Deformation mechanism and hydraulic properties of normal and strike-slip fault zones in porous carbonates outcropping in central and southern Italy

Deformation of porous carbonate grainstones takes generally place by compactive shear banding, which initially produce single bands that may evolve during incremental strain into zones of multiple bands and, eventually, fault zones with discrete slip surfaces. The several processes associated to increased deformation are recorded in the aforementioned structural elements. In fact, they are characterized by a different texture, porosity, dimensional attributes (length, thickness, displacement). Fault zones are characterized by inner fault cores made of cataclastic material surrounded by thicker damage zones including single bands and zones of bands. In this work, we present the results of in situ permeability measurements carried out using a portable field permeameter along both normal and strike-slip fault zones developed in high-porosity carbonate grainstones from the Cretaceous Orfento Formation (Majella Mountain, Abruzzi Region), and in the Lower Pleistocene deposits (Favignana island, Sicily). We studied eight fault zones with displacement ranging from 30 centimeters to 2 meters. Undeformed rocks are poor-to-medium consolidated grainstone. Grains are made up of fragments of carbonates ranging from 0.05 to 1 mm in diameter. The matrix consists of bladed and sparry calcite cement and carbonate fragments smaller than 0.05 mm. Porosity is about 30%, bulk permeability in the order of about 5.6 Darcy. Grainstones in damage zones record nondestructive compactive granular flow and pore collapse, which reduce porosity and pore connectivity. Moreover intergranular pressure solution seams form in the already compacted rock. Permeability measurements show an average value of about 0.6 Darcy. Deformation in fault cores evolves from particulate flow to compactional cataclastic flow. The progressive grain size reduction increases the amount of silt- and clay-size fractions. Hence, porosity was dramatically reduced and the permeability values show an average value of 0.07 Darcy. The aforementioned permeability data indicate a three order of magnitude decrease of permeability from host rocks to cataclastic fault cores. A clear dependence of the fluid circulation paths through porous carbonates is therefore inferred at depth due to orientation, density and connectivity of the shear band fault zones. Accordingly, the results presented in this study may be helpful in applications such as geofluids management for improving the forecasting of carbonate reservoir quality and understanding the extent of reservoir compartmentalization.