Abstract: The sedimentary-tectonic evolution of Baodao sag in the Qiongdongnan basin is complex, with many sets of source-reservoir-seal assemblages in Baodao sag, Qiongdongnan basin. The south-north step-fault zone is the main oil and gas enrichment area in Baodao sag, the high-quality deltaic sand bodies in the Third Member of the Lingshui Formation of Paleogene are the main target beds. Based on the data of logging, seismic and geochemical data, this paper makes a comparative study on the static factors of gas enrichment in the Third Member of Lingshui Formation, and expounds the difference of gas dynamic accumulation by combining with basin simulation technology, the main control factors were summarized. The results show that the methane content is low and the non-hydrocarbon gas content is high in the northern step-fault zone, and the dry gas is the main gas in the southern step-fault zone. In the northern part of the step-fault zone, the medium porosity and low permeability reservoirs are mainly transported by T-type and Z-type mixed reservoirs, while in the southern part of the step-fault zone, the medium porosity, low permeability and ultra-low permeability reservoirs are mainly transported by Z-type reservoirs. The trap of block BD21-1 in the northern fault step zone was formed at the end of the Sanya Formation, and the structural high point of the trap turned to the east-west direction, in the early stage, the oil and gas generated in Yacheng Formation entered the reservoir through secondary faults. The trap in the northern part of block YL10-6 in the southern fault step was characterized as a small anticline trap at the F10-1 fault zone, which was formed at the end of the Sanya Formation and was influenced by compressive stress at the late stage. The Third Member of Lingshui Formation in the south-north step-fault zone experienced three stages of oil and gas charging. The gas differential enrichment depends on the differences in hydrocarbon potential of sub-depression, enrichment degree during primary migration, and preservation situation of secondary migration.

Key words: differential enrichment mechanism, accumulation period, natural gas,  , reservoir, Baodao sag, the south-north step-fault zone, the Third Member of Lingshui Formation, Qiongdongnan basin