| Abstrakt: |
Aiming at assessing the porosity and permeability properties, we present the results of microstructural and laboratory measurements of density, porosity, VP, VS, and electrical resistivity performed in dry and in saturated conditions on 54 blocks of Mesozoic carbonate host rocks and fault breccias. Host rocks consist of carbonate mudstones, wackestones, packstones, and sedimentary breccias pertaining to the Senonian and Vigla formations. These rocks show average density values, low values of porosity, and medium-to-high P- and S-wave velocities. Fault breccias derive from high-angle extensional and strike-slip fault zones, and are characterized by a wider range of density, porosity values up to 5-10 times higher than host rock, and low ultrasonic velocities. Independently on lithology, the carbonate host rocks might include vugs due to selective dissolution. Differently, the fault breccia samples include microfractures. A slight textural anisotropy is documented in the carbonate host rocks, whereas a higher degree of anisotropy characterizes the fault breccias. Selected samples were also tested in pressure vessels with confining pressure up to 80MPa, showing that transport properties along microcracks in fault breccias can significantly increase with increasing depth. In order to assess rock permeability and porosity-permeability relations, three different protocols are employed. Two of them are based on the Effective Medium Theory, so that permeability is computed by inverting ultrasonic measurements and assuming an array of penny-shaped cracks embedded in an impermeable host matrix. Accordingly, the aspect ratio and crack width are obtained by the seismic measurements. Two end terms have been modelled by assuming all cracks isolated, and unconnected or all cracks connected into the network. Application of these two protocols shows a systematic variation of permeability with porosity, whereas the results of the third one, based on the digital image analysis outcomes, do not exhibit systematic variation. We interpret this behavior as due to the not-selective dissolution of the outcropping carbonates causing a wide range of measured fracture aperture values. [ABSTRACT FROM AUTHOR] |
