Spatial distribution of permeability in carbonate fault damage zones

Abstract

Fault conduits often localized fluid flow at specific sites related to fault segment growth and linkage. Understanding these mechanisms is essential for assessing geofluid pathways within reservoirs or leaks to the biosphere. We study here a segmented fault zone with strike-slip kinematics and pluri-decametric displacement, affecting carbonate rocks (Pag island, Croatia). This fault zone has multiple core zones surrounded by a damage zone (DZ), composed of different structures, including wall and link damage. To build discrete fracture networks (DFNs) of these structures, we conducted high-resolution fracture mapping and measurement of aperture in five areas around the main fault system. We also analyzed rock samples from each damage structure using the same method. Fluid flow simulations were performed through the DFNs to quantify the permeability and its anisotropy. We show that link damage is about 10² more permeable than the background damage, and 2 to 5 times more permeable than the wall damage. DZ permeability can be approximated by a tensor at the decametric-scale, but not at the centimetric-scale due to the strong permeability heterogeneity inherent to this scale. In the DZ, decametric-scale fracture patterns are 10–65 times more permeable than the centimetric-scale fractures, providing conduits for fluid flow. Finally, the maximum permeability strongly correlates with the product of mean aperture and connectivity, suggesting that these parameters could be used as proxy of the permeability in fault DZ. These results allow better estimation of fault zone permeability, providing constraints for flow modelling in various applications in the energy transition.