Poro-perm relations of Mesozoic carbonates and fault breccia, Araxos Promontory, NW Greece

Abstract. Aiming at assessing the porosity and permeability properties, we present the results of microstructural and laboratory measurements, including density, porosity, V_P, V_S, and electrical resistivity. These measurements were performed in dry and in saturated conditions on 54 blocks of Mesozoic carbonate host rocks and fault breccias collected in Greece. The host rocks consist of carbonate mudstones, wackestones, packstones, and sedimentary breccias from the Senonian and Vigla formations. These rocks exhibits average density values, low porosity values, and medium-to-high P- and S-wave velocities. Fault breccias originate from high-angle extensional and strike-slip fault zones, displaying a wider range of density, porosity values up to 5–10 times higher than host rock, along with ultrasonic velocities. Regardless of lithology, the carbonate host rocks might include vugs due to selective dissolution. Conversely, the fault breccia samples feature microfractures. Slight textural anisotropy is documented in the carbonate host rocks, while a higher degree of anisotropy characterizes the fault breccias. Selected samples were also tested in pressure vessels with confining pressure up to 80 MPa, revealing that transport properties along microcracks in fault breccias can significantly increase with increasing depth. To assess rock permeability and porosity-permeability relations, three different protocols were employed. Two of them were based on the Effective Medium Theory, where permeability was computed by inverting ultrasonic measurements, assuming an array of penny-shaped cracks embedded in an impermeable host matrix. The aspect ratio and crack width were obtained by the seismic measurements, modeling either by assuming all cracks as isolated or unconnected or all cracks connected into the network. The application of these two protocols showed a systematic variation of permeability with porosity. In contrast, the results of the third protocol, based on the digital image analysis outcomes only, did not exhibit systematic variation. This behavior was interpreted as a result of the not-selective dissolution of the outcropping carbonates causing a wide range of measured fracture aperture values. This study found that carbonate host rocks lacked a clear poro-perm trend due to the presence of stiff, sub-rounded pores and small vugs. On the contrary, fault breccia exhibited a linear increase in permeability with porosity due to a connected pore network including microfractures.