Lavrushin Vasilii¹, Chelnokov Georgii¹, Aidarkozhina Altyn¹

(¹Geological Institute of Russian Academy of Sciences, Moscow, 119017, Russia)

Abstract: Three mud volcanic provinces are known in the Caucasian region: the Kerch–Taman, Azerbaijani, and Georgian provinces. The majority of volcanoes in these provinces are localized in the marginal parts of large petroleum-bearing sedimentary basins (the Western Kuban and Kura basins) adjacent to the Greater Caucasus mountain structure. The areas are marked by petroleum-bearing potential and extensive fault-and-thrust tectonics. However, no signs of recent magmatic activity (based on ³He/⁴He ratios) are found in the mud volcanic gases. In order to study the genesis and formation conditions of the geochemical composition of gases from mud volcanoes in the Caucasian region, isotope-geochemical characteristics (δ¹³C, δ¹⁵N) of gases (CH₄, C₂H₆, CO₂, N₂) from more than 90 mud volcanoes were investigated. The results of these determinations were compared with data from hydrochemical geothermometers and other geochemical characteristics of the aqueous (δ¹³C(HCO₃), δ²H and δ¹⁸O in H₂O) and gas phases of the volcanic fluid systems. This made it possible to gain insight into the temperatures and depths at which the studied gases were formed. The gases exhibit high methane contents, commonly reaching 80–98%, with CO₂ and N₂ occurring as minor components. Nevertheless, in certain volcanoes of the Kerch–Taman area, CO₂-rich gases (with CO₂ up to 20–80%) are occasionally found. In the Caucasian gases, δ¹³C(CH₄) ranges from –61.6 to –25.5‰, while δ¹³C(C₂H₆) varies between –35.4 and –21.4‰. A remarkably wide range of δ¹³C(CO₂) values is observed, from –39 to +24.4‰. The δ¹⁵N(N₂) values, ranging from –6.0 to –0.4‰, indicate the presence of substantial amounts of non-atmospheric nitrogen. The mean δ¹³C and δ¹⁵N values of gases from the three Caucasian mud volcanic provinces do not differ significantly.All the observed variations in δ¹³C and δ¹⁵N values can almost always be explained by gas formation at different depths within the temperature range from 20–40 to 140°C. The correlations between isotopic characteristics of gases and reservoir temperatures are best expressed for the gases of the Kerch–Taman province and somewhat less so for the Azerbaijani and Georgian provinces. It is suggested that this is due to the higher tectonic activity of the latter, which provides better conditions for vertical migration and mixing of gases coming from reservoirs at different depths. It has also been shown that isotopically heavy CO₂ (δ¹³C > +1‰) is characteristic of gases formed at temperatures of 40–80°C, and its genesis may be related to biodegradation processes of organic matter in the upper levels of the geological section of the studied petroleum-bearing basins. This research was funded by the Russian Science Foundation, grant 26-17-00225.

Keywords: mud volcanoes, methane, carbon dioxide, nitrogen, sedimentary basin.

Author Profile (first or corresponding author): Lavrushin V., Male, Chief Researcher, Prof., mainly 

engaged in geochemistry of mineral water and gases. E-mail: wll2@yandex.ru