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Molecular and isotopic composition of free hydrocarbon gases from Sicily, Italy
Author(s) -
Grassa Fausto,
Capasso Giorgio,
Favara Rocco,
Inguaggiato Salvatore,
Faber Eckhard,
Valenza Mariano
Publication year - 2004
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1029/2003gl019362
Subject(s) - methane , hydrocarbon , geology , isotopic signature , anaerobic oxidation of methane , geochemistry , isotope , chemical composition , mantle (geology) , isotopes of carbon , δ13c , isotope analysis , stable isotope ratio , volcano , natural gas , environmental chemistry , mineralogy , earth science , chemistry , total organic carbon , physics , oceanography , organic chemistry , quantum mechanics
Chemical and isotopic data have been used as geochemical tracers for a genetic characterization of hydrocarbon gases from a total of eleven manifestations located in Eastern and Central‐Southern Sicily (Italy). The molecular analysis shows that almost all the samples are enriched in methane (up to 93.2% Vol.), with the exception of four gas samples collected around Mt. Etna showing high mantle‐derived CO 2 content. Methane isotope signatures suggest that these are thermogenic gases or a mixture between thermogenic gases and microbial gases. Although samples from some mud volcanoes in Southern Sicily (Macalube di Aragona) show isotope signatures consistent with a mixing model between thermogenic and microbial, by combining the molecular compositions (C 1 /(C 2 + C 3 )) and the methane isotope ratios (δ 13 C 1 ), such a process seems to be excluded. Therefore, the occurrence of secondary post‐genetic processes should be invoked. Two main hypotheses have been considered: the first hypothesis includes that the gas is produced by microbial activity and altered post‐genetically by microbial oxidation of methane, while according to the second hypothesis thermogenic gas have modified their molecular ratios due to vertical migration.