Hydrocarbon Gas Seepage along the Gydratny Fault (Lake Baikal)

摘要

The Gydratny Fault extends in the SW-NE direction for over 60 km in the central basin of Lake Baikal. During the Class@Baikal-2019 expedition we conducted a multidisciplinary study coupling seismic and gas sampling obtained from bottom sediment profiles intersecting the fault zone. Seismic profiles revealed that the central and northeastern sector of the fault have a pronounced footwall on the lake floor with stronger acoustic anomalies related to gas saturation in the sediments. The southwestern sector of the fault system is instead less pronounced on the lake floor with increasingly thicker hemipelagic deposits further to the west. All sampling station profiles indicate that the pore gas in the sediments is methane-dominated with greater gas concentrations at the localities above the fault. Accordingly, gas molecular and isotope composition revealed that the highest concentrations are present in the central and northeastern segments of the fault zone and in particular associated with the MSU hydrate-bearing structure and the Novosibirsk and Ukhan mud volcanoes. These structures reveal the highest concentrations of methane and C2+ homologues, as well as the highest methane carbon isotope compositions (delta C-13(CH4) = -57 parts per thousand VPDB) and near-surface gas hydrate accumulations. The southwestern segment of the fault is characterized by the lowest gas concentrations in the sediments and the lowest delta C-13(CH4) (-76 parts per thousand VPDB). By combining the geophysical and geochemical data obtained from the profiles, we propose the dominant gas migration mechanisms at various segments of the Gydratny Fault. Where the fault is well-expressed on the bottom relief (the central and northeastern sectors) focused migration is more pronounced and the thermogenic component of the methane is transported by advection from deeper units. Where the fault underlays relatively thick modern sediments (i.e., in the southwestern segment), deeper rising thermogenic gases and a significant portion of microbial methane are transported by combined diffusion and advection mechanisms, respectively. Our results reveal that this laterally extensive tectonic structure is an efficient pathway for fluid migration hosting numerous mud volcanoes and gas hydrate bearing structures. This study provides useful insights for the interpretation of offshore oil and gas geochemical prospects.