Sedimentary characteristics of lacustrine deep-water gravity flow in the third member of Paleogene Shahejie Formation in Dongying Sag,Bohai Bay Basin,China

Many types of sedimentary systems occur in the middle of the third member of the Shahejie Formation(E_(2)S_(3)^(2))of the Paleogene in the Dongying Sag east of the Bohai Bay Basin.Due to the topography and material supply,traction and gravity flow depositions are intertwined in this area,and the sand body types are complex and diverse,making it challenging to improve the accuracy of their description and prediction and restricting oil reservoir exploration and development.Therefore,this paper documents our systematic study of the sedimentary characteristics of the southern slope of Dongying Depression,the formation mechanism of different sand body types,and the prediction of sand body distribution.First,according to the coring well’s single-well facies and vertical rock sequence,nine single lithofacies types and five lithofacies association types were identified.Combined with the well logging facies marks of all wells,the depositional models of delta and gravity flow depositional systems were established in the study area.Then,the gravity flow was divided into slip,collapse,debris flow,and turbidity flow according to its development mechanism.Finally,the distribution law of the gravity flow sedimentary facies type was predicted.Gravity flow sliding deposits are primarily distributed near the delta front,slump and clastic flow deposits are distributed near the far slope,and turbidity current deposits are distributed at the far slope.With the gradual shrinkage of the water body in the north-west direction and the continuous advancement of the river delta,the gravity flow sand body gradually disappears in the late E_(2)S_(3)^(2) and transits to delta plain deposition.