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 CH4and 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 (O2, N2, CH4, CO2, He, Ne, Ar, H2, H2S and C2-C6hydrocarbons) and isotopic (R/RA, \uce\ub413C-CO2, \uce\ub413C-CH4and \uce\ub42H-CH4) composition. Results show that CH4presents a wide range of concentrations (from <0.5 to 925,200 \uce\ubcmol/mol) and isotopic values (\uce\ub413C-CH4from -79.8 to +45.0\ue2\u80\ub0 vs. V-PDB; \uce\ub42H-CH4from -311 to +301\ue2\u80\ub0 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 CH4concentration from EH to HH was recognized, whereas CH4showed intermediate concentrations in VA. The CH4/(C2H6+ C3H8) ratios (from 1.5 to 93,200), coupled with CH4isotopic 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 CH4can 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 (\uce\ub413C-CH4up to +45.0\ue2\u80\ub0 vs. V-PDB and \uce\ub42H-CH4up to +301\ue2\u80\ub0 vs. V-SMOW)