Abstract: The low-permeability reservoir of Huagang Formation in Block X of Xihu Sag is characterized by strong heterogeneity, variable lithology and complex pore structures. These complex factors lead to low permeability calculation accuracy and affect the fine evaluation of the reservoir. With generally poor physical properties and distinct pore structure characteristics among different reservoir types, effective reservoir classification is crucial for accurately calculating physical parameters and establishing relevant computational models across wells in the block. A set of classification standard of reservoir pore structure for each well in the block was established based on flow units by integrating core data and well logs. This standard was applied to permeability curve calculations, yielding favorable results. Flow units enable more precise formation characterization and significantly enhance the understanding of physical properties of low-permeability reservoirs.