OVERPRESSURE RETARDATION OF ORGANIC-MATTER MATURATION AND PETROLEUM GENERATION - A CASE-STUDY FROM THE YINGGEHAI AND QIONGDONGNAN BASINS, SOUTH CHINA SEA

Three superimposed pressure systems developed in the Yinggehai Basin, South China Sea, as indicated by seismic data, well logs, and direct p ressure measurements. The organic maturation profile is nonlinear, wit h three nonparallel segments that correspond to the shallow, normal-pr essured system; the intermediate, overpressured system; and the deep, strongly overpressured system, respectively. The intermediate and deep overpressured systems have abnormally low R(o) gradients. The organic maturity of these overpressured rocks is significantly lower than the maturity of normal-pressured source rocks in nearby wells with simila r thermal histories and does not match the thermal histories of the ro cks. Such an organic maturity anomaly is distinctly different from tho se caused by variation in activation energies, conductivity contrasts, and hydrologic effects, and is confirmed to be the result of overpres sure 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 p ressure, confirming that pressure increases the activation energies of organic-matter maturation reactions. Overpressure retardation has bee n proven to be conditional and quite important for clearly understandi ng petroleum generation, migration, and accumulation in overpressured sedimentary basins.