Isotopic fractionation of methane and ethane hydrates between gas and hydrate phases

Abstract

Isotopic fractionation of carbon and hydrogen in methane and ethane during the formation of gas hydrates was investigated. The gas hydrate samples were experimentally prepared in a pressure cell and isotopic compositions of both residual and hydrate‐bound gases were measured. δD of hydrate‐bound molecules of methane and ethane hydrates was several per mil lower than that of residual gas molecules in the formation processes, while there was no difference in the case of δ13C. These isotopic differences in δD are enough small for discussing the source types of hydrate‐bound gases using the δ13C‐δD diagram of Whiticar et al. [1986]. These results may provide useful insight into the formation process of gas hydrates.

Natural gas hydrates with locally different cage occupancies and hydration numbers in Lake Baikal

“…The confidence limits of the measurements of hydrocarbon composition are 0.005% for C 1 –C 3 hydrocarbons and 0.0005% for C 4 –C 9 hydrocarbons, which were determined by duplicate analyses of standards. The analytical precision of δ 13 C is 0.1‰ and that of δ D is 0.6‰ [ Hachikubo et al , 2007].…”

Hydrocarbon seepage and its sources at mud volcanoes of the Kumano forearc basin, Nankai Trough subduction zone

“…Although molecular and isotopic compositions of gases might be altered during migration through the sediment and incorporation into gas hydrates, hydrates provide information on sources of hydrocarbons formed at greater depth [e.g., Hachikubo et al, 2007;Pape et al, 2010a;Sassen et al, 1999]. Relatively low C 1 /C 2 ratios of hydrate-bound hydrocarbons ranging between 35 and 254 and d 13 C-CH 4 -values >240& V-PDB (Table 1) substantiate that light hydrocarbons expelled from MVs 2, 4, and 10 are predominantly generated by thermal cracking of type II kerogen, which typically originates from marine organic matter ( Figure 4).…”

“…Despite biotic and abiotic molecular and isotopic modifications known to occur during fluid migration and hydrate formation [e.g., Hachikubo et al, 2007;James and Burns, 1984;Pape et al, 2010a;Sassen et al, 1999], the chemical characteristics of those hydrates bear information on the geochemistry of the hydrocarbon source. For instance, a predominance of thermogenic hydrocarbons was reported from MVs in the Nile deep sea fan [Mastalerz et al, 2007], in the Anaximander Mountains [Pape et al, 2010b], or in the Gulf of Cadiz [Hensen et al, 2007].…”

Characterizing lipid biomarkers in methanotrophic communities of gas hydrate-bearing sediments in the Sea of Okhotsk

“…Hachikubo, personal communication). It is possible to infer that the d 13 C of the interstitial methane is about À64&, because there is no kinetic carbon isotopic fractionation during the formation and dissociation of gas hydrates (Hachikubo et al, 2007). The stable carbon isotope fractionations of the archaeol relative to the source methane in the LV39-40H cores, which were calculated with the presumed d 13 C values of the interstitial methane, were different in the above layer (Δd 13 C ¼ from À58& to À37&) than in the hydrate layers (Δd 13 C ¼ from À29& to 16&).…”