Chemical analyses of gas hydrate-bearing marine sediments to study natural methane hydrate formation

由北见技术研究所（日本）、全俄研究所组织的CHAOS（鄂霍次克海碳氢化合物的累积）合作项目。海洋地质和矿物资源研究所（俄罗斯）、韩国极地研究所（韩国）、V.I. I1 'ichev太平洋海洋研究所FEB RAS（俄罗斯）、湖沼研究所SB RAS（俄罗斯）和根特大学雷纳德海洋地质中心（比利时）于2005年和2006年进行了鄂霍次克海天然气水合物研究。在俄罗斯库页岛近海取芯，利用重力和水力取芯器采集鄂霍次克海海底含天然气水合物、无天然气水合物和参比沉积物，利用KIT设计和建造的挤压器采集孔隙水样品。采用离子色谱法、电感耦合等离子体原子发射光谱法和火焰原子吸收光谱法对孔隙水中的硫酸根、钠等离子进行了测定。用质谱仪测定了含甲烷水合物水的水样的6180和6D等稳定同位素比值，根据含水合物岩心的离子分析，未观察到深部水的排出。水合物水的b180值比研究区海底5 m处海水的b180值重1.4%。这些结果表明，天然气水合物形成的主要流体有两种不同的可能性，即，(i)不仅游离气体被排出，而且具有光6180的水也被排出，并且它是主要的气体水合物形成流体，以及（i i）海水是主要的气体水合物形成流体，因为在气体渗流结构处只有游离气体被排出而没有任何上升流体。

The CHAOS (hydro-Carbon Hydrate Accumulations in the Okhotsk Sea) cooperative project organized by Kitami Institute of Technology (KIT, Japan), All Russia Research Institute. for Geology and Mineral Resources of the Ocean (Russia), Korea Polar Research Institute (Korea), V. I. I1'ichev Pacific Oceanological Institute FEB RAS (Russia), Limnological Institute SB RAS (Russia), and Renard Centre of Marine Geology, Ghent University (Belgium) with the aim of the natural gas hydrate studies at the Okhotsk Sea was conducted in 2005 and 2006. The core sampling areas were located offshore Sakhalin Island, Russia.The gas hydrate-bearing, gas hydrate-free and reference marine sediments were retrieved from the bottom of the Okhotsk Sea by using gravity-and hydro-corers.The pore water samples were obtained by a squeezer designed and constructed at KIT. Determination of ions such as sulfate, sodium etc. in the pore water was carried out by ion chromatography, inductively coupled plasma atomic emission spectrometry and flame atomic absorption spectrometry. Stable isotope ratios such as 6180 and 6D of the water samples including dissociated methane hydrate water were measured by a mass spectrometer.Discharging of deeper sourced water was not observed based on ion analyses at the gas hydrate-bearing cores. The b180 value of the hydrate water was up to +1.4% heavier than that of the seawater obtained at 5 m above the sea floor of the studied area. These results suggested the two distinct possibilities for the main gas hydrate-forming fluids, i.e., (i) not only free gas was expulsed but also the water with light 6180 was discharged and it was the main gas hydrate-forming fluid and (ii) seawater was the main gas hydrate-forming fluid since only free gas was expulsed without any ascending fluid at the gas seepage structures.