Geophysical characteristics and aggregative accumulation modes of gas hydrate in Qiongdongnan sea area of the northern South China Sea

Gas hydrate in the Qiongdongnan sea area has garnered significant scientific interest. However, a comprehensive understanding of its geophysical characteristics and accumulation mechanisms is not deep enough. This study integrates a multi-disciplinary dataset, including 3D seismic data, multi-beam bathymetry and sediment core, to elucidate the fundamental characteristics of gas hydrate in this region. Seismic reflection characteristics, amplitude variations, frequency attributes, velocity anomalies, and preand post-stack attributes are examined to delineate gas hydrate distribution characteristics and aggregative accumulation modes. Some differences in gas hydrate seismic information are observed at W08 and W09 stations. The fluid migration channel beneath station W08 exhibits a strong amplitude string of beads analogous to the cold seepage seismic reflections observed in the Songnan low convex region of the Qiongdongnan sea area. Conversely, the fluid migration channel beneath station W09 displays reverse polarity and strong amplitude, reminiscent of the Haima cold seepage documented in the Lingnan low convex region. Sediment cores retrieved from both stations provide evidence of gas hydrate leakage, including the presence of mussels. Subsequent drilling results confirmed that both stations characterized by quaternary shallow surface sediments unformed rock muddy silt, or sandy sediment were rich in gas hydrate. These findings underscore the significance of cold seepage leakage systems and pathways as important indicators of gas hydrate reservoir presence. Based on the integrated drilling results and geophysical exploration data analysis, we proposed a conceptual model for gas hydrate aggregative accumulation in the study area. Gas hydrate is transported upward to the high-pressure, low-temperature stability zone through gas chimneys and fault conduits. Concurrently, fluid leakage occurs at the shallow seabed surface through these established transport and aggregation channels, contributing to the formation and evolution of the gas hydrate reservoir. This model emphasizes the critical role of gas supply, especially the shallow surface transport system and its enrichment processes, in controlling gas hydrate reservoir development. This study significantly advances our understanding of the geophysical characteristics and aggregative accumulation modes of gas hydrate in the Qiongdongnan sea area. The insights gained provide valuable guidance for future exploration and development strategies targeting gas hydrate resources in the northern South China Sea. © 2024 Geological Society of China. All rights reserved.