Integrated analysis of geophysical and geochemical data from cold fluid seepage system along the Gydratny Fault (Lake Baikal)

Lake Baikal is a part of large intracontinental rift zone. Baikal sedimentary infill is more than 7 km thick and was developed under predominantly extensional tectonics. Large number of faults of different geometry is imaged by several seismic surveys carried out in the region. The fault systems serve as fluid discharge pathways from deep sources to surface. A number of active seepage structures were mapped and studied during the Class@Baikal expeditions along the major fault system of the Central Baikal basin, which extends in SW-NE direction over 40 km and was named recently as the Gydratny Fault. Irregular distribution of these seeps, differences in their morphology and activity rate imply a variable permeability of the fault and different characteristics of migration pathways along its segments.

High-resolution seismic sections were acquired across the Gydratny Fault during the Class@Baikal cruises. The survey was followed by extensive bottom sediments and gases sampling. Hydrocarbon gases and isotopic characteristics as well as sediment pore water composition were analysed. Methane was detected in sediments along the whole fault extend in concentrations of more than 100 ml/l, exceeding background values (<15 ml/l), suggesting that the fault plane acts as regional fluid migration path. The highest methane content (>275 ml/l) and the presence of its homologues were observed at several local sites situated along the fault and associated with mud volcanoes and gas hydrate bearing seeps. The carbon isotopic composition varies from -72 to -57‰ VPDB for methane and from -21 to -31 ‰ VPDB for ethane, suggesting that these are thermogenic gases that migrate from deep layers of sedimentary infill of the basin.

Seismic data show well-established segmented nature of the Gydratny Fault system, which is believed to be a reason for observed variations of fluid discharge rates. Integrated analysis of the collected geophysical and geochemical data allowed evaluating contributions of different structural elements of the Gydratny Fault to fluid migration pattern in the area. NE segment of the fault system is a well expressed normal fault propagating to the lake bottom which is associated with higher methane concentrations (150-200 ml/l), elevated methane homologues content of up to 40 ml/l and heavier carbon isotopic composition in gas samples. The SW segments is either faintly expressed in the bottom relief or does not reach the surface at all. The methane concentrations in sediment samples collected from the segment are 100-150 ml/l and its carbon isotopic composition is normally lighter. We suggest that deeper parts of the SW fault segment are still highly conductive and concentrated hydrocarbon fluids migrate from the source upwards but some near-surface dispersal of migrated fluids occurs at places where the fault does not reach the lake bottom. The Gydratny master fault is accompanied by numerous subsidiary faults developed within hanging wall while footwall is less faulted. The associated faults are believed to enhance the main fluid migration system and this interpretation is supported by observations of normally higher methane concentrations in bottom sediments of the hanging block.

This study was funded by RFBR Grant № 18-35-00363.