Genesis, evolution and reservoir identification of a Neogene submarine channel in the southwestern Qiongdongnan Basin, South China Sea

A rarely reported middle−late Miocene−Pliocene channel (incised valley fill), the Huaguang Channel (HGC), has been found in the deep-water area of the southwestern Qiongdongnan Basin (QDNB). This channel is almost perpendicular to the orientation of another well-known, large, and nearly coeval submarine channel in this area. Based on the interpretation of high-resolution 3D seismic data, this study describes and analyzes the stratigraphy, tectonics, sedimentation, morphology, structure and evolution of HGC by means of well-seismic synthetic calibration, one- and two-dimensional forward modeling, attribute interpretation, tectonic interpretation, and gas detection. The HGC is located on the downthrown side of an earlier activated normal fault and grew northwestward along the fault strike. The channel is part of a slope that extends from the western Huaguang Sag to the eastern Beijiao Uplift. The HGC underwent four developmental stages: the (1) incubation (late Sanya Formation, 20.4–15.5 Ma), (2) embryonic (Meishan Formation, 15.5–10.5 Ma), (3) peak (Huangliu Formation, 10.5–5.5 Ma) and (4) decline (Yinggehai Formation, 5.5–1.9 Ma) stages. The channel sandstones have a provenance from the southern Yongle Uplift and filled the channel via multistage vertical amalgamation and lateral migration. The channel extended 42.5 km in an approximately straight pattern in the peak stage. At 10.5 Ma, sea level fell relative to its lowest level, and three oblique progradation turbidite sand bodies filled the channel from south to north. A channel sandstone isopach map demonstrated a narrow distribution in the early stages and a fan-shaped distribution in the late stage. The formation and evolution of the HGC were controlled mainly by background tectonics, fault strike, relative sea level change, and mass supply from the Yongle Uplift. The HGC sandstone reservoir is near the Huaguangjiao Sag, where hydrocarbons were generated. Channel-bounding faults and underlying faults link the source rock with the reservoir. A regionally extensive mudstone caprock overlies the channel sandstone. Two traps likely containing gas were recognized in a structural high upstream of the channel from seismic attenuation anomalies. The HGC will likely become an important oil and gas accumulation setting in the QDNB deep-water area.