Hydrocarbon gas seepage along the Gydratny Fault (Lake Baikal)

The Gydratny Fault, extending in SW-NE over 60 km, is situated in the central basin of Lake Baikal. This fault governs a distribution of hydrocarbon gas discharge zones on the lake bottom, many of them are hydrate-bearing. During the Class@Baikal-2019 expedition gas samples were obtained from bottom sediments along the fault zone, molecular and isotopic properties of the gases were studied. It is concluded that the fault zone is permeable for hydrocarbons and serves as an important fluid flow conduit from deep sources to the surface. The most intensive gas seepage was determined along the central and north-eastern segments of the fault zone where it is associated with the MSU hydrate-bearing structure and the Novosibirsk and Ukhan mud volcanoes. Within these structures the highest concentrations of methane and C2+ components were detected, the heaviest isotopic composition of methane carbon (–57‰ VPDB) were identified and near-surface gas hydrate accumulations were observed. The south-western segment of the fault is characterized by the lowest concentrations of gases in sediments with light carbon isotopic composition of methane (–76‰ VPDB).A dominant gas migration mechanism (advection or diffusion) in the upper part of the basin sedimentary section conditions the registered concentrations of methane and its homologues in sediments and their carbon isotopic composition. Diffusion and dispersion of migrated thermogenic gases and significant portion of microbial methane in seeping gases are characteristic for the areas where the fault is buried under relatively thick modern sediments. Focused gas migration followed by its focused discharge are specific for the areas where the fault propagates to the lake floor and is well-expressed in a bottom relief. Two models of hydrocarbon gases migration to the surface explaining the observed gas-geochemical characteristics of bottom sediments are described and discussed. Obtained results can help in enhancing interpretation of data of gas geochemical prospecting for offshore oil and gas.