Gas-Hydrate Horizons Detected in Seismic-Profiler Data from the Western North Atlantic
Gas-Hydrate Horizons Detected in Seismic-Profiler Data from the Western North Atlantic
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DOI：
10.1306/c1ea3dc5-16c9-11d7-8645000102c1865d
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发表时间：
1977-05
期刊：
影响因子：
3.5
通讯作者：
B. Tucholke;G. Bryan;J. Ewing
中科院分区：
文献类型：
--
作者：
B. Tucholke;G. Bryan;J. Ewing
据报告，在美国东海岸外的两个地点，一个是沿着布莱克外脊脊的顶部，另一个是在新泽西和特拉华州外的上大陆隆起之下，存在振幅极高、与海底相一致的反射层。对这些层位的详细测绘表明：（1）这些层位穿过沉积物中的层面;（2）这些层位的浅海底深度随着海底深度的增加而增加，从而随着海底（底层水）温度的降低而增加;（3）这些层位仅限于沉积物地层向陆倾斜的地区;这种异常层位在正常的向海倾斜的大陆隆起地层中并不常见。在布莱克外脊（102、103、104号地点）异常层位或其附近进行的深海钻探发现了富含甲烷的沉积物。在这两个地区的异常层位的沉积柱内的压力/温度条件是适当的气体水合物的形成到几百米的深度，从而表明，一个区域的气体水合物覆盖的异常层位。沉积柱中甲烷/海水系统的水合物/气相转化图，使用地质学上合理的海底温度和沉积物中的热梯度和声速值，显示相变深度与异常反射层的最小深度之间有良好的相关性。因此，这些层位被认为是由沉积物中天然气水合物向下转变为天然气所引起的增强对比。在异常层位区，倾向陆地的地层与天然气水合物层形成游离气的圈闭，天然气水合物层阻挡天然气向海运移，倾向地层限制天然气向陆地运移。
Reflecting horizons which have anomalously high amplitude and which are conformable to the seafloor at about 500 to 600 m subbottom have been reported in two locations off the United States east coast--one along the crest of the Blake Outer Ridge, and another beneath the upper continental rise off New Jersey and Delaware. Detailed mapping of these horizons shows that: (1) the horizons cut across bedding planes in the sediment; (2) subbottom depth of the horizons increases with increasing seafloor depth and thus with decreasing seafloor (bottom water) temperature; and (3) the horizons are restricted to areas where sediment strata dip landward; such anomalous horizons are uncommon within the normal seaward-dipping continental-rise strata. Deep-sea drilling into or close to the anomalous horizon on the Blake Outer Ridge (Sites 102, 103, 104) recovered methane-rich sediment. Pressure/temperature conditions within the sediment column in both areas of anomalous horizons are appropriate for formation of gas hydrates to several hundred meters depth, thus suggesting that a zone of gas-hydrates overlies the anomalous horizons. A plot of the hydrate/gas phase transformation of the methane/seawater system in the sedimentary column, using geologically reasonable values for seafloor temperature and for thermal gradient and sound-velocity in the sediment, shows a good correlation between the depth of the phase change and the minimum depth of the anomalous reflecting horizons. The horizons therefore are thought to represent an imped nce contrast caused by the downward change from gas hydrate to gas in the sediment. Landward-dipping strata and the gas-hydrate layers in the areas exhibiting anomalous horizons appear to form traps for free gas, whereby the gas-hydrate layer blocks seaward gas migration and the dipping strata restrict landward migration.