Petroleum accumulation history of Nanbaxian belt — Study of gas generation and fluid phase, northern margin of Qaidam Basin, West of China

Abstract

To study the phase evolution and the fluid accumulation history of Nanbaxian petroleum field, in this study, a series of anhydrous closed-system gold-tube thermal simulation experiments have been performed on a carbonaceous mudstone (type Ⅱ kerogen) from the northern margin of the Qaidam Basin in the Xiaomeigou Formation (J1x). The hydrocarbon generation kinetics research was used to simulate the gas generation history of Yiliping depression, and the phase characteristics of the evolution were analysed by PVTsim. The influence of kerogen generated CO2 on the fluid phase was discussed that CO2 has a significant influence on the fluid phase at low maturity stage that the fluid phase could be a gas phase under geological condition. As a result, the fluid was migrated to Lower Ganchaigou Formation as gas phase, in the early stage of hydrocarbon accumulation. The fluid phase was gradually transformed into oil phase with the CO2 loss and accumulated as primary reservoir. With the increase of the maturity, the effect of CO2 on the fluid phase gradually decrease. During the Shizigou Formation depression, the tectonic action and the hydrocarbon generated from Yiliping depression led to the evaporation fractionation of the primary reservoir and the gas washing which cause the reconstructed of the primary reservoir in Lower Ganchaigou Formation and form of the secondary reservoir in Shizigou Formation.