Controls On The Evolution Of The Sedimentary Environment Of The First Member Of The Qingshankou Formation In The Bayanchagan Area Of The Songliao Basin,NE China

In recent years,shale oil has gradually become the focus of global oil and gas exploration.The first member of the Cretaceous Qingshankou Formation(henceforth called K₂qn¹)in the Songliao Basin has demonstrated a tremendous shale oil potential.However,the evolution of the sedimentary environment,the main controlling factors and their effects on the organic matter accumulation of the mud(shale)in the K₂qn¹have not yet been identified.In particular,there is still a great controversy about the sedimentary period of this section:1)whether transgression occurred or not;2)if it did,what kind of influence it has on the formation of high-quality shale.These problems greatly affect the distribution prediction of high-quality shale and restrict the exploration and development of shale oil in the K₂qn¹.In order to address this problem,the author observed and sampled typical drilling wells with relatively complete core taking in the K₂qn¹of the Bayanchagan area in the Songliao Basin,and carried out systematic shale quality evaluation,chromatography,mass spectrometry,major and trace element analysis.The results show that the TOC content of the K₂qn¹ranges from 0.31%to 3.45%.The distribution of(S₁+S₂)is in the range of 0.35-18.36 mg/g.The type of organic matter is mainly type II,and it is in the mature stage.The evolution trend of TOC content in the profile shows that the quality of shale in the K₂qn¹is not balanced,and the shale with good quality and poor quality are distributed.The whole rock fluorescence observation shows that there are not only algae from aquatic organisms,but also vitrinite and sporophyte from terrestrial higher plants in the organic macerals of the shale.But biomarker analysis(including C₂₄tetracyclic terpane/C₂₆tricyclic terpane,regular sterane/17?-hopane,TAR,(C₂₇+C₂₈)/C₂₉regular steranes)confirmed that aquatic organisms were the main source of organic matter in the K₂qn¹.Inorganic geochemical characteristics(including abnormally Sr content and high Sr/Cu ratio(4.40-41.73),high Fe/Mn ratio(15.08-97.37),positive correlation between Sr content and Sr/Ba and Ca/Mg)and organic geochemical characteristics(C₂₆/C₂₅,C₂₄/C₂₃and other tricyclic terpane ratios)indicate that transgressive events are the main factors leading to salinization of the paleosongliao lake.At least five transgressions occurred in this period,which can be expressed as T_I,T_II,T_III,T_IV,and T_V.Higher V/V+Ni(0.73-0.87),V/Cr values(1.65-2.73)and lower Th/U values(1.53-4.12)suggest that transgression may contribute to the formation of oxygen poor water environment.Further study shows that under the background of frequent transgression events,the shale quality of the K₂qn¹is mainly controlled by paleoproductivity.However,the influence of transgression events in different periods on shale quality(paleoproductivity)is quite different.In the early transgression events(e.g.T_Iand T_II),the enrichment intensity of organic matter is negatively correlated with the transgression intensity,while in the later transgression events(e.g.T_IIIand Tv),the enrichment intensity of organic matter is positively correlated with the transgression intensity.The possible reason is that it is difficult for freshwater organisms to adapt to salinization during the early transgression,which leads to the decline of primary productivity and is not conducive to the enrichment of organic matter.After multi-stage salinization,salt tolerant or halophilic organisms are more dominant,and transgression is conducive to the improvement of paleoproductivity and the enrichment of organic matter.This study provides an important basis for the determination of high-quality shale and further exploration and development of shale oil in the first member of the Qingshankou Formation of Cretaceous in the Songliao basin.