Hydrocarbon Generation in Source Rocks with Low Organic Matter as the Primary Mechanism for Overpressure Development: A Case Study from the Yinggehai Basin, South China Sea

Overpressure is developed in both the reservoirs and source rocks of the Huangliu Formation, with pressure coefficients reaching up to 2.31. This study investigates the mechanisms responsible for overpressure in the source rocks of the Huangliu Formation and reconstructs their temporal evolution. Overpressured mudstones have anomalously high acoustic velocity and low resistivity values. However, the absence of low-density values indicates that disequilibrium compaction is not responsible for the observed overpressure. The source rocks in this formation predominantly contain type III kerogen (gas-prone), with total organic carbon(TOC) values ranging from 0.4% to 2.56% and an average value of 0.79%. Overpressured mudstones are buried at depths above 3300-4300m, and subjected to formation temperatures ranging from 135℃ to 200℃ and the relevant vitrinite reflectance (R o ) of 0.7%-1.57%. Models of maturity and hydrocarbon generation history show that mudstones started to become overpressured at 3.5 Ma and that pore pressure is still increasing. This study provides new insights revealing that hydrocarbon generation, rather than disequilibrium compaction, is the primary mechanism driving the overpressure development despite of the low-organic matter content of the source rocks.