Abstract Three depth-related pressure systems are developed in the Banqiao Sag, Eastern China, as indicated by well logs and direct pressure measurements. There are hydropressure, mild overpressure and high overpressure within the overpressured sediments, the organic maturation profile is nonlinear, with five nonparallel segments. Segments 1 and 3 have abnormally low R o gradients. Such an organic maturity anomaly is distinctly different from those caused by variation in activation energies, thermal conductivity contrasts, and hydrologic effects, and is believed to result from overpressure retardation. The division criteria of diagenetic stages published by China National Petroleum Corporation (CNPC) most widely used in China cannot be applied to overpressured clastic reservoirs in the Banqiao Sag. Improved division criteria of diagenetic stages are established according to vitrinite reflectance, spore pollen colour, TAI, alteration of clay minerals and geotemperature. Overpressure retardation of maturation is suggested to be important for clearly understanding deep exploration in overpressured sedimentary basins.