Pressure evolution and hydrocarbon migration in the Baiyun sag, Pearl River Mouth basin, China

Abstract

The Pearl River Mouth basin is one of the most important offshore basins in China. Petroleum exploration in the deep-water area is to start in the Baiyun sag in the basin. Due to the high cost of exploration in deep-water areas, the hydrocarbon migration and pressure evolution needs to be thoroughly probed before any actual exploration is to be done. Drilling results have indicated that the pore pressure is hydrostatic in shallow-water area. The mud diapirs found in the Baiyun sag indicate that there may be overpressure in the deep-water area. At present, little is known about hydrocarbon migration and pressure distribution in deep-water areas. On the basis of geological data from wells and basin modeling, the research comes to the following conclusions. (1) At least three episodic accumulations and releases of pressure had happened in the Cenozoic, which were closely related to the three regional tectonic movements: the Zhu-Qiong movement in the Late Eocene, the Nanhai movement in the Middle Oligocene and the Dongsha movement between the latest Middle Miocene and Late Miocene. The pressure release and associated hydrocarbon migration occurred primarily during the Dongsha movement, when most of the overpressure was released to hydrostatic pressure except for some overpressure in the deep-water area. (2) Both the measured pressure and the modeling results indicate that the pore pressure is hydrostatic pressure in the shallow-water area. Abnormally high pressure in the deep-water area is noted to be present. The difference of pressure distribution is caused by the lithologies, facies and faults. (3) The diapirs in the deep water cannot be driven by the overpressure because the maximum pressure is less than 9.0 MPa in the course of pressure evolution. (4) The fluids migrated to the shallow-water area and deep-water area along faults. Some faults connected the deep-water fans with the hydrocarbon source. Huge oil and gas fields may be formed in the deep-water area more favourably than in the shallow-water area.