AAPG Bulletin, Vol.103, No.6, 1401-1442, 2019
Petroleum charge history in the Baiyun depression and Panyu lower uplift in the Pearl River Mouth Basin, northern South China Sea: Constraints from integration of organic geochemical and fluid inclusion data
The source, thermal maturity, and charge history of oil and gas condensate in the Baiyun depression and the Panyu lower uplift were determined using organic geochemical and fluid inclusion data. Nearly all the gas condensates and oil in the study area originated from Eocene Enping Formation source rocks, whereas the condensate fluids in the eastern area originated from the Enping and Zhuhai Formations. The condensate fluids have a relatively low thermal maturity with a calculated vitrinite reflectance equivalent that typically ranges from 0.8% to 1.0% based on the C-29 sterane isomerization ratios and aromatic hydrocarbon maturity parameters. Four types of oil inclusions with various fluorescence colors (yellow-blue) and API gravities (25 degrees-44 degrees) were recognized in the Oligocene and lower Miocene reservoir units, which suggest multiple oil charging events. The homogenization temperatures of the fluid inclusions recorded the oil charge events accompanying large-scale hot fluid movement, which was most likely caused by the charging of high-temperature natural gas from the deeper Wenchang Formation. The trapping pressures of the oil inclusions and the reconstructed burial histories were used to determine the timing of oil charge. The main oil charge occurred from 16-7 Ma throughout the Baiyun depression and Panyu lower uplift, whereas the Panyu lower uplift experienced an additional oil charge from 5-0 Ma. The natural gas charge occurred during the main oil charging period and may continue to the present day. Multiple physical processes, such as gas injection (without gas exsolution), gas washing (with gas exsolution), and migration fractionation, have played important roles in gas condensate formation and controlling the petroleum distribution in the Baiyun depression and Panyu lower uplift.