Differences and Genesis of High-Quality Reservoirs of Mixed Siliciclastic-Carbonate Rocks in the Es₁₂ around Bozhong Depression, Bohai Sea

The mixed siliciclastic-carbonate rock reservoirs are developed in the Es₁₂ around Bozhong depression, Bohai Sea, in which a large amount of oil and gas have been found. The reservoir physical properties show great differences, and strong heterogeneity. However, there are few studies on the differences and causes, restricting further exploration evaluation of this type of reservoir. In this paper, the differences and genesis of the high-quality reservoirs of the Es₁₂ around Bozhong depression in Bohai Sea are discussed based on the data analyses of drilling, conventional physical properties, casting thin sections, whole rocks and clay minerals. The results show follows. (1) There are three types of reservoirs, namely bioclastic-based mixed siliciclastic-carbonate rocks, mixed siliciclastic-carbonate rocks dominated by terrigenous clastics, and mixed-rock reservoirs dominated by chemically precipitated carbonates, respectively, in Es₁₂ around Bozhong area. A large number of snail and ostracod fossils are developed in the first two types of reservoirs. Among them, the mixed siliciclastic-carbonate rock reservoir dominated by bioclastic development has a large number of biological cavity pores, and the dolomitization has strong effect and the best physical properties. The mixed rock reservoir dominated by terrigenous clastics is mainly composed of dissolution pores, followed by biological cavity and primary pores, which is characterized by the development of mud-crystal cladding, strong dolomitization, containing a large amount of intermediate-acid volcanic rock debris, and shallow buried reservoirs have good physical properties. Calcite in cemented rock reservoirs dominated by chemical precipitation has strong cementation and poor physical properties. There are obvious differences in reservoir types and physical properties of different structural aggregate rocks. (2) The differences of mixed siliciclastic-carbonate rock reservoirs in the study area are mainly controlled by the parent rock type, basement lithology, sedimentary water environment, sedimentary facies, atmospheric freshwater leaching, buried depth compaction, cementation, dissolution and tectonic action. However, each constructed high quality reservoir has an independent combination of control factors. Non-carbonate mother rock area, the primary sedimentary period-igneous rock base, a large amount of bioclastic mixed rock, quasi-syngenetic period-atmospheric freshwater leaching, early dolomization, and deep burial period-strong dissolution, fracture development is the commonality of the formation of high-quality mixed rock reservoirs of the Es₁₂ around Bozhong depression. The high-quality reservoir differences and genesis analysis of mixed siliciclastic-carbonate rock are important for the prediction of high-quality reservoirs in the middle and deep oil and gas exploration and evaluation process of continental fault basins.