PERMEABILITY STRUCTURE OF A LARGE, BASIN-BOUNDING, NORMAL FAULT IN PLATFORM CARBONATES, CENTRAL ITALY

We address the permeability of the Venere normal fault, which borders the southeastern side of the Fucino basin in central Italy, by deciphering its structural components and then determining their petrophysical properties. The fault is about 10 kilometers long and has as much as 0.8 kilometers of throw. Along its central portion, the internal structure of the fault zone is made up of deformed sediments and cemented slope scree in the hanging wall, main slip surfaces that are coated with low-Mg calcite, and meter-thick fault rocks and tens-of-meters-thick deformed carbonates of the damage zone in the footwall. The fault rocks are both matrix- and cement-supported. The damage zone is comprised of pulverized (5% of the clasts larger than 1 millimeter in size), intensely fractured (5% of the clasts larger than 4 millimeters), and moderately fractured (11% of the clasts larger than 16 millimeters) carbonates that flank the fault rocks. The thickness of the entire fault zone and of the individual fault components decreases toward the lateral terminations, which consist of polished, intra-carbonates slip surfaces. The platform carbonate host rock is a Mesozoic boundstone with a porosity of 0.8% and permeability of 7∙10-4mD. The moderately and intensely fractured host rocks have higher porosity (up to 3%) and permeability (up to 79.9mD) than the boundstone host rock. Based on the presence of cataclastic shear bands, the porosity of the pulverized carbonates is higher than 15%. The fault rocks have lower values of porosity (down to 0.6%) and permeability (down to less than 1∙10-4mD), relative to the carbonates of the damage zone. The structural architecture and petrophysical properties are thus consistent with the central portion of the fault being characterized by fault rocks that inhibit cross-fault fluid flow and a wide damage zone that forms a conduit for both along- and cross-fault. Conversely, towards the lateral terminations the fault behaves as a localized conduit for both along-fault and cross-fault fluid flow. Transient pathways form near and along the main slip surfaces due to dilation and opening of subvertical cracks, which are eventually sealed by diagenetic cement of low-Mg calcite.