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Tiber delta CO2-CH4 degassing: A possible hybrid, tectonically active Sediment-Hosted Geothermal System near Rome

TitoloTiber delta CO2-CH4 degassing: A possible hybrid, tectonically active Sediment-Hosted Geothermal System near Rome
Tipo di pubblicazioneArticolo su Rivista peer-reviewed
Anno di Pubblicazione2016
AutoriCiotoli, G., Etiope G., Marra F., Florindo F., Giraudi C., and Ruggiero L.
RivistaJournal of Geophysical Research B: Solid Earth
Volume121
Paginazione48-69
ISSN21699313
Parole chiavealkane, Belemnoidea, Carbon dioxide, degassing, Fiumicino, geothermal system, greenhouse gas, isotopic composition, Italy, Lazio, methane, Roma [Lazio], Rome, soil gas, Tiber Delta, Tiber River
Abstract

Fiumicino town in the Tiber River delta, near Rome International Airport (Italy), is historically affected by large amounts of carbon dioxide (CO2) in the ground and gas eruptions triggered by shallow drilling. While it is known that CO2 originates from carbonate thermometamorphism and/or mantle degassing, the origin of methane (CH4) associated with CO2 is uncertain and the outgassing spatial distribution is unknown. Combining isotope gas geochemistry, soil gas, and structural-stratigraphic analyses, we provide evidence for a hybrid fluid source system, classifiable as Sediment-Hosted Geothermal System (SHGS), where biotic CH4 from sedimentary rocks is carried by deep geothermic CO2 through active segments of a half-graben. Molecular and isotopic composition of CH4 and concentration of heavier alkanes (ethane and propane), obtained from gas vents and soil gas throughout the delta area, reveal that thermogenic CH4 (up to 3.7 vol% in soil gas; δ13CCH4: -37 to -40‰ VPDB-Vienna Peedee Belemnite, and δ2HCH4: -162 to -203‰ VSMOW - Vienna Standard Mean Ocean Water in gas vents) prevails over possible microbial and abiotic components. The hydrocarbons likely result from known Meso-Cenozoic petroleum systems of the Latium Tyrrhenian coast. Overmaturation of source rocks or molecular fractionation induced by gas migration are likely responsible for increased C1/C2+ ratios. CO2 and CH4 soil gas anomalies are scattered along NW-SE and W-E alignments, which, based on borehole, geomorphologic, and structural-stratigraphic analyses, coincide with active faults of a half-graben that seems to have controlled the recent evolution of the Tiber delta. This SHGS can be a source of considerable greenhouse gas emissions to the atmosphere and hazards for humans and buildings. ©2015. American Geophysical Union. All Rights Reserved.

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cited By 1

URLhttps://www.scopus.com/inward/record.uri?eid=2-s2.0-84958074213&doi=10.1002%2f2015JB012557&partnerID=40&md5=868a1b565bcbca1e9b2e96f9f99eaba1
DOI10.1002/2015JB012557
Citation KeyCiotoli201648