Gas generation and main gas source rocks in Baiyun Sag and the Pearl River Mouth Basin of the northern South China Sea

Due to the limited number of exploration wells in Paleogene mudstones, as well as the complex and diverse types of delta, lacustrine, and marine source rocks in locations in the marine–continental transitional zone, there have been few studies of the gas generation mechanism of source rocks in Baiyun Sag. This has resulted in a long-term dispute about the types and stratigraphic sequences of the main gas source rocks. In this study, gas generation simulation experiments were performed using a high-temperature and high-pressure gold tube closed system on various types of source rocks (i.e., semi-deep lacustrine, shallow lacustrine, delta, and marine transgression/marine facies) from three sets of strata from the Wenchang, Enping, and Zhuhai Formations drilled in Baiyun Sag. Then, the hydrocarbon-generation processes and production characteristics of different types of source rocks at different stratigraphic sequences were rebuilt, and the composition and carbon isotope characteristics of the natural gas generated from these source rocks at different evolution stages were identified. Based on established natural gas generation kinetic modeling of virtual well points in different structural parts of Baiyun Sag, and constrained by the present gas reservoir characteristics and accumulation periods, it was demonstrated that the third and fourth members of the Wenchang Formation in the slope–depression area of the main subsag would have been the main gas source kitchens in Baiyun Sag. However, due to the overall high degree of thermal evolution in the depression area, the generation of natural gas with maturity comparable to the present gas reservoirs was too early—that is, before 20 Ma—to be effectively captured.