Three superimposed pressure systems developed in the Yinggehai Basin, South China Sea, as indicated by seismic data, well logs, and direct pressure measurements. The organic maturation profile is nonlinear, with three nonparallel segments that correspond to the shallow, normal-pressured system; the intermediate, overpressured system; and the deep, strongly overpressured system, respectively. The intermediate and deep overpressured systems have abnormally low Ro gradients. The organic maturity of these overpressured rocks is significantly lower than the maturity of normal-pressured source rocks in nearby wells with similar thermal histories and does not match the thermal histories of the rocks. Such an organic maturity anomaly is distinctly different from those caused by variation in activation energies, conductivity contrasts, and hydrologic effects, and is confirmed to be the result of overpressure retardation. The degree to which the organic-matter maturation is retarded, expressed as the difference between predicted and measured vitrinite reflectance, increases exponentially with increasing fluid pressure, confirming that pressure increases the activation energies of organic-matter maturation reactions. Overpressure retardation has been proven to be conditional and quite important for clearly understanding petroleum generation, migration, and accumulation in overpressured sedimentary basins.