Abstract: Within the Beibuwan Basin in the western part of the South China Sea, there are a large number of low-resistivity reservoirs in gravelly sandstones, with complex pore structure and fluid distribution, and the water saturation of the formation calculated by conventional methods is on the high side, which makes it difficult to distinguish between irreducible water, movable water and movable oil, and makes it difficult to discriminate between the fluid properties of the formation and to evaluate the oil saturation of the reservoir. In view of this, the cause of low resistivity in gravelly sandstone reservoirs was analyzed from internal and external factors based on core assay data such as mercuric pressure and thin section, and combined with logging and mud-logging characteristics. On the basis of the reservoir classification, the petroelectric parameters of the low-resistivity and high-resistivity reservoirs were finely etched to obtain the evaluation parameters applicable to different microporous structures. At the same time, the variable T₂ cutoff value processing was performed based on the "morphology method" and in conjunction with nuclear magnetic resonance (NMR) logging. Compared with the traditional fixed T₂ cutoff value, the obtained results can effectively evaluate the content of irreducible and movable fluids, which significantly improves the accuracy of formation fluid analysis and the compliance rate of logging interpretation of low-resistivity reservoirs, and then forms a set of comprehensive fluid identification and reservoir evaluation method for low-resistivity reservoirs with complex pore structure in gravelly sandstone. By applying this evaluation method, a large number of low-resistivity reservoirs were found in the old wells, which confirms that this method is highly practical and operable. This study is of great significance to the fluid identification and evaluation of the low-permeability and low-resistivity reservoirs with complex pore structures.

Key words: pore structure, low-resistivity reservoir, petroelectric parameters, capillary irreducible water, variable T₂ cutoff value, gravelly sandstone, Beibuwan Basin