Abstract: In this paper, the G structure, which is not ideal in oil and gas exploration in the southern part of Santan Depression in Xihu Sag, is regarded as the research target, the differences in hydrocarbon generation conditions, migration conditions, lithofacies association and fault system characteristics between Pinghu Formation and Huagang Formation are analyzed from the perspective of basic petroleum geological conditions. Based on the comprehensive analysis of igneous rock age, vitrinite reflectance, inclusion homogenization temperature, organic geochemistry and other laboratory data, it is concluded that Pinghu Formation of G structure had the favorable conditions of self-generation , self-storage and compartment reservoir forming before Longjing movement. After Longjing movement, deep oil and gas migrated vertically to the upper member of Pinghu Formation, which has the possibility of lateral sand body dredging and formation of lithologic reservoir. The unfavorable position of fluid potential in Huagang Formation is the main reason for the difficulty of hydrocarbon accumulation in structural traps of Huagang Formation. Combined with the analysis of the accumulation and evolution process of G structure, it is suggested that the next step can be to focus on the structure of the downthrown block of the western wing of the central anticlinal zone with fault lateral seal or the low gentle anticline around the main structure of the central anticlinal zone.