Genetic mechanism of overpressure and its significance on petroleum exploration in Baiyun sag in the deep water zone of Pearl River Mouth Basin

Recently, an important breakthrough has been made in the oil-gas exploration of Zhuhai Formation and Wenchang Formation of Baiyun sag in the deep water zone of Pearl River Mouth Basin, and the sandbody, i.e., oil and gas, was found in overpressure section, revealing that overpressure might be one of the key controlling factors for the formation and distribution of this type of oil-gas reservoirs. However, there are still few stuies on the overpressure genesis and the relationship between it and oil-gas reservoir formation. Therefore, three typical overpressure wells in the sag were selected for systematic shale sampling throughout the well section, and the experiments such as field emission scanning electron microscopy, N2/CO2 absorption and X-ray diffraction were carried out, clarifying the shale diagenesis in the main depression, north slope and the east of Baiyun sag, as well as the causes for the difference in overpressure logging response. Meanwhile, based on the determination of overpressure distribution characteristics according to the pressure prediction results obtained by drilling and Bowers method, multiple methods were adopted to this paper thoroughly analyzes the overpressure genesis mechanism using multiple methods and explores the hydrocarbon accumulation effect under overpressure and the oil-gas exploration fields in the near future. The research results show that there is a regional difference in the logging response model of overpressure section in Baiyun sag; the sonic transit time and density remain constant and the electrical resistivity is increased in the main depression and north slope of Baiyun sag; in the east of Baiyun sag, the sonic transit time and resistivity show an obvious reversal with the increasing of depth, while the density first increases and then is slightly reversed. The reason for such a difference is that there are differences in mineral composition of the shale in the two regions, thus resulting in various pore structures and compaction modes. The overpressure in Zhuhai Formation-Wenchang Formation of Baiyun sag is not caused by undercompaction, and the source of overpressure primarily lies in fluid expansion caused by hydrocarbon generation; the overpressure in the east of Baiyun sag may also be related to clay mineral transformation, and reservoir overpressure is resulted from pressure transfer. As overpressure is the driving force for hydrocarbon accumulation, and the formation and distribution of oil-gas reservoirs are closely related to the distribution of overpressure in ancient and modern times. On this basis, the oil-gas exploration fields can be divided into the atmospheric pressure oil-gas reservoir at the edge of overpressure sealing compartment or outside the compartment and the overpressure oil-gas reservoir inside the compartment. Specifically, the former is mainly controlled by the effective coupling of reservoir-forming elements, such as pressure releasing pathway and high-quality trap, while the latter is closely related to such factors as good hydrocarbon generation and hysteresis conditions, overpressure strength and high-quality reservoirs and sweet spots. © 2022, Editorial Office of ACTA PETROLEI SINICA. All right reserved.