Comparison of the characteristics for natural gas hydrate recovered from marine and terrestrial areas in China

摘要

China has carried out several drilling campaigns for natural gas hydrate both in marine and terrestrial areas and successfully obtained the samples. The marine gas hydrate samples were firstly recovered from Shenhu area in 2007 and then from the Pearl River Mouth basin in 2013 in South China Sea (SCS). The terrestrial gas hydrate samples were recovered from Qilian Mountain permafrost (QMP) in 2009 and 2013, respectively. In this paper, systematic analyses have been carried out on these gas hydrate samples to compare the characteristics of gas hydrates from SCS with those from QMP. The results indicate that the characteristics of occurrence, structure and gas composition are obviously different. Marine gas hydrate from SCS shows different kinds of occurrence and demonstrates a typical structure I (sI), with cage occupancy of more than 99% methane in large cage and 90% in small cage, respectively, corresponding to hydration numbers of approximately 6.0 by thermodynamic calculation. The guest molecules are predominantly methane (>99%) from biogenic origin produced by CO2 reduction. However, the terrestrial gas hydrates from QMP occur as a thin layer within the cracks of fine-grained sandstones, siltstones and mudstones, showing a possible structure II (sII) hydrate based on its Raman spectra and gas composition. The molecular composition of hydrate-bound gas indicates that CH4 only accounts for similar to 60% of the guests while the others are heavier hydrocarbons (e.g. C2H6, C3H8 and C4H10).The cage occupancy ratio of methane in small and large cage (theta(S)/theta(L)) is around 7.5, suggesting that larger molecules preferentially occupy the large cage of the hydrate. The isotopic analysis shows that hydrate-bound gases in QMP are from thermogenic origin. (C) 2015 Elsevier B.V. All rights reserved.