## 4 Result and Interpretation
### Natural Gas Preservation Condition of the Central Anticline Belt  
Sandstone Group-1 and Group-2 of the Huagang Formation (H1-H2) sand-mudstone-interbedded units in the Central Anticline Belt of the Xihu Sag are the most stable regional mudstone caprock, with a thickness of approximately 600 m, mainly consisting of micro-facies of flood plains in the delta plain. This set of regional caprocks is mainly composed  
Figure 3: Three single-well diagrams for three different structures (Q, N, and Z) in the Central Anticline Zone, including gamma ray (GR) curves, lithological association, deep and shallow lateral resistivity (RD, RS) curves, and logging interpretation of oil, gas, and water layers. The location of the three wells is shown in Figure 1a. of thick mudstone intercalated with thin siltstone, with a high argillaceous content and a mud-to-land ratio of approximately 60%. Most of the oil- and gas-bearing strata in the Central Anticline Belt are located beneath this set of regional caproks (Figure 4). However, under the background of strong compression during the Longjing Movement, a series of late-stage EW-trending fault systems are commonly developed in the anticline cores of the northern medium-strong compression zone (Structure-Z) and the southern strong compression zone (Structure-N) (Figure 4). Their planar distribution scale is relatively small. Vertically, these faults can extend downward through the regional mudstone caprock and cut into the main oil- and gas-bearing strata, such as H3 and H5, and, upward, they can reach the T10 interface at the top of the Miocene strata. The late-stage fault activities have significant adjustment, transformation, and destructive effects on the natural gas reservoirs of the Huagang Formation, resulting in the large-scale escape of natural gas and local adjustment and accumulation in the effective traps of the shallow Longjing Formation. In the southern strongly compressed zone, under the intense compression background of the Longjing Movement, Structure-N has generally developed near E-W trending transverse tensile destructive faults in its anticline core, which have damaged the original gas reservoirs of the deep H3-H5 sand groups. Natural gas has been adjusted and accumulated in the effective traps of the shallow Longjing Formation. This has led to multiple oil- and gas-bearing strata in the vertical direction in this structural area, with small gas reservoir scales and common water presence. For instance, in well-N1, the current H3 is a bottom water gas reservoir with a gas layer thickness of approximately 8.4 m (Figure 5). The LINKAM THMS600-type heating and cooling stage of the nuclear industry was used to conduct gains containing oil inclusions (GOI) analysis and quantitative grain fluorescence analysis for the gas-water layer and the entire water layer section of the H3b gas layer in well-N1. Generally, it is considered that a hydrocarbon-bearing inclusion GOI \(\geq\) 5% and QGF-index \(\geq\) 4 are the basis for paleo-oil and gas reservoirs [30; 31; 32; 33; 34]. It can be seen that the GOI of hydrocarbon-bearing inclusions in H3 of well-N1 is \(<\)5% at 3198 m, and 3155 m is taken as the paleo gas-water interface. Through the analysis of the paleo-oil and gas reservoir interface, the thickness of the paleo gas layer of H3b is 72 m, indicating that H3 of Structure-N was a damaged residual gas reservoir of the Huagang Formation. Under  
Figure 4: Seismic profile A–A\({}^{\prime}\) of different compression zones of the Central Anticline Belt, showing the features of regional cap rock, and NE-trending and EW-trending fault systems developed in various structures. The location of the seismic profile A–A\({}^{\prime}\) is shown in Figure 1a. the destructive effect of the late near-E-W-trending faults, a large amount of natural gas escaped into the shallow Longjing Formation. In the northern medium-strong compression zone, the anticline core of Structure-Z also developed late-stage near-EW-trending transverse tensile faults, but its compression degree was relatively weak.