Sedimentary environment of the Middle Jurassic Bi Qu Formation and its controls on petroleum geology in the Zaring area, southern Qiangtang basin

Abstract：Located on the southern margin of the southern Qiangtang basin, the area is a "gap area" for hydrocarbon investigation. Detailed reconnaissance of 1:50,000 petroleum geological structure was carried out in the Zaring block of the Qiangtang basin, and according to the reconnaissance results the authors analyze two fossil oil accumulations in the Bi Qu Formation of the study area. Based on the types, textures, authigenic mineral assemblages and fossils of carbonate rocks in the sections as well as the characteristics of carbonate rock associations in the area, the Bi Qu Formation in the area is classified into four major sedimentary environments, namely, semi-restricted platform, restricted platform, open platform and shoal. Grain limestone showing the shoal environment is characterized by widespread occurrence, a great thickness, high porosity and high permeability, and its occurrence controls the size of reservoirs in the area. Dolostone showing the restricted platform environment forms another reservoir in the area. It is an effective reservoir, but its capability is related to the occurrence of the overlying strata. Micritic limestone with micritic calcarenite and calcirudite interbeds that formed in an open platform environment is a better source rock in the area, and its spatial distribution is strictly controlled by sedimentary facies. Micritic limestone with bioclastic limestone and micritic calcarenite interbeds formed in a semi-restricted platform environment is a favorable cover in the area, and the occurrence of the cover is also controlled by the nature of the semi-restricted platform environment.