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Marine-controlled source electromagnetic study of methane seeps and gas hydrates at Opouawe Bank, Hikurangi Margin, New Zealand

机译:新西兰Hikurangi Margin的Opouawe Bank对甲烷渗透和天然气水合物进行海洋控制的电磁源研究

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摘要

Marine controlled source electromagnetic (CSEM) data have been collected to investigate methane seep sites and associated gas hydrate deposits at Opouawe Bank on the southern tip of the Hikurangi Margin, New Zealand. The bank is located in about 1000 m water depth within the gas hydrate stability field. The seep sites are characterized by active venting and typical methane seep fauna accompanied with patchy carbonate outcrops at the seafloor. Below the seeps, gas migration pathways reach from below the bottom-simulating reflector (at around 380 m sediment depth) toward the seafloor, indicating free gas transport into the shallow hydrate stability field. The CSEM data have been acquired with a seafloor-towed, electric multi-dipole system measuring the inline component of the electric field. CSEM data from three profiles have been analyzed by using 1-D and 2-D inversion techniques. High-resolution 2-D and 3-D multichannel seismic data have been collected in the same area. The electrical resistivity models show several zones of highly anomalous resistivities (>50 Ωm) which correlate with high amplitude reflections located on top of narrow vertical gas conduits, indicating the coexistence of free gas and gas hydrates within the hydrate stability zone. Away from the seeps the CSEM models show normal background resistivities between ~1 and 2 Ωm. Archie's law has been applied to estimate gas/gas hydrate saturations below the seeps. At intermediate depths between 50 and 200 m below seafloor, saturations are between 40 and 80% and gas hydrate may be the dominating pore filling constituent. At shallow depths from 10 m to the seafloor, free gas dominates as seismic data and gas plumes suggest.
机译:收集了海洋控制源电磁(CSEM)数据,以调查新西兰Hikurangi边缘南端的Opouawe Bank的甲烷渗漏点和相关的天然气水合物沉积物。堤岸位于天然气水合物稳定区内约1000 m的水深处。渗漏点的特征是活跃的通风和典型的甲烷渗漏动物区系,在海底还伴有片状碳酸盐露头。在渗漏下方,气体迁移路径从模拟海底反射层的下方(沉积深度约380 m)到达海底,表明自由气体向浅水合物稳定区域输送。 CSEM数据是通过海底拖曳电多偶极子系统获取的,该系统测量电场的线内分量。通过使用一维和二维反演技术分析了来自三个剖面的CSEM数据。在同一地区已收集了高分辨率的2-D和3-D多通道地震数据。电阻率模型显示出几个高异常电阻率(> 50Ωm)区域,这些区域与位于狭窄垂直气体导管顶部的高振幅反射相关,表明水合物稳定区内自由气体和气体水合物共存。 CSEM模型远离渗漏,显示出正常的背景电阻率在1-2Ωm之间。已应用阿奇定律估算低于渗流的天然气/天然气水合物饱和度。在海底以下50至200 m之间的中间深度处,饱和度在40至80%之间,而天然气水合物可能是主要的孔隙填充成分。从地震数据和气柱表明,在距海底10 m的浅层深度中,自由气体占主导地位。

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