## 5 Discussions  
### Natural Gas Enrichment Processes of the Central Anticline Belt  
It is currently widely believed that the large-scale charging and accumulation of natural gas in the Central Anticline Belt of the Xihu Sag are closely related to the Longjing Movement, especially with the late accumulation at 13 Ma being the main process. By comparing the characteristics of oil and gas reservoirs of the three typical anticline structures of N, Q, and Z in the Central Anticline Belt, this study believes that there are significant differences in the natural gas accumulation processes of different anticline structures. Generally, there are two natural gas accumulation models: one is where the hydrocarbon convergence occurs first and then oil and gas transport and accumulate into the upper structural reservoirs; the other one is where the hydrocarbon migration and oil and gas accumulation occur simultaneously, followed by hydrocarbon adjustment. Model 1 of natural gas enrichment process  
Model 1 is referred to as \"the hydrocarbon convergence occurs first and then oil and gas transport and accumulate into the upper structural reservoirs\", which is the typical pattern of the natural gas enrichment process. The main hydrocarbon-generating sub-sags in the Xihu Sag began to generate and expel hydrocarbons around 25 Ma [40]. During the hydrocarbon generation and expulsion period, the source rock strata first experienced a short-distance migration and convergence of source rocks in the pre-existing paleo-structures of the basin to form oil and gas reservoirs. When the source rock strata are in low-amplitude structures, gentle steps, or nose-like structures, hydrocarbons will also undergo migration and convergence. These accumulated or semi-accumulated hydrocarbons can further undergo vertical migration during late tectonic movements, be further transported upward, and accumulate and form reservoirs in traps. Before the Longjing Movement in Structures-Q and -Z, the source rock strata of the Pinghu Formation already had a certain paleo-structural background, so oil and gas were preferentially transported and accumulated in the direction of the paleo-structure, while the traps of the Huagang Formation were formed late and the ancient and modern structures were connected by faults, forming a \"the hydrocarbon convergence occurs first and then oil and gas accumulate into the upper structural reservoirs\" pattern (Figure 11). The previous text has comprehensively discussed oil and gas inclusions, natural gas methane carbon isotopes, etc., under a certain paleo-structural background, effectively confirming this natural gas enrichment process. Figure 10: The hydrocarbon inclusion characteristics of sandstone sampled from Huagang Formation of well-N1. In the first process of natural gas enrichment, the deep gas reservoirs have the characteristics of early natural gas charging based on the pre-convergence and also have a higher gas saturation (Figure 11). By comparing the microscopic reservoir characteristics of and differences in the P2 layer and H7 layer of well-Q1 (Figure 12), although both layers belong to ultra-low-permeability reservoirs, under the background of early oil and gas convergence in the P2 layer, the formation of illite was inhibited (Figure 12d-f). Illite K-Ar dating indicates that the hydrocarbon charging time is approximately 25 Ma. Therefore, the overall pore-throat structure of the P2 layer sand body is relatively clean, and the maximum gas saturation is 52%, while the oil and gas charging of the H7 layer is related to the Longjing Movement, and the oil and gas charging time is relatively late (about 13 Ma). The maximum gas saturation of the gas reservoir in the Huagang Formation is only 45%. Model 2 of the natural gas enrichment process is related to the fact that \"The hydrocarbon migration and accumulation occur simultaneously, followed by later hydrocarbon adjustment\".