## 4 Result and Interpretation  
### Differential Enrichment Characteristics of Various Anticline Structures  
According to the intensity of the compressive stress and the scale of the anticline, the Central Anticline Belt in this research can be divided into three different structures, including a northern medium-strong compression zone (Structure-Z), a southern strong compression zone (Structure-N), and a western weak compression zone (Structure-Q; Figure 2). Due to different structural evolution processes, preservation conditions, hydrocarbon migration and accumulation processes, etc., the gas enrichment characteristics of each anticline structures show differences in the gas column, gas filling degree, and reserve abundance (Figure 3).  
Structure-Z is fully saturated and located in the northern medium-strong compressed zone of the northern Central Anticline Belt (Figure 1a), covering an area of 40 km\({}^{2}\). It is an inversion anticline formed during the Longjing Movement, with an amplitude of approximately 400 m. The hydrocarbon-bearing strata (H1-H7) are located below the regional mud-seal H1-H2 in the Huagang Formation (Figure 3). The gas column of Structure-Z is 400 m, with a 100% gas filling degree and 12.4 m\({}^{3}\)/km\({}^{2}\) reserve abundance (Figure 2).  
Structure-N is located in the southern strongly compressed zone of the Central Anticline Belt (Figure 1a), with an anticline area of 700 km\({}^{2}\) and an anticline amplitude of approximately 850 m. It is the largest anticline in the offshore areas of China. Hydrocarbons have been encountered in multiple horizons vertically, ranging from the Huagang Formation to the Longjing Formation (Figure 3). However, the gas column is generally less than 50 m, and most of the gas reservoirs are underlain by water. The proportion of the volume filled by hydrocarbons in this anticline is less than 20%, with 2.5 m\({}^{3}\)/km\({}^{2}\) reserve abundance (Figure 2).  
Figure 2: Differential enrichment characteristics of natural gas in the anticline structure Q, N, and Z of the Central Anticline Belt.  
Structure-Q is located in the western weakly compressed zone of the Central Anticline Belt (Figure 1a). It is a low-amplitude anticline structure formed under the compressive background of the Longjing Movement, with an anticline area of 60 km\({}^{2}\) and an amplitude of about 100 m. The main hydrocarbon reservoir strata of this structure are the H4 and deeper strata of the Huagang Formation (Figure 3). The anticline core and flank of this structure have different gas-water interfaces. The maximum height of the gas column is 65 m, the gas saturation or filling degree is 40%, and the reserve abundance is approximately 1.4 m\({}^{3}\)/km\({}^{2}\) (Figure 2).  
It is worth noting that thick-layer, ultra-low-permeability gas reservoirs are developed in the deep layers of these three anticline structures (Z, N, and Q), with huge reserve scales. However, there are significant differences in the gas-bearing properties of the gas reservoirs. Among them, in Structure-Z, the permeability average of Sandstone Group-5 of the Huagang Formation (H5) is 0.28 mD, and the average gas saturation is 53%. In Structure-N, the permeability average of Sandstone Group-7 of the Huagang Formation (H7) is 0.25 mD, and the average gas saturation is only 42%. In Structure-Q, the permeability average of Sandstone Group-2 of the Pinghu Formation (P2) is 0.38 mD, and the average gas saturation is 48%.