—We investigated the molecular and stable isotope compositions of hydrocarbon gases of the Gorevoi Utes oil–gas seep (Lake Baikal) characterized by a simultaneous discharge of oil and gas from the lake floor. It has been found that these hydrocarbon gases are mostly thermogenic methane (δ13C–C1 = –53.9 to –38.8‰; δ13C–C2 = –23.4 to –33.3‰). At the same time, the gases have a minor amount of C2+, seldom reaching 10%. The C1/C2+ value varies from 9 to 2700, with the average value being 807, which is atypical of petroleum hydrocarbons. We suggest that degassing of oil leaking to the lake floor affects the molecular composition of such gases. Some secondary processes, primarily molecular fractionation of gas during its migration into the subsurface sediments and anaerobic biodegradation of methane homologues, contribute to the decrease in C2+ content.