Gas Microseep Differences Between The Thyrrenian And Adriatic Coasts

From a geological point of view Italy is characterized by active and recent volcanoes (from the Pliocene to Present) and geothermal fields along the Thyrrenian margin. Two mountain chains (the Alps and the Apennines) transect the country and sedimentary basins (the Padanian Foredeep, Adriatic sea and other small peri-thyrrenian basins) host oil and natural gas reservoirs and saline aquifers. Therefore there is a very strong geological complexity which controls the gas leakage from deep reservoirs (Ciotoli et al., 1997; 1998; 1999; 2002). Within this framework various CO2 reservoirs are located throughout the different geological domains, and many are considered as natural analogues of the geological storage of anthropogenic CO2. These are characterized by different pressures, temperatures and leakage rates, and thus represent a large range of situations which may eventually occur in an engineered system. This combination of great depths and long time periods mean that the natural analogue sites can help us better understand eventual system evolution, which is difficult to model or to represent experimentally.

The experience accumulated in Italy has shown widespread macro-scale (gas vents, fumaroles, mineralised aquifers) and micro-scale (micro seeps) gas leakage from oil, natural gas and geothermal reservoirs along active faults (Ciotoli et al., 1998; Etiope and Lombardi, 1994). In active volcanic areas the CO2 flux from degassing activity ranges from 25 at Etna, to 1-2 at Stromboli to 0.066 Mton y-1 at Vulcano (Morner and Etiope, 2002). In the sedimentary basins along the Tyrrhenian coast, where geothermal reservoirs also occur, the flux of CO2 derives from diffuse soil degassing and from gas vents. The output of diffuse degassing ranges from 7300 ton y-1km-2 (Cava deSelci, Alban Hills Volcanic District), to > 73,000 ton y-1km-2 (Latera, Sabatinian Volcanic District), to 0.83-1,123 kg m-2 y-1 (in the sedimentary basins of the Siena Graben). In the case of gas vents the measured output ranges from 1,800 ton y-1 (at Massa Martana and Montecastello Vibio) to 310,000 ton y-1 (Mefite d'Ansanto). The regionally evaluated output of the central Apennine chain is 4.4-13.2 x106 ton y-1 (over 12,564 km2) (Chiodini et al., 2000).

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