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Based on the practice of oil and gas exploration and the analysis of shallow lithologic reservoirs, combined with the allocation relationship and enrichment law of oil and gas accumulation factors, main controlling factors and models of hydrocarbon accumulation of large lithologic reservoirs in shallow strata around the Bozhong sag are summarized, and favorable exploration areas are proposed. The coupling of the four factors of “ridge-fault-sand-zone” is crucial for the hydrocarbon enrichment in the shallow lithologic reservoirs. The convergence intensity of deep convergence ridges is the basis for shallow oil and gas enrichment, the activity intensity of large fault cutting ridges and the thickness of cap rocks control the vertical migration ability of oil and gas, the coupling degree of large sand bodies and fault cutting ridges control large-scale oil and gas filling, the fault sealing ability of structural stress concentration zones affects the enrichment degree of lithologic oil and gas reservoirs. Three enrichment models including uplift convergence type, steep slope sand convergence type and depression uplift convergence type are established through the case study of lithologic reservoirs in shallow strata around the Bozhong sag.

Abstract: By using thin section identification, cathodoluminescence, major and trace elements and fluid inclusion tests and authigenic illite dating, based on observation of core cracks, combined with the microscopic characteristics and imaging logging characteristics of fractures, the stages of the fractures in the Huagang Formation of the central reversal tectonic belt of the Xihu Sag in the East China Sea, and the matching relationship between the fracture development stages and the oil and gas charging stages are clarified. There are diagenetic fractures and tectonic fractures in the reservoirs of the Huagang Formation in the study area. The diagenetic fractures developed during the diagenetic stage of the reservoirs and have less effect on oil and gas migration and transport. The tectonic fractures are divided into three stages based on tectonic movements controlling the fractures and their relationships with hydrocarbon charging: The first stage of fractures was generated in the early stage of the Himalayan Movement–Longjing Movement (12–13 Ma ago), when the tectonic stress caused the sutures and shale strips to twist, deform, and break. Tectonic microfractures generated in this period had short extension, narrow width, and poor effectiveness, and had little effect on oil and gas migration and transport. The second stage of fractures came up during the middle-late period of Himalayan Movement–Longjing Movement (9–12 Ma ago), when tectonic movements caused the development of tectonic fractures in the central reversal tectonic belt, these fractures are of large scale, long extension, and good effectiveness, and matched with the first stage of large scale oil and gas charging (9–12 Ma ago), so they play an important role in oil and gas migration, transportation, and accumulation. The third stage of fractures were created from Himalayan Movement–Okinawa Trough movement to the present day (0–3 Ma ago), the fractures are tectonic ones developing successively; matching with the second stage (0–3 Ma ago) of large-scale oil and gas charging, they created conditions for continuous natural gas migration and transportation. All these prove that the development of reservoir fractures in the Huagang Formation of Xihu Sag can provide seepage space and continuous and effective channels for efficient migration and accumulation of oil and gas.