Origin of methane and light hydrocarbons in natural fluid emissions: A key study from Greece

Greece, a country characterised by intense seismic and volcanic activity, has a complex geodynamic and geological setting that favours the occurrence of many gas manifestations. In this study, we address the origin of CH4 and light hydrocarbons in cold and thermal emissions discharging along the Hellenic territory. Also, we investigate their possible relationship with the main geochemical composition of the gases and the different geological settings of the sampling sites. For this purpose we collected 101 new samples that were analysed for their chemical (O-2, N-2, CH4, CO2, He, Ne, Ar, H-2, H2S and C-2-C-6 hydrocarbons) and isotopic (R/R-A, delta C-13-CO2, delta C-13-CH4 and delta H-2-CH4) composition. Results show that CH4 presents a wide range of concentrations (from < 0.5 to 925,200 mu mol/mol) and isotopic values (delta C-13-CH4 from -79.8 to + 45.0 parts per thousand vs. V-PDB; delta H-2-CH4 from -311 to + 301 parts per thousand vs. V-SMOW). Greece was subdivided in four geologic units (External [EH] and Internal [IH] Hellenides, Hellenic Hinterland [HH] and active Volcanic Arc [VA]) and a decreasing CH4 concentration from EH to HH was recognized, whereas CH4 showed intermediate concentrations in VA. The CH4/(C2H6 + C3H8) ratios (from 1.5 to 93,200), coupled with CH4 isotopic features, suggest that the light alkanes derive from different primary sources and are affected by secondary processes. An almost exclusive biotic, mainly microbial, origin of CH4 can be attributed to EH gases. Cold gases at IH have mainly a thermogenic origin, although some gases connected to continental serpentinization may have an abiogenic origin. Methane in gases bubbling in thermal waters of IH, HH and VA and fumarolic gases of the VA seem to have an abiogenic origin, although their chemical and isotopic characteristics may have been produced by secondary oxidation of thermogenic CH4, a process that in some of the sampled gases causes extremely positive isotopic values (delta C-13-CH4 up to + 45.0 parts per thousand vs. V-PDB and delta H-2-CH4 up to + 301 parts per thousand vs. V-SMOW).