## 3 Materials and Methods
### Reservoir Characteristics and Distribution Pattern  
#### 4.5.1 Reservoir Diagenesis  
Based on the staged evolution of hydrocarbons, combined with diagenetic petrology, characteristic processes, and types of secondary pores, the diagenetic processes of carbonate reservoirs can be classified into different stages. (1) Syndepositional Stage--Submarine Diagenetic Environment  
The submarine diagenetic environment represents the earliest diagenetic setting experienced by marine sediments. The primary diagenetic processes occurring in this environment include mechanical compaction, submarine cementation, and pencontemporaneous dolomitization. Syndepositional diagenesis is closely linked to the sedimentary environment and depositional processes; therefore, different sedimentary environments exhibit distinct syndepositional diagenetic characteristics. (2) Early Diagenetic Stage--Shallow to Intermediate Burial Environment  
The early diagenetic stage corresponds to a shallow to intermediate burial environment, where the major diagenetic processes include the following: 1 The earliest burial dissolution and cementation, occurring after early meteoric freshwater leaching and dissolution during subaerial exposure. 2 Seepage-reflux dolomitization, following the pencontemporaneous evaporative concentration of seawater. In lagoonal or tidal flat settings behind reef and shoal complexes, concentrated brines percolate downward through early dissolution channels formed during reef-shoal exposure, leading to dolomitization of the carbonate deposits. (3) Middle Diagenetic Stage--Intermediate to Deep Burial Environment  
The middle diagenetic stage corresponds to an intermediate to deep burial environment, where the dominant diagenetic processes include tectonic fracturing and dissolution induced by organic acids generated during hydrocarbon generation and migration. The key diagenetic processes in this stage are as follows: 1 Re-equilibration or adjustment of early-formed dolomite in response to evolving diagenetic fluids in a progressively closed system. 2 Recrystallization, which enhances the pressure resistance of carbonate grainsand interstitial fluids. As burial depth increases, early-stage pressure dissolution intensifies, disrupting the initial equilibrium, leading to the formation of stylolites and pressure-solution seams. This process facilitates later-stage dolomitization, forming intercrystalline pores, inter-crystalline dissolution pores, and dissolution fractures in grainy dolostones developed within back-reef shoals. (4) Late Diagenetic Stage--Deep Burial Environment  
The late diagenetic stage corresponds to a deep burial environment, where the dominant diagenetic processes include the infilling of reservoirs with pyrobitumen due to hydrocarbon thermal cracking and burial dissolution caused by the generation of CO\({}_{2}\) from overmature organic matter. Additionally, pressure solution, dolomitization adjustment, and recrystallization also occur during this stage. Overall, as burial depth increases, the diagenetic environment progressively becomes more closed, transitioning into an alkaline and reducing system. However, within subsurface diagenetic settings, variations in temperature, pressure, and organic matter maturation over time lead to spatial and temporal differences in pore water chemistry. For the Changxing Formation reservoir, dolomitization and dissolution are the key diagenetic processes that have significantly modified the reservoir quality. The process of reservoir formation in the Changxing Formation of the study area is as follows:  
Syntefmentary period: The rock structure is loose with high pore water content, and compaction is weak. Primary porosity is well developed, including intraganm25(we5r)18 hs20 rmostly formed after strong dolomitization, with high dolomite content (>85%), and the original internal bio-morphology and structure have been damaged and are difficult to identify (Figure 8e,f).