Deformation Mechanism and Its Depression Controlling-Source Controlling Effect of X Sag Fault System in Pearl River Mouth Basin

To clarify the migration characteristics of the X Sag trough in the Zhuyi depression of the Pearl River Mouth Basin and understand the distribution patterns of hydrocarbon source rocks, this study employs newly processed high-resolution 3D seismic data and integrates techniques such as fault activity period determination, fault displacement-distance curve analysis, and balanced section methods. It systematically investigates the deformation mechanism of the X Sag Fault in the Zhuyi Depression and its control over hydrocarbon source rock distribution. The study reveals: ① The X Sag is a north-dip, south-lobe scoop-shaped half-graben controlled by the NE-NEE-NWW multi-trend arc-shaped F1 fault, with three sets of secondary faults (NE, NEE, EW) developing within the depression. Based on the segmentation characteristics of the main boundary fault F1 and its combination patterns with secondary faults, the study area is divided into eastern and western sub-basins. ② The X Sag underwent multiple phases of tectonic evolution under the influence of multi-phase, multi-directional stress fields, primarily comprising four stages: Fracture Stage I, Fracture Stage II, Fracture Stage III, and the Tilting Stage. Based on the long-term activity characteristics of the main boundary fault F1 and the activity features of different phases of the secondary faults within the sag, five sets of fault systems were delineated: faults active only during the Early Wenchang Period, faults active during the Early Wenchang-Enping Period, faults active during the Late Wenchang-Enping Period, faults active only during the Enping Period, and long-term active faults. ③ The secondary faults within the X Sag are collectively controlled by a pre-existing arc-shaped NE-NEE-NWW-trending reverse-transform fault system. During basin formation, the western sub-sag underwent extensional deformation along pre-existing NE-NEE-trending faults, forming a series of secondary faults aligned with the main boundary fault strike. These appear in cross-section as structures reverse-cutting the main boundary fault. Conversely, the eastern sub-sag underwent extensional-torsional deformation along pre-existing NWW-trending strike-slip faults, generating a series of near-EW-trending secondary faults. During deformation, a “V”-shaped structural pattern formed in the profile. ④ The segmented growth and differential activity characteristics of different control-depression faults within the basin governed the migration of the depression trough sedimentary center from northwest to southeast and from the basin margin toward the basin interior, thereby influencing the distribution of hydrocarbon source rocks during the Early and Late Wenchang periods.