Fracture porosity and equivalent horizontal permeability computed for shallow-water carbonates of the southern Apennines fold-and-thrust belt, Italy

Fracture and fault networks are characterised by complex spatial and dimensional properties that might affect the flow and accumulation of subsurface fluids. In geological applications, DFN models are commonly employed to compute the multiscale properties of fractured rock volumes in terms of porosity and equivalent permeability. In the present contribution, we focus on an outcrop-to-reservoir investigation of Mesozoic shallow-water carbonates exposed along the axial zone of the southern Apennines fold-and-thrust belt, FTB, Italy. The carbonates were originally deposited in lagoon-to-proximal ramp settings of the Paleo Apenninic Platform during Lower Jurassic–Upper Cretaceous times and include well-layered and massive associations of bed packages several m-thick. By integrating the results of both field and digital structural analyses, we build multiple DFNs to assess the hydraulic behaviour of geocellular volumes representative of the different scales of observation, and stochastically populated with high-angle fractures, small, and medium faults. Fractures are either strata-bound, SB, or non-strata-bound, NSB, and results compartmentalized within single-bed packages. Small faults crosscut multiple bed-packages and show a few cm-to-m throws, whereas medium faults displace multiple bed-package associations and have throws > 1m. Both small and medium faults exhibit high peaks of fracture density, P₂₀, and intensity, P₂₁, in correspondence with the releasing jogs disrupting mechanical interfaces such as bed package boundaries and pre-existing low-angle thrust faults. As data input for DFN modeling, the aperture values of the stochastic fractures are set as proportional to either fracture length (most favourable flow conditions) or to the square root of fracture length (least favourable conditions). At the outcrop scale, 5m-side DFN models show the highest values of fracture porosity among those considered in this work. These results are therefore consistent with both SB and NSB fractures forming the main repository for fluid accumulation. At larger scales, the 50m-side and 500m-side DFN models including small and medium faults, are characterized by higher values of equivalent permeability, which range between 10^-2 and 10^-1 mD. Considering the computed Kxx and Kyy values for the single geocellular volumes, near-isotropic horizontal conditions are assessed across all scales of investigation. Accordingly, altogether, high-angle SB and NSB fractures, small and medium faults form a system of well-connected network through the shallow-water carbonates. Interpreting these data in light of published values for platform carbonates in Italy, we interpret this multiscale horizontal permeability isotropy due to the severe exhumation (~ 4 to 5km) the studied carbonates went through during the Quaternary downfaulting of the southern Apennines FTB.