Organic Geochemical Characteristics and Geological Significance of Anoxic Event Bed of the Upper Cretaceous Qingshankou Formation in the Northern Songliao Basin

Abstract

AbstractThe Cretaceous period in Songliao Basin is the largest inland petroliferous basin in ancient Asia, filled with volcanics, volcaniclastic rock and sand-mudstone deposition of up to 10,000 m thick. Qingshankou formation is the product of the first large-scale lake transgression in the basin, the biological species is rich unusually, its lower develop dark mudsone, shale and oil shale mainly, is an important source rock in the basin, and the mark layer of stratigraphic division and correlation. Organic geochemistry characteristics show that the organic carbon and kerogen carbon isotope of Qingshankou formaiton have positive excursion characteristics, and black shale is the product of lacustrine anoxic events in the Cretaceous greenhouse effect time window, which provides an important material basis for the formation of high-abundance high-quality hydrocarbon source rock. The latest zircon U-Pb age data show that the age of volcaniclastic rock in the lower part of Qingshankou formation is 91.06 ± 0.69 Ma, indicating that its age has entered the lower Turonian. Combined with the results of biostratigraphy, it is concluded that the anoxic event layer of Qingshankou formaiton can be compared with the global ancient ocean Cenomanian-Turonian boundary event layer, thus establishing a new comparison scheme between continental Cretaceous in the Songliao Basin and sea phase standard, which provides an important scientific basis for the standard of middle-shallow geological era in the Songliao Basin.KeywordsSongliao BasinQingshankou formationHydrocarbon source rockAnoxic eventsZircon SHRIMP dating