HR: 0800h
AN: B31B-0987    [Abstracts]
TI: Origin of methane expelled from the "Macalube di Aragona" mud volcanoes area (Central Sicily, Italy): thermogenic or bacterial?
AU: Grassa, F
EM: f.grassa@pa.ingv.it
AF: Istituto Nazonale di geofisica e Vulcanologia, Sezione Geochimica di Palermo, Via Ugo La Malfa,153, Palermo, 90146 Italy
AU: * Inguaggiato, S
EM: s.inguaggiato@pa.ingv.it
AF: Istituto Nazonale di geofisica e Vulcanologia, Sezione Geochimica di Palermo, Via Ugo La Malfa,153, Palermo, 90146 Italy
AU: Favara, R
EM: r.favara@pa.ingv.it
AF: Istituto Nazonale di geofisica e Vulcanologia, Sezione Geochimica di Palermo, Via Ugo La Malfa,153, Palermo, 90146 Italy
AB: The "Macalube di Aragona" area is located in Central Sicily (Italy) within a thick accretionary wedge (up to 6 km), developing along the frontal part (Southern margin) of the Sicilian thrust belt. Mud volcanoes are among the most important natural emissions of waters, oil and gaseous hydrocarbons from buried sediments. We have periodically collected gases from one mud volcano and one pool with the aim to investigate the origin of the hydrocarbons from the "Macalube di Aragona". Gas released from mud volcano and mud pool from the "Macalube di Aragona" show very similar chemical and isotopic composition. They consist of mostly CH4 (90 to 92.5 Vol.) with δDCH4 and δ13CCH4 around -190 and -48 respectively. N2, O2 and CO2 are only minor components with N2/O2 ratios very close to same ratio of Air. The carbon isotope composition of methane straddles the fields relative to thermogenic and microbial gas, thus giving ambiguous indication on the origin of gases. However, coupling the molecular and the isotopic composition of hydrocarbons, the hypothesis of a mixing process seems to be not realistic. Then, the occurrence of secondary post-genetic processes which masks the original molecular composition and/or the isotope signature should be invoked. Three post-genetic processes have been considered and evaluated. The first process suggests that gas are thermogenic in origin and they are selectively enriched in CH4 with respect to heavier molecular gas during migration. The remaining processes include that gas are preferentially originated by bacterial reduction and the change in the composition are due (1) to the depletion of substrate, which cause a progressive enrichment in 13C in the residual reservoir and consequently in the produced gases or (2) to the preferential loss of CH4 as a consequence of oxidation processes. In fact, being methane oxidized much more easily than heavier hydrocarbons such a process causes an enrichment of heavier isotope in the residual methane. According to the first hypotheses, the Bernard parameter (C1/(C2+C3) ratio) can be modified during gas migration from reservoir towards surface. Because of CH4 rises more quickly that heavier hydrocarbons, an increasing of the C1/(C2+C3) ratio is then expected. Hydrocarbon can be also selectively adsorbed on clays and/or organic matter causing a shift of the Bernard parameter. In the case of a microbial origin of gas, the methanogenic pathways can be identified considering the carbon isotope signature of CH4 and the coexisting CO2 and/or the hydrogen isotope ratios of methane and of formation water. Both these isotope pairs indicate that hydrocarbon have been generated by carbonate reduction. Late stage of hydrocarbon generation are compatible with intermediate values for methane between and the unusual positive isotope values for the isotopic composition of Carbon dioxide.
DE: 0414 Biogeochemical cycles, processes, and modeling (0412, 0793, 1615, 4805, 4912)
DE: 0466 Modeling
DE: 0499 New fields (not classifiable under other headings)
SC: Biogeosciences [B]
MN: Fall Meeting 2005