HR: 1340h
AN: B33A-1008 INVITED     [Abstracts]
TI: Microscale gas Leaks in Different Geological Settings of Central Italy: Implications for Geological Sequestration of CO2.
AU: * Lombardi, S
EM: salvatore.lombardi@uniroma1.it
AF: Dipartimento di Scienze della Terra Universita' di Roma "La Sapienza", P.le Aldo Moro, 5, Roma, 00185 Italy
AB: The structural setting of Italy is characterized by the overprinting of extensional tectonics related to the opening of the Tyrrhenian basin onto compressional tectonics caused by the NE migration of the Apennine front. While subduction is still active, there is a debate regarding the mechanisms which allow for the coexistence of both extensional and compressional motion. The main forces acting underneath the central Apennines have been due to either slab pull and/or eastward mantle flow, or to slab detachment. The development of this structural setting caused the formation of major deep fault and fracture systems which provide a direct conduit for deep fluids (water and gas) upwelling towards the surface. The analysis of these fluids at surface provides the opportunity to understand the geochemical factors affecting the conditions and mechanisms that allow for their transfer to the near-surface environment. The addressing of these goals is very important in the field of geological CO2 sequestration, where long term isolation is necessary for its successful application. Over the last 20 years the present authors have been surveyed a large part of central Italy for the concentration and distribution of gas species in the vadose zone. While the majority of samples can be referred near-surface biological processes, a significant number can be related to microseeps from deep gas sources. The studied areas included different geological provinces of Italy, resulting in the creation of a unique, gas geochemistry database. The present paper applies statistical analyses to more than 10,000 soil gas samples to compare the micro-scale degassing processes occurring across the Italian Peninsula. These results are examined in light of choosing suitable sites, developing safety assessment tools for the geological sequestration of CO2 via the characterization of migration effects induced by tectonics, the selection of natural analogues to study gas migration from buried reservoirs and the study of sedimentary-cover gas permeability. Results highlight extensive micro-scale, as well as spot, CO2 and 222Rn degassing in the western extensional sector and high He, Rn and CO2 values in the Apennine chain, whereas He and CH4 microseeps occur in the compressional foredeep Adriatic area. Multiple origins can be proposed for soil gas CO2 in the western sector. A possible crustal CO2 source could be due to water-rock interaction with extensive Mesozoic limestones in the geothermal areas of the western side of the peninsula. In the same areas, where high He contents occur, CO2 could have a partial mantle origin, as suggested by the 3He/4He ratios of some gas vents and by their association with eruptive centers. A third origin could be linked to hydrocarbon oxidation and/or bacterial activity. In the Apennine Chain high CO2 and Rn values are linked to gas migration along active faults of the intermontane basins (e.g. Fucino Plain, San Vittorino Plain, Comino Plain, Roveto Valley). These zones occur along a regional fault line supposed as a crustal discontinuity corresponding to a deep step of the Moho. Finally the observed high He and CH4 concentrations in the Adriatic foredeep suggests a possible deep helium source associated with the hydrocarbon reservoirs, with leakage and distribution at the surface influenced by the presence of more permeable pathways such as regional faults and associated fractures.
DE: 0428 Carbon cycling (4806)
DE: 0468 Natural hazards
DE: 0478 Pollution: urban, regional and global (0345, 4251)
DE: 0490 Trace gases
SC: Biogeosciences [B]
MN: Fall Meeting 2005