Characteristics of hydrate-bound gas retrieved at the Kedr mud volcano (southern Lake Baikal)

Abstract

We reported the characteristics of hydrate-bound hydrocarbons in lake-bottom sediments at the Kedr mud volcano in Lake Baikal. Twenty hydrate-bearing sediment cores were retrieved, and methane-stable isotopes of hydrate-bound gases (δ13C and δ2H of − 47.8‰ to − 44.0‰ V-PDB and − 280.5‰ to − 272.8‰ V-SMOW, respectively) indicated their thermogenic origin accompanied with secondary microbial methane. Powder X-ray diffraction patterns of the crystals and molecular composition of the hydrate-bound gases suggested that structure II crystals showed a high concentration of ethane (around 14% of hydrate-bound hydrocarbons), whereas structure I crystals showed a relatively low concentration of ethane (2–5% of hydrate-bound hydrocarbons). These different crystallographic structures comprised complicated layers in the sub-lacustrine sediment, suggesting that the gas hydrates partly dissociate, concentrate ethane and form structure II crystals. We concluded that a high concentration of thermogenic ethane primarily controls the crystallographic structure of gas hydrates and that propane, iso-butane (2-methylpropane) and neopentane (2,2-dimethylpropane) are encaged into crystals in the re-crystallisation process.

Carbon Isotope Fractionation during the Formation of CO2 Hydrate and Equilibrium Pressures of 12CO2 and 13CO2 Hydrates

“…Since CO 2 comprises carbon and oxygen atoms, several isotopic species of CO 2 exist, depending on the combination of stable isotopes (isotopologues: 12 C, 13 C, 16 O, 17 O, and 18 O). For example, the abundance ratio of 13 CO 2 is~1.1% of the total CO 2 and the rest is almost 12 CO 2 .…”

“…For example, the trend of hydrogen and carbon isotope fractionation during the formation of synthetic methane and ethane hydrates was Molecules 2021, 26, 4215 2 of 8 investigated [13]. Moreover, the results were applied to estimate the formation process of natural gas hydrates [14][15][16]. Carbon isotope fractionation during the formation of CO 2 hydrates was reported [17], revealing that the CO 2 δ 13 C in the hydrate phase was 0.9‰ lower than that in the gas phase at 268 K [17].…”

Phase Equilibrium of Methane Hydrate Encapsulated Isotopologue 13CH4 and Singly Deuterated Methane

“…By contrast, little-to-no carbon isotope fractionation (the difference in δ 13 C between encapsulated and residual CH 4 ) was observed. − Isotopic fractionation for ethane hydrate and CO 2 hydrate , was also investigated. These results for synthetic CH 4 hydrates were used as a reference for estimating the formation process of a natural gas hydrate by comparing the isotopic data for hydrate-bound and dissolved gas in the lake/sea bottom sediment. − …”

Molecular Indicators of Sources and Biodegradation of Organic Matter in Sediments of Fluid Discharge Zones of Lake Baikal

“…Gas hydrates and authigenic carbonates were detected in sediments of the MV Kedr. It was also shown that the area around the MV has moundy bottom relief with pockmarks, which testify to the gas fluid discharge from greater depths [52]. Moreover, the coal-bearing beds of the Tankhoy Formation of the Oligocene-Miocene age in this part of the lake may serve as a source of gas for hydrates formation.…”

“…The study of the southern basin included the sampling of sediments from the mud volcano Malenky (site 13, Figure 1, Table 1; [6,7,13,28,[48][49][50]) and gas hydrate-bearing sediments from the mud volcano Kedr (site 14, 15, Figure 1, Table 1; [49,51,52]). The former study of gas fluid from the MV Malenky [6] has suggested its possible migration from the deep strata.…”