AAPG Bulletin, Vol.91, No.10, 1467-1498, 2007
Hierarchies of overpressure retardation of organic matter maturation: Case studies from petroleum basins in China
The effects of overpressure on different aspects of organic matter maturation have been examined in three basins using multiple parameters. Overpressure in the Yinggehai Basin has retarded kerogen maturation, hydrocarbon generation, and thermal cracking of long-chain normal hydrocarbons, as well as evolution of isoprenoid hydrocarbons. Overpressure in the Dongpu depression, Bohai Bay Basin, seems to have retarded the maturation of bulk kerogens reflected by Rock-Eval T-max, the thermal cracking of long-chain normal hydrocarbons, and the evolution of isoprenoid hydrocarbons, but seems to have had no detectable effects on vitrinite reflectance. Overpressure in the Qiongdongnan Basin has had no detectable retardation effects on all aspects of organic matter maturation. The observed phenomena suggest differential retardation of organic matter maturation (that is, different organic matter maturation reactions and different maturity parameters have been retarded by overpressure to different degrees in the same overpressured system) and four hierarchies of overpressure retardation. Hierarchy I retardation is defined as the state where the overpressure has retarded all aspects of organic matter maturation and is proposed to have been caused by the combination of high formation pressure and excess pressure, a high-water/organic matter ratio, and strong retention of reaction products in an early-developed, strong, persistent overpressured system. Hierarchy II retardation refers to the case in which all aspects of organic matter maturation except the maturation of hydrogenpoor kerogens, especially vitrinite, have been retarded and seem to have been caused by early-developed, moderate to strong, persistent overpressure. Hierarchy III retardation refers to the situation where overpressure has retarded only the thermal cracking of liquid hydrocarbons, but has had no detectable effect on the maturation of both hydrogen-rich and hydrogen-poor kerogens, and is proposed to have been caused by moderate to strong, persistent overpressures that began to develop around peak oil generation. Hierarchy IV retardation is defined as the situation where overpressure has had no detectable retardation on all aspects of organic matter maturation and may have been caused by cases wherein the overpressure developed too late, the intensity of the overpressure was too low, the duration of the overpressure was too short, or overpressured fluids had been expelled frequently. Resulting from both the diversities of organic matter maturation reactions and the complexities of overpressure generation and development, the hierarchies of overpressure retardation in different basins may be quite different, and it is necessary to use multiple parameters to understand the evolution of the petroleum systems in overpressured basins.