Multi-scale fracture networks of faulted and fractured Apulian carbonates, Italy

The work is aimed at deciphering the contribution of both background and fault related deformation on the fluid flow properties of tight platform carbonates. Taking advantage of 3D exposures present in the Murge area of southern Italy, we are able to analyze, at different scales, the fracture networks that crosscut the layered Cretaceous limestone of the Altamura Fm (Korneva et al., 2014). The background deformation consists of (i) stratabound fractures, consisting of bed-perpendicular joints and sheared joints and (ii) non-stratabound fractures, represented by incipient strike-slip faults with cm offsets, which crosscut several beds and confined within individual bed-packages. Persistent fracture zones, which are made up of small strike-slip faults that offset several bed-packages and display tens of cm-offset, are also present along the study outcrops. The latter features include thin damage zones and very discontinuous, if not absent, cataclastic fault cores. At larger scales, well-develop faults, characterized by m to tens of m-offset, form a conjugate system of sub-vertical strike-slip faults. These larger faults include 10’s of m-thick fractured and faulted damage zones, which surround up to 1 m-thick cataclastic fault cores. For each fracture/fault set documented along the study outcrops, we assess the dimensional, spatial and scaling properties. Data were gathered in the field by mean of scan line and scan area measurements. By computing the mean orientation, size distribution, aspect ratio, aperture, N (number of fractures per sample volume) and fractal dimension of each fracture/fault set, we build up multi-scale DFN (Discrete Fracture Network) models of representative rock volumes. In particular, concerning the background deformation, different DFN models are constructed based upon the investigated bed thicknesses (stratabound fractures) and bed-packages thicknesses (non-stratabound fractures). One larger model is generated for the persistent facture zones crosscutting several bed-packages, in which both stratabound and non-stratabound fractures are included. Finally, DFN models representing the structural architecture of larger strike-slip faults are also constructed. Outputs of individual models are used to compute, at different scales, the overall P32, porosity and 3D permeability (Kx, Ky, Kz) values of the limestone rocks: from single beds to multiple bed-packages and, finally, to large fault zones. The results enable us to assess the multi-scale properties of the fracture network at different scales and, hence, to discuss the role exerted by brittle deformation on susburface fluid flow within tight layered carbonates.