Formation pressure modeling in the Baiyun Sag, northern South China Sea: Implications for petroleum exploration in deep-water areas

The Baiyun Sag lies in the deep-water area of the Pearl River Mouth Basin. It is the largest and deepest sag in the northern South China Sea with the highest hydrocarbon potential. The pressure distribution and pressure evolution of the Baiyun Sag have been explained by combining analysis of measured pressure data, prediction of overpressure using the seismic velocity volume and two-dimensional basin modeling. Today, shallow formations are mainly normally pressured but the deep Enping and Wenchang Formations are over-pressured. Rapid sedimentation and hydrocarbon generation are two mechanisms that lead to compaction disequilibrium and subsequent overpressure generation in the Baiyun Sag, the former effect being considered more important than the latter. The multiple identified episodes of overpressure release through the basin history allows the conclusion to be made that rapid mudstone sedimentation and hydrocarbon generation are considered to be the factors leading to overpressure generation and preservation while fault activity is considered to be a factor leading to overpressure release. Two major overpressure release episodes: the Baiyun Event (23 Ma) and the Dongsha Event (10 Ma) are considered to have been important in allowing deep over-pressured hydrocarbon fluids to migrate into shallow normally pressured formations and accumulate. The primary pathways for migration are interpreted as being directions where pressure reduced fastest, and this being linked to faulting related to tectonic events.