High Frequency Paleolake Level Change And Its Controlling Factors Of Qingshankou Formation In Cretaceous Songliao Basin

Oil shale at the bottom of Qingshankou Formation in Songliao Basin is the main source rock of Daqing Oilfield,which has important industrial value.At the same time,the Qingshankou Formation is dominated by dark fine-grained sediments,which record abundant Cretaceous paleoclimate signals and have important research value.Whether transgression occurred in Qingshankou Formation has been a hot topic in recent years in the study of Cretaceous continental paleoclimate.Based on the INPEFA curves of more than 40 wells in nine secondary tectonic units of Songliao Basin,the high frequency lake level evolution of Qingshankou Formation has been reconstructed by constructing a cross section of connecting wells.On the whole,the lake level of Qingshankou Formation in Songliao Basin shows a downward trend,and the lake level is the highest in the middle of the second and third member of Qingliao Basin.The Lake anoxic event(LAE1)and oil shale horizon at the bottom of the first member of Qinghai Formation have a significant lake level decrease.Through CEEMD analysis,the IMF components of INPEFA signals of Well SK1 and Well SK2 in Songke are obtained,and the lake level change signals are more clear.Based on the geological age of SK1 well,the IMF component is combined with the total sulfur isotope,pyrite isotope and biomarker of SK1 well and compared with the global sea level.The conclusion that transgression and rainfall control the lake level together is further obtained.At the location of oil shale and lake anoxic events at the bottom of Qingshankou Formation,transgression controlled the lake level rise to reach the first lake flooding surface of Qingshankou Formation,and subsequently the decline of sea level resulted in the weakening of lake-sea communication and the decrease of Lake salinity.From the upper part to the lower part of Qingshankou Formation,the changes of lake level and rainfall(indicated by Al/Si of ECS logging in SK2 well)are consistent,indicating that rainfall controls the changes of Lake level.Intermittent weak transgression events may have an impact on the peak lake level during this period.The lake level at the bottom of the Qingshankou Formation is again contrary to the rainfall index,which is consistent with or influenced by the Cretaceous Ocean Anoxia Event(OAE3).