Methane hydrate investigations on sediment core samples obtained in the Sea of Okhotsk

2003年，由Kitami理工学院领导的CHAOS（鄂霍次克海碳氢水合物聚集）项目的成员在鄂霍次克海库页岛近海进行了高分辨率地震声学剖面的侧扫声呐调查。调查结果以独特的海底/近海天然气渗流结构和垂直流体通道图像揭示了天然气水合物聚集的特征分布。在10 × 20公里的调查范围内发现了40多个渗水结构。所观察到的渗流结构最大直径约为600 m。通过水声剖面探测到渗漏结构释放到上述水体中的甲烷气体，形成耀斑图像。通过气相色谱、液相色谱、氧/氢/碳同位素、13C-NMR、热分析、物理分析等分析技术，从这些渗流结构中获得天然气水合物和沉积物岩心样品，并将其运送到北上工业大学进行实验室研究。在海底以下2-3米深的粉质粘土、海洋沉积物岩心中观察到块状、层状和/或小块的甲烷水合物。天然气水合物样品在岩心回收后立即储存在液氮温度容器中，防止水合物样品解离。所得主要结果总结如下。水合物中的客气主要是甲烷，只有少量的乙烷、丙烷和其他气体。水合物的晶体结构为笼形水合物结构I，水化数接近6，表现出非常高的天然气水合物质量。碳同位素和气体成分测定结果表明，天然气水合物中的甲烷气体是微生物成因的。测定的天然气水合物潜热与人工纯甲烷水合物的潜热相当接近。

Side-scan-sonar survey with high-resolution seismo-acoustic profiling was performed offshore Sakhalin, Okhotsk Sea by the members of CHAOS (hydro-Carbon Hydrate Accumulations in the Sea of Okhotsk) Project lead by Kitami Institute of Technology in 2003. The survey results revealed characteristic distributions of gas hydrate accumulations with unique images of gas seepage structures and vertical fluid channel at/near sea bottom. More than 40 seepage structures were found within a 10 x 20 km survey area. The maximum size of seepage structure observed is about 600 m in diameter. Methane gas released from the seepage structures into the above water was detected as flare images by hydro-acoustic profiling. Gas hydrate and sediment core samples were obtained from those seepage structures and transported to Kitami Institute of Technology for laboratory studies by gas chromatography, liquid chromatography, oxygen/hydrogen/carbon isotopes, 13C-NMR, thermal analysis, physical analysis and other analytical techniques. Methane hydrates of massive forms, layers and/or small pieces were observed in silty-clay, sea sediment cores within a depth of 2-3 meter below the sea floor. Gas hydrates samples were stored in a liquid-nitrogen-temperature container immediately after core recovery, preventing hydrate samples from the dissociation. Main results obtained are summarized as below. Guest gases trapped in hydrates are mainly of methane and only small amount of ethane, propane and others. Crystal structure of hydrates is of clathrate hydrate structure I, and the hydration number is close to six, showing very high quality of gas hydrates. The results of carbon isotope and gas composition measurements show the methane gas in gas hydrates is of microbial origin. Latent heat of gas hydrate measured is found to be quite close to that of artificial, pure methane hydrate.