OS3A-4:CHARACTERIZATION OF NATURAL GAS HYDRATE RECOVERED FROM BOTH MARINE AND TERRESTRIAL REGIONS IN CHINA: LABORATORY STUDIES

Changling LIU1, Yuguang YE1, Qingguo MENG1, Xingliang HE1, Chengfeng LI1, Guangxue ZHANG 2, Jinqiang LIANG2, Zhenquan LU3, Youhai ZHU3
1. Key Laboratory of Gas Hydrate, Qingdao Institute of Marine Geology, CHINA; 2. Guangzhou Marine Geological Survey, CHINA; 3. Oil and Gas Survey, China Geological Survey,  CHINA

    In recent years, China has carried out several drilling campaigns for gas hydrate both in marine and terrestrial regions with financial support of the national gas hydrate project, and successfully obtained the natural gas hydrate samples in these regions. For example, marine gas hydrate samples were firstly recovered from Shenhu area in 2007 and then from Dongsha area in 2013 in South China Sea. The terrestrial gas hydrate samples were recovered from Qilian Mountain permafrost, northwest China in 2009 and 2013, respectively. In this paper, systematic analyses have been carried out on these gas hydrate samples in laboratory to investigate their structural characteristics and gas composition. The gas hydrates from South China Sea (Shenhu and Dongsha areas) show different kinds of occurrences (i.e. dispersive, veiny, nodular and massive). The results show that the gas hydrates demonstrate a typical structure I (sI), with cage occupancy of methane is more than 99% in large cage, and ~86% in small cage, respectively, corresponding to hydration numbers of approximately 6.0 by instrumentation methods such as Raman spectroscopy and powder X-ray diffraction (RXD). The guest molecules are predominantly methane (> 99%), and isotopic analysis showed that the methane was of biogenic origin. As for the gas hydrate samples from Qilian Mountain (QLM-GH) permafrost, the field observations indicate that the hydrate samples occur as a thin layer within the cracks of fine-grained sandstones, siltstones and mudstones. The results show that the gas hydrate from Qilian mountain permafrost (QLM-GH) is formed by different gases. Methane accounts only for ~ 60% of the guests while the others are higher hydrocarbons (e.g. C2H6, C3H8 and C4H10). The QLM-GH shows a possible structure II (sII) hydrate based on its Raman spectra and gas compositions. The cage occupancy ratio of methane in small and large cage (θS/θL) is around 26, much larger than that of SH-GH (0.87), suggesting larger molecules preferentially occupy the large cage of hydrates. The isotopic analysis shows that guest gases are most likely of thermogenic origin. Additional work is still needed to address the presence of minor gas components and the heterogeneity of natural hydrate samples.