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The regional gas geochemical profile was first performed in the South China Sea in the north–south direction in January 2017 (42nd cruise of the R/V Akademik Boris Petrov). Five zones with methane emission into the atmosphere with a maximum of 30 mole km–2 day–1, were revealed above the oil- and gas-bearing structures in the southern part of the sea, in the Nam Con Son Basin. Methane emission into the atmosphere is generally related to ascending migration of methane from hydrocarbon deposits along fault zones. The expedition was set up by the Council for the Earth’s Hydrosphere of the Federal Agency for Scientific Organization as part of the international project “The 2nd International Indian Ocean Expedition (IIOE-2).”

This paper reports the results of the third Russian–Vietnamese expedition (V.I. Il'ichev Pacific Oceanological Institute, Far East Branch, Russian Academy of Sciences and the Institute of Marine Geology and Geophysics, Vietnam Academy of Science and Technology) in the Gulf of Tonkin, South China Sea (April 2016). The studies revealed new specific features of the distribution and origin of gasgeochemical fields in sediments within the rift zone of the Red River along a 150-km profile. Four zones with high-amplitude anomalies of hydrocarbon gases, helium, hydrogen, carbon dioxide, and carbon monoxide were revealed. The distribution of the anomalies reflects the tectonic structure of the area and points to the presence of several lithospheric sources of gases including gases of deep origin. The studies were carried out within the scope of the Joint Vietnamese–Russian Laboratory for Marine Geoscience (V.I. Il'ichev Pacific Oceanological Institute, Far East Branch, Russian Academy of Sciences and the Institute of Marine Geology and Geophysics, Vietnam Academy of Science and Technology).