Occurrence and accumulation characteristics of natural gas hydrate in the eastern Nankai Trough, Japan
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摘要: 根据日本南海海槽东部天然气水合物钻探与综合大洋钻探计划研究成果,分析总结了该区天然气水合物形成的地质背景与赋存特征。南海海槽区作为构造活动强烈的会聚式大陆边缘,大型增生楔与断裂系统十分发育,富含重力流沉积物。特别是该海槽北部向陆斜坡水深2 000 m以浅海域,发育一系列弧前盆地,盆地内第四系未固结沉积物广泛分布,沉积速率较高,为水合物的形成提供了良好的环境。水合物钻探取心分析揭示,南海海槽东部天然气水合物为Ⅰ型结构,水合物分解气中甲烷组分占比普遍高达99.9%以上,甲烷碳同位素分析显示为典型的生物成因气。水合物主要以孔隙填充型产出,优先选择在富砂层中富集,富砂层段水合物饱和度一般为50%~60%,可高达80%~90%,泥质层中水合物饱和度极低。Abstract: The geological setting and occurrence characteristics of natural gas hydrate in the eastern Nankai Trough offshore Japan have been analyzed and summarized based on the research of gas hydrate drilling expeditions and the Integrated Ocean Drilling Program conducted in this area. As part of the convergent continental margin with strong tectonic activity, the Nankai Trough has well-developed fault and fracture systems, and is rich in gravity flow sediments. Especially in the continental slope of the northern trough with water depth shallower than 2 000 m, a series of fore-arc basins exist, filled with widely distributed unconsolidated Quaternary sediments. The deposition rate of these young sediments is rather high, providing favorable conditions for the formation of gas hydrate. Analyses of the drilling cores taken from this area indicate that the gas hydrate is type I, comprising 99.9% or more methane. The carbon isotopic analysis shows that methane forming the gas hydrate is typically microbial. The gas hydrate occurs commonly in a dispersive state, filling in the pores of the sediment, preferentially accumulating in the sand-rich sediments. The hydrate saturation in the sand-rich intervals is commonly 50%-60% and can reach as high as 80%-90%; whereas the hydrate saturation in the shale layers is extremely low.
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Key words:
- gas hydrate /
- developing settings /
- occurrence characteristics /
- accumulation zone /
- Nankai Trough /
- Japan
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