methods of oil content of two types of source rocks suitable for the study area.Based on the analysis of lithologic association characteristics,the influence of lithologic association characteristics on the hydrocarbon generation potential and oil-bearing distribution characteristics of shale in the Chang 7 member of the study area is discussed.The results show that the organic matter abundance of dark mudstones varies greatly,with the TOC value of dark mudstone from 0.17%to 25.49%in Chang 7 Member of the study area where the poor to good source rocks are distributed widely,and the main source rocks are good ones whose organic matter types are mainlyⅡ₁andⅡ_2.The organic matter abundance of black shale is generally high,with the TOC value of black shale from 2.17%to 33.68%,which belongs to good to very good source rock,and the organic matter type is mainlyⅠandⅡ₁.The vitrinite reflectance（Ro）of the source rocks in the Chang 7 Member ranges from 0.59%to 1.1%,which is generally in the stage of low maturity to maturity.The hydrocarbon generation potential of source rocks in different sub segments of Chang 7 member in the study area is different.Specifically,the average TOC value of two types of source rocks increases from first sub segment to third sub segment of Chang 7 member.The organic matter type of dark mudstone gradually transits from typeⅡ₂to typeⅡ₁,and the organic matter type of black shale gradually transits from typeⅡ₁to type I.Hydrocarbon generation potential and organic matter type of the source rocks also differentiates in different exploration blocks.The abundance of organic matter of dark mudstone and black shale of Chang 7 Member is relatively high in Longdong and Huachi areas,whose organic matter types are mainlyⅡ₁ andⅠ,respectively.The abundance of organic matter of dark mudstone and black shale of Chang7 member is higher in Jiyuan area,whose organic matter type is mainlyⅡ₂ andⅡ₁,respectively.The abundance of organic matter in dark mudstone of Chang 7 Member in Shanbei area is relatively low,whose organic matter type is mainlyⅡ₁.The oil content of black shale in Chang 7 member of the study area is significantly higher than that of dark mudstone,and the distribution range of oil content of the two is13.46mg/g～48.65mg/g and 1.65mg/g～11.94mg/g,respectively.The enrichment degree of shale oil in different sub members of Chang 7 Member in the study area is quite different.The oil content of dark mudstone and black shale increase from first sub member to third sub member of Chang 7 member.The shale oil enrichment intervals are mainly distributed in second sub member and third sub member of Chang 7 member.The oil content of shale also differentiates in different exploration blocks in the study area.The oil content of dark mudstone of the Chang 7 member in Huachi area is relatively high,followed by Longdong and Jiyuan areas,and relatively low in Shanbei area.The oil content of black shale of Chang 7 member in Longdong area is relatively high,followed by Huachi and Jiyuan areas,and relatively low in Shanbei area.The shale system of Chang 7 member in the study area can be divided into five types of lithologic combination:the combination of thin sand and thin shale interbedding,the combination of thick shale interbedding,the combination of thick sand interbedding thin shale,the combination of thick sand interbedding thick shale interbedding,the combination of mudstone and shale interbedding.The first sub member of Chang 7member is mainly composed of the combination of thin sand and thin shale interbedding;the second sub member of Chang 7 member is dominated by the combination of thin sand and thin shale interbedding,and thick shale with thin sand;and the third sub member of Chang 7 member is dominated by the combination of thick shale with thin sand,mudstone and shale mixed layer.High quality source rocks are mainly distributed in the combination of thick sand and thick shale interbedding,and the combination of mudstone and shale mixed layer.The distribution of oil content in the shale of Chang 7 member in the study area is obviously affected by the characteristics of lithologic combination,among which the oil content of shale in the combination of mudstone and shale mixed layer and the combination of thick sand and thick shale interbedding is the highest,followed by the oil content of shale in the combination of thick shale with thin sand and the combination of thin sand with thin shale interbedding,and the oil content of shale in the combination of thick sand with thin shale interbedding is the lowest.In addition,the oil content of shale and the mobility of shale oil are also affected by the abundance,type and maturity of organic matter in source rocks.The higher the abundance,type and maturity of organic matter,the more conducive to the enrichment and flow of shale oil in Chang 7 shale.