CO2 degassing generally occurs in volcanic and geothermal areas. CH4 degassing (seepage) is typical of petroleum-prone sedimentary basins. In both cases, gas migration typically occurs along active gas-bearing faults and/or fractured zones. Hybrid systems, where CH4 from sedimentary rocks is transported upward by deep crustal or mantle CO2, are called Sediment-Hosted Geothermal (or Hydrothermal) Systems (SHGS; e.g., Mazzini et al., 2011). We propose that the SHGS model can explain the CO2-CH4 emissions recently observed in the Fiumicino area, close to the Rome International Airport, within the Tiber Delta. The origin of CO2 in surface manifestations at Fiumicino (eruptions after drilling, natural bubbling vents, gas-enriched soil) is, in fact, related to deep geothermal process, likely including either crustal thermo-metamorphism or magma degassing, similar to that of adjacent volcanic areas of the Roman Magmatic Province (Ciotoli et al., 2013; Pio Sella et al., 2014; Bigi et al., accepted). Substantial amounts of methane (concentrations of 1-8 vol.% in the soil-air and gas vents) are however associated to CO2 and, based on isotopic data, the origin of CH4 appears to be thermogenic, i.e. from organic-rich sedimentary rocks. Indeed, the presence of hydrocarbons in the Tiber Delta sedimentary sequence is known based on drillings for petroleum exploration. Accordingly, we hypothesise that deep geothermal CO2 can strip CH4 from the shallower hydrocarbon-system, as typically occurs in SHGSs. The Tiber Delta appears to be characterized by the presence of neo-tectonic faults (Ciotoli et al., 2013; Milli et al., 2013) and our soil-gas surveys have revealed that the CO2-CH4 degassing mainly follows these tectonic discontinuities.
Delta CO2-CH4 degassing: a possible sediment-hosted geothermal system.
G Ciotoli;L Ruggiero;F Stigliano
2014
Abstract
CO2 degassing generally occurs in volcanic and geothermal areas. CH4 degassing (seepage) is typical of petroleum-prone sedimentary basins. In both cases, gas migration typically occurs along active gas-bearing faults and/or fractured zones. Hybrid systems, where CH4 from sedimentary rocks is transported upward by deep crustal or mantle CO2, are called Sediment-Hosted Geothermal (or Hydrothermal) Systems (SHGS; e.g., Mazzini et al., 2011). We propose that the SHGS model can explain the CO2-CH4 emissions recently observed in the Fiumicino area, close to the Rome International Airport, within the Tiber Delta. The origin of CO2 in surface manifestations at Fiumicino (eruptions after drilling, natural bubbling vents, gas-enriched soil) is, in fact, related to deep geothermal process, likely including either crustal thermo-metamorphism or magma degassing, similar to that of adjacent volcanic areas of the Roman Magmatic Province (Ciotoli et al., 2013; Pio Sella et al., 2014; Bigi et al., accepted). Substantial amounts of methane (concentrations of 1-8 vol.% in the soil-air and gas vents) are however associated to CO2 and, based on isotopic data, the origin of CH4 appears to be thermogenic, i.e. from organic-rich sedimentary rocks. Indeed, the presence of hydrocarbons in the Tiber Delta sedimentary sequence is known based on drillings for petroleum exploration. Accordingly, we hypothesise that deep geothermal CO2 can strip CH4 from the shallower hydrocarbon-system, as typically occurs in SHGSs. The Tiber Delta appears to be characterized by the presence of neo-tectonic faults (Ciotoli et al., 2013; Milli et al., 2013) and our soil-gas surveys have revealed that the CO2-CH4 degassing mainly follows these tectonic discontinuities.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
