Sedimentary characteristics and controlling factors of hyperpycnal flow in the Lower Shahejie Formation of Qikou Sag, Bohai Bay Basin

This study clarified the sedimentary characteristics of hyperpycnal flow sediments, investigated the plane and vertical characteristics and the differences in hyperpycnal flow sedimentary systems from different provenances, and finally discussed the controlling factors of hyperpycnal flow in rift basins. The sedimentary characteristics of hyperpycnal flow in the Lower Shahejie Formation of the Qikou Sag, Bohai Bay Basin have been studied through the comprehensive analysis of drilling core, logging, seismic attribute, and grain data.The results show that there are three typical recognition features for the hyperpycnal flow deposits in the study area: ① The vertical sedimentary sequence is composed of a reverse grain grading in the lower part, a normal grain grading in the upper part and a relatively thick layer with no obvious change in grain size; ② The sediment is dominated by a gradual suspension. The types of cumulative probability curves include a complex multistage style, an upper arch style and a low slope two-stage style; ③ The lithofacies is characterized by erosional filling deposition and various bedding structures with the development of maroon argillaceous layers, maroon gravels, abundant plant debris and carbonaceous debris. The channelized hyperpycnal flow sediments have multiple sources. The most significant source is the Gegu source, which is characterized by a long-distance transportation path, multiple flower bodies and widespread deposits along the transportation path.Two types of sedimentary models are established in this study, and the controlling factors of hyperpycnal flow include paleogeomorphology, paleoprovenance and paleoclimate. Considering that hyperpycnal flow widely exists in rift lacustrine basins with its channel sandstone bodies being good reservoir properties, our study suggests that hyperpycnal flow is a breakthrough point for oil and gas exploration in continental rift lacustrine basins.