Characteristics of gas-bearing fluid migration and accumulation system and their control on gas hydrate accumulation in the Jianfengbei Basin of South China Sea

The Jianfengbei Basin, located on the northern slope of the South China Sea, has high potential of gas hydrate accumulation. There are abundant gas sources and good conditions for migration and accumulation of gas-bearing fluids. In order to reveal the geological characteristics of gas hydrate accumulations in the basin, various types of data, such as high-precision 3D multi-channel seismic data, sub-bottom profiles and multi-beam data are collected and analyzed, and the geological and geophysical characteristics of the deep and shallow pathways for gas-bearing fluid migration and accumulation as well as their controlling effects on gas hydrate accumulation are carefully studied. Morphology, size and distribution patterns of the main gas-bearing fluid migration pathways in both the deep and shallow parts and their controlling factors are described in details. The combination characteristics of the deep and shallow gas-bearing fluid migration pathways and their coupling relationship with gas hydrate distribution are analyzed. Finally, in consideration of the geological conditions for the formation of gas hydrate, accumulation models and influencing factors are discussed. The study shows that the pathways for gas-bearing fluid migration in the Jianfengbei Basin are dominated by faults. All the features, such as bright-spot reflection, reflective blank zone, BSR, acoustic blank, acoustic turbidity and other geophysical characteristics, suggest that gas-bearing fluid migration and gas hydrate mostly occur on the top and/or in the places near the source connected faults, paleo-uplifts associated faults or polygonal faults. Bounded by T3 reflection interface, the gas-bearing fluid migration pathways composed of the faults mentioned above linked vertically the deep gas source layer to the gas hydrate stability zone in the shallow part. Based upon the above, two gas-bearing fluid migration and hydrate accumulation models, “source -connected fault- polygonal fault” and “paleo-uplift associated faults -polygonal fault” are proposed in this paper. The existence of polygonal faults has two functions, On one hand, it promotes gas-bearing fluids “inherited migration” into the shallow layers and controls the accumulation of gas hydrates; on the other hand, in the layer where polygonal faults densely developed, strongly fluid charging may cause the destruction of local temperature and pressure balance which leads to gas hydrate decomposition and leakage, and "intermittent" BSR. The continuing effect of shallow gas leakage will modify the seabed morphology, and lead to the formation of submarine micro-geomorphology, such as seabed slumping, seabed faults, pockmarks and mound-like features.