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

Abstract

Fault zones in carbonates control the storage and transport fluid properties. Their physical state can be very complex due to the interplay of diagenetic processes and multiple deformation episodes, leading to complex pore spaces given by overlapping voids space and crack damage. Aiming to assess key physical properties such as porosity and permeability for fault zones in the Araxos Promontory (Greece), we present the results of microstructural and laboratory measurements, including density, porosity, VP, VS and electrical resistivity for 54 blocks of Mesozoic carbonate host rocks and fault breccias originating from high-Angle extensional and strike-slip fault zones. The host rocks consist of carbonate mudstones, wackestones, packstones, and sedimentary breccias from the Senonian and Vigla formations (Ionian zone) and include vugs due to selective dissolution. These rocks exhibit average density values, low porosity values, and medium-To-high P-and S-wave velocities. In contrast, fault breccia samples feature microfractures and display a wider range of density and porosity values, up to 5-10 times higher than those of the host rock, and a higher degree of anisotropy. To assess rock permeability and porosity-permeability relations, three different workflows were employed: (i) using Effective Medium Theory, inverting ultrasonic measurements; (ii) modelling a constant crack aperture of 1 μm; and (iii) using crack density values obtained from 2D image analysis. Selected samples were also tested in pressure vessels with confining pressure up to 80 MPa to reproduce in situ conditions. The application of the first two workflows showed a systematic variation of permeability with porosity. In contrast, the results of the third workflow, based solely on the outcomes of the digital image analysis, did not exhibit systematic variation. This behaviour is interpreted as a result of the non-selective dissolution of the outcropping carbonates, which causes a wide range of measured fracture aperture values and a lack of a clear poro-perm trend for host rocks due to the presence of stiff, sub-rounded pores and small vugs. On the contrary, the fault breccia exhibits a linear increase in permeability with porosity due to a connected pore network including microfractures.