Characteristics of gas-bearing fluid migration and accumulation system and their control on gas hydrate accumulation in the Jianfengbei Basin of South China Sea
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摘要:
南海北部陆坡尖峰北盆地发育良好的气源及含气流体运聚疏导条件，具备较好的天然气水合物成藏潜力。为深入揭示尖峰北盆地水合物的成藏地质特征，基于高精度三维多道地震、浅地层剖面、多波束资料，深入分析了研究区深、浅部含气流体运聚疏导通道的地质、地球物理特征及对水合物成藏的控制作用。详细刻画了研究区深、浅部主要含气流体疏导通道的形态特征、发育规模、分布特点及对含气流体运聚的控制作用；重点剖析了深、浅部含气流体疏导通道组合特征及与水合物分布的耦合关系，最后结合水合物成藏地质条件，探讨了研究区水合物的成藏模式及影响因素。研究结果表明：尖峰北盆地的含气流体疏导通道主要以断裂型为主，亮点反射、反射空白带、BSR、声空白、声浑浊等含气流体运聚及水合物赋存指示标志多出现在沟源断层、古隆起伴生断层、多边形断层的顶部及邻近区域。以T3反射界面为界，其下伏沟源断层、古隆起伴生断层与上覆多边形断层构成的深、浅部含气流体疏导通道在垂向上相连通，沟通了深部气源层与浅层水合物稳定域，形成了“沟源断层—多边形断层”与“古隆起伴生断层—多边形断层”两种含气流体运移与水合物成藏模式。多边形断层的存在一方面促进了含气流体向浅层发生“中继疏导”，控制水合物富集成藏；另一方面，在多边形断层密集发育段，强烈的流体充注会引起局部温压平衡破坏和水合物分解、渗漏，导致“断续型”BSR的产生。浅层气体的渗漏和扩散可以持续作用至海底并对海底形态进行改造，导致海底滑塌、断裂、麻坑、丘状体等一系列海底微地貌的形成。
Abstract: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.
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Key words:
- polygonal fault /
- gas-bearing fluid /
- migration pathway /
- gas hydrate /
- Jianfengbei Basin /
- northern South China Sea
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