Characteristics And Difference Analysis Of Oil Shale In The First Member Of Qingshankou Formation Of Upper Cretaceous In Songliao Basin

Songliao Basin,located in Northeast China,is rich in oil shale resources.Oil shale is mainly developed in the first member of Qingshankou Formation of Upper Cretaceous.This paper analyzes the petrological characteristics,industrial quality characteristics,organic geochemistry characteristics,biomarker characteristics,sedimentary environment and spatial distribution law of oil shale in the first member of Qingshankou Formation in Songliao Basin,and analyzes the difference of oil shale between southeast uplift area and central depression area,so as to provide certain scientific basis for the exploration and development of oil shale in Songliao Basin.Through core observation and sample test analysis,it is considered that the oil shale of the first member of Qingshankou Formation in Songliao Basin is mainly grayish black and dark gray,followed by grayish brown,and horizontal bedding is developed.The mineral composition of oil shale includes clay minerals,felsic minerals and a small amount of carbonate minerals.The oil content of oil shale ranges from 3.50 to 18.10%,with an average of 4.78%,and the average contents of ash,volatile matter and total sulfur are 82.40%,14.70%and 0.82%.Therefore,the oil shale in Songliao Basin is characterized by low to medium oil content,high ash content,low volatile matter and ultra-low sulfur.According to the data of oil content,TOC and rock pyrolysis analysis,it can be seen that there is a good positive correlation between TOC and oil content of oil shale in Songliao Basin.The average value of TOC is 7.36%,and the average value of S₁+S₂ is 56.64mg/g,indicating that the organic matter abundance is high.The source of organic matter in oil shale is mainly type I kerogen,followed by type II.The terrigenous organic matter of oil shale in southeast uplift area is more than that in central depression area.The average T_max values of oil shale in the southeast uplift and the central depression are 443?and 449?,respectively,indicating that the oil shale in the southeast uplift is in the low mature stage,while that in the central depression is in the low mature stage.Based on sequence stratigraphy and sedimentology,combined with core observation and geochemical testing,it can be known that the first member of Qingshankou Formation in Songliao Basin was mainly lacustrine facies,which can be further divided into muddy shallow lake microfacies,beach bar microfacies and semi deep lake deep lake microfacies according to lithology,structure,primary structure and contents;a third-order sequence was identified in the first member of Qingshankou Formation,which developed transgressive system tract and high water level system among them,the high water system tract is the main formation period of oil shale because of its dark color,large thickness and large number of ore beds.Through geochemical tests and biomarker characteristics,it can be seen that from the southeast uplift to the west into the central depression,the overall buried depth of oil shale gradually increases,the single-layer thickness and cumulative thickness of oil shale ore layer gradually decreases,and the oil shale gradually transits from low to medium quality to low-quality;the average gamma wax index of oil shale in Southeast Uplift and central depression is 0.40 and0.27 respectively,which indicates that the salinity of water in southeast uplift is relatively higher.The cross plot of Pr/n-C₁₇ and Ph/n-C₁₈ shows that the reducibility of water in the central depression is relatively stronger,indicating that both of them are conducive to the preservation of organic matter.Because the central depression area is deeply buried,the maturity of organic matter is high,and the quality of oil shale becomes worse after hydrocarbon expulsion of some organic matter.In addition,the central depression area is far away from the source area,and the content of terrigenous organic matter is relatively small,so the thickness of oil shale is thin.Near the junction of the southeast uplift and the central depression is the main development area of oil shale in the first member of Qingshankou Formation in Songliao Basin.