Genesis of High-Quality Mixed Rock Reservoir and Its Exploration Significance in Es₁ around Southeast Margin of Qinnan Sag, Bohai Sea

In order to ascertain the genesis of the high-quality reservoirs of the Es₁ in the southeast margin of the Qinnan sag, using conventional physical properties, cast thin sections, scanning electron microscope, carbon and oxygen isotopes and other analytical laboratory data, the characteristics of the Es₁ in Q29, L25 and Q30 structures and the control factors of high-quality reservoirs were studied. The results show follows. (1) The tectonic background controls the type, thickness, composition and overall physical properties of the first member of Shahejie mixed rock reservoir, which is the main reason for the difference of high-quality mixed rock reservoir. The Q29 structure mainly develops mixed fans in steep slopes. Longitudinally, it is composed of cycles of fan delta with bioclastic beaches. It has the characteristics of coarse particle size, high gravel content, the highest content of medium-acid volcanic rocks in terrestrial detritus, and low quartz content. Its overall physical properties are poor, the thickness is large, the dissolution pores are dominant, and a large number of biological cavity pores are developed. The L25 structure mainly develops mixed beaches in gentle slopes. Vertically, it is composed of the interaction between fan delta and bioclastic shoal, or bioclastic shoal with fan delta. Development of dissolution pores, primary pores and a large number of biological cavity pores. The Q30 structure mainly develops bioclastic shoals in the local uplift zone under water. It has the characteristics of low content of terrestrial debris, relatively poor overall physical properties, thinnest thickness, dissolution pores and biological cavity pores. (2) Controlled by differences in different tectonic backgrounds and sedimentation, the diagenesis suffered by the Q29, L25, and Q30 structures is both similar and different. Salty lakes, strong wave elutriation, low mud content, early dolomization, containing a large amount of biological debris, the development of mud crystal cladding, and strong acidic dissolution are the common characteristics of the formation of high-quality mixed-rock reservoirs. However, the strength of atmospheric freshwater leaching, compaction, cementation, and dissolution suffered by different structures show obvious differences. (3) The mixed rock reservoir has a large lower limit of burial depth. Constrained by the factors affecting the quality of high-quality mixed rock reservoirs, searching for large-scale and medium-sized oil and gas reservoirs in large-scale steep slope zone mixed rock reservoirs and good physical properties in gentle slope zone mixed rock reservoirs is currently one of the important targets of near-middle-deep exploration in the Paleogene Bohai Sea area.