Geochemical characterization of petroleum migration of Yacheng Gasfield, offshore South China

Abstract

The pressure of the reservoir zone of the Yacheng Gasfield is normal, but its temperature and maturity are anomalously high compared with those of the zones above and below it. In the gasfield both of them show a trend from the west to the east where they become lower even though the depths of the reservoir zone become greater. The organic matter of the mudstone in the reservoir zone is detected, heated, dissolved and extracted The trend is agreed by the geochemical data of the gas and condensate samples: the carbon isotope composition, the composition of C 6 in paraffin, naphthathene and the aromatic form, the C 15+ composition and the oil's physical properties. An interpretation can be made that the gas and condensate would migrate into and through the reservoir zone with hot water from the deep and hot Yinggehai Basin where gas and oil would be dissolved in water. In the reservoir zone, the gas and oil enter into a gaseous phase, which has the ability to dissolve and extract hydrocarbons from the organic matter of the reservoir zone. Simultaneously, a great amount of heat would be transported by the hot water and significantly raise the temperature of the reservoir zone and the maturity of its organic matter. As the gas and oil migrate in the gaseous phase, the unusual chemical properties of the gas and condensate will become more normal along the trend.