Geochemical Characteristics and Hydrocarbon Accumulation Model of Natural Gas in the Third Member of the Oligocene Lingshui Formation in the Baodao Sag, Qiongdongnan Basin, South China Sea

The deep-water area of the Qiongdongnan basin is currently a hot topic for exploration. The discovery of gas fields in the Baodao sag confirms its abundant oil and gas resources and potential, making it of significant economic and strategic importance. The complexity of sedimentary structural evolution within the Baodao sag makes the process of oil and gas accumulation in the area extremely complex, and the law of natural gas enrichment is difficult to grasp, resulting in unclear exploration directions. Therefore, this study focuses on the third member of the Lingshui Formation in the Paleogene of the Baodao sag. Based on the abundant thin section, scanning electron microscopy, 3D seismic and geochemical analysis data in the area, through analyzing the density of natural gas, the proportion of hydrocarbon and non-hydrocarbon components, the dryness coefficient carbon, and the isotopic characteristics, combined with the deep natural gas genesis discrimination chart, the types and genesis types of natural gas and organic matter in the sag are clarified. In addition, combined with the package and BasinMod 2009 software, the filling period and reservoir-filling process were clarified and restored. At the same time, the reservoir formation characteristics of the different fault-step zones inside the sag were dissected and the primary and secondary migration of natural gas were analyzed in order to clarify the types and characteristics of different fault-step zone transport systems. Finally, the research findings indicate that there are two reservoir formation modes developed within the depression, as follows: “multiple hydrocarbon generation and control sources—continuous vertical control of large faults—lateral sand body convergence (T + Z-type transport)—multiple cap layer closure” and “mixed-source hydrocarbon supply—continuous vertical control of large faults—short lateral sand body convergence (Z-type transport)—multiple cap layer closure”, providing an important basis for the next exploration of the basin.