Sequence stratigraphy and sedimentary response of the First Member of Yinggehai Formation in Ledong Area, Yinggehai Basin

The Ledong area in the Yinggehai Basin is a typical region with high subsidence and intensive filling in the northern South China Sea. Since the Pliocene, very thick clastic deposits formed in this area, characterized by strong intrabed heterogeneity and frequent transitions of sedimentary systems, which brings great challenges to fine reservoir prediction. Based on sequence stratigraphy and seismic sedimentology, by integrating well logs, mud logs, core samples, and 3D seismic data, a high-resolution high-frequency sequence framework was established and the sedimentary filling architectures and their genetic dynamic mechanisms were explored. Results show that: (1) The First Member of the Yinggehai Formation in the Ledong area comprises two long-term cycles (LSC1 and LSC2), six medium-term base-level cycles (MSC1-MSC6), and nine high-frequency short-term base-level cycles (SSC1-SSC9). A typical forced regressive systems tract (FRST) was identified within LSC2. (2) During the deposition of the 1stMYF, the accommodation-to-sediment supply ratio (A/S) was generally less than 1, indicating a continuous state of sedimentary overcompensation. The lowest A/S ratio and peak sediment supply co-occurred during the period represented by the T₂₃–T₂₀ interfaces, which led to low-angle, long-distance accretional growth of the shelf margin by 40~50 km, formed a forced regressive sedimentary succession, and effectively controlled sediment dispersal and the distribution of reservoir bodies. (3) Against the background of decelerating tectonic subsidence, the synergistic control of subtle paleo-uplifts formed by diapirism and the migration trajectory of the shelf margin governed the spatial configuration of a composite system comprising beach-bar, slope fan, and gravity flow channel complexes. In addition, the beach-bar sandbodies in the shelf-margin littoral zone and the slope fan complexes were identified as the most important targets for future oil and gas exploration in this area.