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Interpretation of high-resolution two-dimensional (2D) and three-dimensional (3D) seismic data collected in the Qiongdongnan Basin, South China Sea reveals the presence of polygonal faults, pockmarks, gas chimneys and slope failure in strata of Pliocene and younger age. The gas chimneys are characterized by low-amplitude reflections, acoustic turbidity and low P-wave velocity indicating fluid expulsion pathways. Coherence time slices show that the polygonal faults are restricted to sediments with moderate-amplitude, continuous reflections. Gas hydrates are identified in seismic data by the presence of bottom simulating reflectors (BSRs), which have high amplitude, reverse polarity and are subparallel to seafloor. Mud diapirism and mounded structures have variable geometry and a great diversity regarding the origin of the fluid and the parent beds. The gas chimneys, mud diapirism, polygonal faults and a seismic facies-change facilitate the upward migration of thermogenic fluids from underlying sediments. Fluids can be temporarily trapped below the gas hydrate stability zone, but fluid advection may cause gas hydrate dissociation and affect the thickness of gas hydrate zone. The fluid accumulation leads to the generation of excess pore fluids that release along faults, forming pockmarks and mud volcanoes on the seafloor. These features are indicators of fluid flow in a tectonically-quiescent sequence, Qiongdongnan Basin.

In this article, we document a large number of focused fluid escape structures using high quality 2D seismic reflection data and multibeam bathymetry data from a poorly known area at the intersection of the northern South China Sea (SCS) and the western SCS. Three types of focused fluid escape systems are identified and described: mud volcanoes, pipes and associated pockmarks. The mud volcanoes occur singly or as clustered groups. The overpressure driving the mud volcanism is argued herein to be related to the generation of thermogenic hydrocarbons. The clustered distributions are related to localized tectonic uplift in the basin. Pipes mainly occur within the Guangle Uplift or accompany the mud volcano clusters. The pipes located within the Guangle Uplift are attributed to carbonate dissolution caused by hydrothermal fluids. Fluids ascended through these structures and were expelled at the palaeo-seabed or present seabed forming palaeo-pockmarks and present-day pockmarks. Some 'mega-pockmarks' show evidence of enlargement due to bottom currents. The marginal basins of the SCS are petroliferous, with attention gradually shifting to the deep-water area. Our results show that fluid migration must be taken into account when assessing seabed stability. This analysis also improves our understanding the petroleum geology in the study area, and is also useful for predicating where chemosynthetic ecosystems may be located.

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