Pore Structure Characteristics and Their Diagenetic Influence: A Case Study of Paleogene Sandstones from the Pinghu and Huagang Formations in the Xihu Depression, East China Sea Basin

This study evaluated the effects of different experimental methods and pore structure characterisation methods to establish a comprehensive pore structure evaluation index (PSI) and analyse the influence of reservoir diagenesis on pore structures. Various experimental methods, such as nuclear magnetic resonance (NMR), high-pressure mercury intrusion (HPMI), NMR cryoporometry and X-ray computed tomography, were used. Moreover, the characterisation effects of various pore structure characterisation methods were compared. The results showed that the NMR fractal method and the HPMI fractal method could effectively characterise the fractal pore structure characteristics when the pore radii exceeded 13 to 200 nm and 5 to 2926 nm, respectively. However, both the methods had certain limitations. Therefore, the PSI was established using a method for reconstructing the NMR T 2 spectrum with a bimodal Gaussian function. The analyses showed that the PSI provided a good correlation with pore structure parameters and the fractal dimension. Thus, it can be effectively used as a parameter for characterising pore structure quality. The relationship between the PSI and logging curve was used to predict the pore structure at the reservoir scale. Diagenesis is the main factor affecting the pore structure of reservoirs in the Xihu Depression. Dissolution is the key factor for improving the seepage capacity of the reservoir. However, compaction and the formation of quartz overgrowth, carbonate cementation, and illite can occupy pore spaces and cause a deterioration of pore connectivity. The method of PSI acquisition and prediction has broad application prospects in the prediction and evaluation of pore structure.