Origin of light hydrocarbons in gases from mud volcanoes and CH4-rich emissions

This paper presents new chemical data of hydrocarbon-rich gases discharged from mud volcanoes and CH4- rich emissions located in different areas of Italy (Northern Apennines and Sicily). The determination of C5–C10 alkanes, cyclics and aromatics was carried out by GC–MS, while the main gas species and C1–C4 hydrocarbons were analyzed by GC-TDC and GC-FID, respectively. Methane is by far the most abundant component of all the investigated gas emissions, with the exception of part of those discharging close to Mt. Etna volcano, which are CO2-rich. The gas samples collected from the Emilia Apennine and Sicily show δD–CH4 and δ13C–CH4 values typical of thermogenic gases. Most gases from the Romagna Apennine discharge methane with a biogenic isotopic signature. Non-methane hydrocarbons of biogenic gases are almost exclusively C2– C4 alkanes, with minor amounts (fraction of μmol/mol) of C4+ compounds, including few aromatics and cyclics, likely deriving from minor thermogenic contribution not evidenced by the δ13C–CH4 and δD–CH4 values. The Etnean gases, whose R/Ra and δ13C–CO2 values indicate a strong contribution from a hydrothermal fluid source related to the nearby volcanic system, show a significant enrichment of aromatics compounds (up to 36% of the non-methane organic gas fraction), which is likely produced by catalytic reforming processes, such as dehydrocyclization of alkanes. The thermogenic gases from the Apennines and southwestern Sicily are characterized by the presence of more than 20 different cyclic compounds with concentrations up to several μmol/mol. Cyclic compounds are likely formed by i) thermal cracking and ii) uncompleted aromatization of alkanes occurring at depth>3 km and temperatures not exceeding 120–150 °C.