Characteristics of Mixed Sedimentary Development and Main Control Factors in Lower Third Member of Shahejie Formation of Southern Slope Belt of Laizhouwan Sag, Bohai Sea

Mixed sediments were widely distributed in the Bohai Sea during the first and second periods of Shahejie Formation, but few were found in the third period. In this paper, taking the lower third member of Shahejie Formation of the Lainan slope belt as an example, the mixed deposit characteristics of the study area were comprehensively studied by means of various data such as wall core, thin section, drilling log and three-dimensional seismic data, etc., the distribution of mixed deposition plane and vertical direction were predicted, and the main controlling factors of the development of mixed deposition were discussed. The results show that the mixed sediments associated with lacustrine facies and volcanic eruption are developed in the lower third member of Shahejie Formation of southern slope belt of Laizhouwan sag, and that there are two types of mixed sediments associated with fractional mixing and interbedding mixing in the vertical formation. There are three types of sedimentary facies, including mixed facies, braid delta facies and pyroclastic rocks. Among them, the early continental detritus has a great influence on the development of far-shore mixed micritic beach. The influence of land-based detritus at late stage was weak, but the wave action was enhanced, and the mixed detrital beach developed. The warm and humid paleoclimate, relatively closed shallow-water environment, and the nutrients from volcanic formation during the same period provided favorable conditions for the development of mixed deposition. The supply of far source in the west and near source in the east controlled the development types of mixed deposition. Underwater paleouplift controls its plane distribution and enrichment degree. Local volcanic activities formed special mixed sedimentary lithology and its association. The results not only improve and deepen the new understanding of mixed deposition in Bohai Sea, but also provide guidance for the search for similar mixed deposition in the third member of Shahejie Formation, and also provide reference for the study of high-quality reserve utilization and well pattern deployment in the research area.