Isotopic composition of gas hydrates in subsurface sediments from offshore Sakhalin Island, Sea of Okhotsk

摘要

Hydrate-bearing sediment cores were retrieved from recently discovered seepage sites located offshore Sakhalin Island in the Sea of Okhotsk. We obtained samples of natural gas hydrates and dissolved gas in pore water using a headspace gas method for determining their molecular and isotopic compositions. Molecular composition ratios C-1/(C2+) from all the seepage sites were in the range of 1,500-50,000, while delta C-13 and delta D values of methane ranged from -66.0 to -63.2aEuro degrees VPDB and -204.6 to -196.7aEuro degrees VSMOW, respectively. These results indicate that the methane was produced by microbial reduction of CO2. delta C-13 values of ethane and propane (i.e., -40.8 to -27.4aEuro degrees VPDB and -41.3 to -30.6aEuro degrees VPDB, respectively) showed that small amounts of thermogenic gas were mixed with microbial methane. We also analyzed the isotopic difference between hydrate-bound and dissolved gases, and discovered that the magnitude by which the delta D hydrate gas was smaller than that of dissolved gas was in the range 4.3-16.6aEuro degrees, while there were no differences in delta C-13 values. Based on isotopic fractionation of guest gas during the formation of gas hydrate, we conclude that the current gas in the pore water is the source of the gas hydrate at the VNIIOkeangeologia and Giselle Flare sites, but not the source of the gas hydrate at the Hieroglyph and KOPRI sites.