Geochemistry of oil-and-gas seepage in Lake Baikal: towards understanding fluid migration system

Baikal is a Cenozoic syn-rift sedimentary basin with many surficial manifestations of distinct hydrocarbon system. Focused gas seeps, gas-hydrate accumulations, and various mud volcanoes are abundant all over the lake bottom and were recently studied in order to characterize an upward fluid migration from deeper strata. Highly concentrated oil seeps which can provide detailed information on basin fluid migration pathway configurations are mostly developed at the east coast and rift flank of Lake Baikal.

Herewith, we report results of detailed geochemical studies (gases, organic matter, bitumen, pore waters, and sediments) completed on samples collected from an area of active oil and gas seepage, asphalt/tar edifices and subbottom gas-hydrates occurrences located 18 km offshore the Gorevoy Utes cape (the eastern coast of the lake) at the depth of 850-950 m.

As a part of the Class@Baikal-2018 expedition, two high-resolution seismic profiles (total length of about 10 km) crossing the fluid discharge zone in transverse directions were acquired to locate 22 bottom sampling stations and to retrieve samples. Four more seismic lines and 12 sampling cores were collected during the follow up Class@Baikal-2019 cruise.

The highest concentrations of all gases and a fresh crude oil in sediments are characteristic for a spot of only about 500 m in diametre, marking a probable centre of the most intense deep fluid migration to the surface. The elemental composition characteristic of sampled oil was determined as follow: C=83.84%, H=10.67%, N=0.37%, and S<0.08% by wt. And its molecular compounds are 15% asphaltenes, 20% resins, 35% aromatic hydrocarbons, and 30% saturates.

High concentration of methane was also detected in samples at the distance from this central spot all around the studied field. According to isotopic analyses, this indicates lateral redistribution of thermogenic methane ongoing together with enhanced bacterial methane generation in surrounding sediments. δ13С of methane from the peripheries varies from -70.98 ‰ to -88.46 ‰, whereas the δ13С of methane from the central spot is heavier (up to -41.00 ‰). The high content of methane homologues (ethane and propane) and carbon dioxide is characteristic and indicative for all samples taken from the central spot. A few samples collected outside of the central zone demonstrated the high thermogenic methane concentration, carbon dioxide content and presence of some methane homologues as well. Most likely this points out at existence of locally permeable segments aside of main conduit, probably some fractures accompanying the central pathway. Interestingly, no fresh oil was found in those samples.

Rock-Eval pyrolysis, isotopic analyses and biomarker studies revealed that the source rocks for both hydrocarbon gases and oil are terrigeneous and contain predominant humic organic matter components (kerogen type III). These strata belong to different maturation stages, ranging from low-mature to peak-mature, which is well explained by the complex structure of the rift sedimentary infill and documented presence of local thermal anomalies in the region.

Results of geochemical studies are incorporated into an integrated model of source-to-surface fluid migration to explain the observed peculiarities of the Gorevoy Utes seepage area.