Identification of the vertical sealing of faults by geochemical methods—taking the Zhu-3 sub-basin of the Pearl River Mouth Basin as an example

The vertical sealing of faults affects the migration and accumulation of oil and gas, which can be visually identified by geochemical methods applied in the Zhu-3 sub-basin of the Pearl River Mouth Basin. Faults penetrate different horizons vertically, and the characteristics of source rocks and crude oil are diverse. On this basis, experimental analysis of Rock–Eval, carbon isotope analysis, kerogen macerals, saturated gas chromatography-mass spectrometry, sand inclusion analysis were carried out. The results of the carbon isotope and biomarker characteristics indicate that the crude oil at the top of the Zhuhai Formation comes from the source rock of Member 2 of Wenchang Formation (WC2), and the crude oil at the bottom of the Zhuhai Formation and the Enping Formation comes from the source rock of member 1 of Wenchang Formation (WC1). The homogenization temperature of the inclusions indicates that the crude oil has the characteristics of multi-stage accumulation. Based on the above analysis, it can be found that the faults gradually shifted from open to closed after WC2 source rock matured and expelled hydrocarbons and before the WC1 source rock matured, which finally led to the present vertical difference in the distribution of crude oil.