This study examined the geochemical features of pore water in the diapiric area of the Yinggehai Basin, northwestern South China Sea, and illuminated the origin and evolution of basin fluids. Pore water with low salinity occurs in marine sediments in the diapiric area even without meteoric water infiltration. The presence of low-salinity water within deep, overpressured compartments is assumed to be due to smectite-illite transformation. Howerver, in shallow portions(less than 2 000 m) of diapiric areas with normal pressure, pore water has a much wider variation and much lower salinity than that in the overpressured intervals. Its total dissolved solid(TDS) content is ~5 336 to 35 939 mg/L. Moreover, smectite and chlorite content sharply decreases as kaolinite and illite content increase in shallower intervals. The geochemical variation of pore water in diapiric structures indicates the expulsion of low-salinity, overpressured fluids along vertical faults. Strong injection of hot fluids from deep overpressured sediments results in rapid clay mineral transformation in shallow reservoirs. Consequently, fluid mixing due to fluid expulsion from deeper overpressured deposits leads to variation in salinity and ionic composition as well as some diagenetic reactions. This includes transformation of clay minerals caused by the higher temperatur of deeper hot fluids, e.g., the transfromation of smectite to illite and chlorite to kaolinite. Therefore, variations in salinity and ionic compositions in various pressured systems provide a clue to flow pathways and associated diagenetic reactions. This study examined the geochemical features of pore water in the diapiric area of ​​the Yinggehai Basin, northwestern South China Sea, and illuminated the origin and evolution of basin fluids. Pore water with low salinity occurs in marine sediments in the diapiric area even without meteoric water infiltration. The presence of low-salinity water within deep, overpressured compartments is assumed to be due due smectite-illite transformation. Howerver, in shallow portions (less than 2 000 m) of diapiric areas with normal pressure, pore water has a little Its total dissolved solid (TDS) content is ~ 5 336 to 35 939 mg / L. Moreover, smectite and chlorite content sharply decreases as kaolinite and illite content increase in shallower intervals. The geochemical variation of pore water in diapiric structures indicates the expulsion of low-salinity, overpressured fluids along vertical faults. Strong injection of hot fluid s from deep overpressured sediments results in rapid clay mineral transformation in shallow reservoirs. This includes transformation of clay minerals caused by the higher temperatur of deeper hot fluids, eg, the transfromation of smectite to illite and chlorite to kaolinite. Thus, variations in illinity and ionic compositions in various pressured systems provide a clue to flow pathways and associated diagenetic reactions.