Characteristics and varieties of gases enclathrated in natural gas hydrates retrieved at Lake Baikal

Molecular and stable isotope compositions of hydrate-bound gases collected from 59 hydrate-bearing sites between 2005 to 2019 in the southern and central sub-basins of Lake Baikal are reported. The delta H-2 of the hydrate-bound methane is distributed between - 310 parts per thousand and - 270 parts per thousand, approximately 120 parts per thousand lower than its value in the marine environment, due to the difference in delta H-2 between the lake water and seawater. Hydrate-bound gases originate from microbial (primary and secondary), thermogenic, and mixed gas sources. Gas hydrates with microbial ethane (delta C-13: - 60 parts per thousand, delta H-2: between - 310 parts per thousand and - 250 parts per thousand) were retrieved at approximately one-third of the total sites, and their stable isotope compositions were lower than those of thermogenic ethane (delta C-13: - 25 parts per thousand, delta H-2: - 210 parts per thousand). The low delta H-2 of ethane, which has rarely been reported, suggests for the first time that lake water with low hydrogen isotope ratios affects the formation process of microbial ethane as well as methane. Structure II hydrates containing enclathrated methane and ethane were collected from eight sites. In thermogenic gas, hydrocarbons heavier than ethane are biodegraded, resulting in a unique system of mixed methane-ethane gases. The decomposition and recrystallization of the hydrates that enclathrate methane and ethane resulted in the formation of structure II hydrates due to the enrichment of ethane.