A multidisciplinary study of a natural example of CO2 geological reservoir in central Italy

Storage of CO2 and, in particular, geological storage is one of the most promising solutions to counteract the continued increase of anthropogenic greenhouse-gas emissions. Here we present a multidisciplinary study of a natural CO2 geologic reservoir where fluid overpressure, measured at 85% of the lithostatic load, is trapped at similar to 4700 m depth in the Northern Apennines of Italy. Deep borehole data and seismic reflection profiles show that the observed CO2 overpressure: (a) is hosted in dome shaped structures that are the result of the interplay between the compressional and extensional tectonic phases of the area and (b) occurs within dolostones that are sealed by Ca-sulphates (anhydrites) horizons. Field studies on outcropping evaporites, that represent exhumed analogues of the lithologies found at depth in the pressurized reservoir, show dolostones affected by fracturing and faulting and Ca-sulphates characterized by ductile folding without macroscopic fractures. Borehole and laboratory P-wave velocities coincide for anhydrites (6.2-6.3 km/s) but are different for dolostones (6.2-6.3 km/s in situ and similar to 7.2 km/s in lab), confirming the different mode of deformation in the two rock types. Since anhydrites are not dominated by fractures, the very low permeability values, similar to 10(-19) to similar to 10(-22) m(2), recorded in laboratory experiments, can also be representative for in situ conditions. These data confirm the sealing ability of the anhydrites also for high values of fluid pressure. The integration of these datasets suggests that: (1) tectonics and structural position, and (2) lithology and associated mode of deformation, play a key role for channelling and trapping deep-seated CO2-rich crustal fluids.