## 4 Result and Interpretation
### Paleo-Structure Characteristics of Differential Enrichment of Gas  
The paleo-structural background is one of the important controlling factors for the degree of oil and gas enrichment in the Central Anticline Belt of the Xihu Sag. A large number of oil and gas exploration practices have proved that, in the dynamic process of oil and gas migration and accumulation, paleo-structures are often favorable target areas for paleo-oil and gas migration [36; 37; 38; 39]. Meanwhile, paleo-structures provide important sites and trapping conditions for the accumulation of paleo-oil and gas, and also have an important impact on the formation and distribution of current oil and gas reservoirs. The Xihu Sag has undergone multiple tectonic movements, among which the Longjing Movement since the Miocene has the greatest influence on the formation and evolution of the Central Anticline Belt and the accumulation of oil and gas. Before the Longjing Movement, the Xihu Sag experienced a series of tectonic movements, such as the Yuquan and Huagang movements, which controlled the paleo-structural background of the early Xihu Sag. By restoring the paleo-structure of the top surface of the source rock strata of the Pinghu Formation during the period of 16 Ma (Figure 7b), it can be seen that the paleo-structure had already formed before the large-scale charging and accumulation of natural gas, and it is quite different from the current structural map (Figure 7a). Among them, Structure-Q and Structure-Z  
Figure 6: Typical cross-wells section of Structure-Q, showing hydrocarbon charging and the hydrocarbon migration features with various-level fault systems. The location of the two wells is shown in Figure 1a. developed nosing paleo-structures, while Structure-N was a negative unit before the oil and gas charging and did not develop a paleo-structure during 16 Ma (Figure 7b). The paleo-structural background of Structure-Q was formed by the compression of the Longjing Movement (Figure 7b), providing a favorable site for the early oil and gas charging or trapping. According to the analysis of the P2-layer sandstone samples from well-Q1, two stages of oil and gas inclusions were developed (Figure 8). The first stage of oil and gas inclusions developed in the early to middle stage of the secondary enlargement of quartz grains in sandstone, with a low development abundance and a GOI of about 3%. Most of the first-stage inclusions are distributed in bands along the inner side of the enlarged edge of quartz grains and along the micro-fractures during the diagenesis of quartz grains (Figure 8), and the homogenization temperature of the inclusions is 120 \({}^{\circ}\)C to 130 \({}^{\circ}\)C. The second stage of oil and gas inclusions developed after the secondary enlargement of quartz grains in sandstone, with a high development abundance and a GOI of about 15%. Most of the second-stage inclusions are distributed in bands along the post-diagenetic micro-fractures that cut through quartz grains and their enlarged edges (Figure 8), and the homogenization temperature of the inclusions is 140 \({}^{\circ}\)C to 150 \({}^{\circ}\)C. In addition, from the analysis of the methane carbon isotopes of natural gas in the Pinghu Formation and Huagang Formation of the Q structure, it is also confirmed that the Q structure has the characteristics of multi-stage charging. Vertically, from the deep Pinghu Formation to the shallow Huagang Formation, the methane carbon isotope differentiation of natural gas is relatively large, exceeding 3%. Gradually, the methane carbon isotope is shown heavier upward, increasing from \(-\)30.7% to \(-\)33.8% (Figure 9a), reflecting the gradually increasing maturity of the source rocks and the multi-stage charging characteristics of natural gas. Figure 7: (**a**) The present structural map and (**b**) the paleo-structure map of top Pinghu formation at 16 Ma of the study area in the Central Anticline Belt.