The purpose of the study was to assess the nature of the gases of various volcanic systems using known genetic criteria developed by many years of research by a number of scientists around the world. A comprehensive processed and analyses of the collected large amount of literature data and the author's own research results made the following main conclusions:

– the isotope composition (IC) of CO₂ and argon (⁴⁰Ar/³⁶Ar) in igneous volcanoes varies within very narrow limits, in comparison with sedimentary volcanoes. However, the use of these parameters as an unambiguous genetic criterion is not possible;

– more objective genetic indicators are: helium isotope ratio (R/Ra) and IC of hydrocarbon (HC) gases. According to these parameters, the HC gases of the considered volcanic systems have different genetic sources: a) abiogenic in igneous and carbonic sedimentary volcanic systems; b) predominantly abiogenic-biogenic - in the hydrothermal system; c) biogenic - in methane sedimentary volcanoes;

– amount of abiogenic methane in the gas of the igneous volcanic system is about 1% on average.

The results are of interest in connection with the discussion about the genesis of commercial accumulations of HCs in the sedimentary strata of the earth's crust.

Keywords: igneous / hydrothermal and sedimentary volcanic systems, gas, isotopic composition, comparative genetic analysis

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