Discrete Fracture Network modelling of a hydrocarbon-bearing, oblique-slip fault zone: Inferences on fault-controlled fluid storage and migration properties of carbonate fault damage zones

Abstract

Discrete Fracture Network (DFN) modelling is a useful tool to assess both fluid storage and migration properties of naturally fractured carbonate rocks. This work presents the results of multiple DFN models, which represent the structural network associated with a ca. 100 m–offset, oblique-slip, normal fault zone crosscutting Miocene ramp carbonates of the Majella Mountain, Italy. Specifically, by taking advantage of a previously published conceptual model of fault nucleation and growth, the DFN models are aimed at computing the amount of fracture porosity and equivalent permeability of representative portions of the evolving fault damage zone. In fact, by first considering only background structural elements, those pre-dating the fault nucleation stage, all fault-related structural elements such as joints, small and medium faults are added, one by one, according to their relative time of formation. Small faults are characterized by several cm-offset, whereas the latter ones have several 10's of cm-offset. The following fracture parameters are inputted for each structural element: (i) length; (ii) aspect ratio; (iii) mechanical and hydraulic aperture; (iv) fracture intensity and (v) orientation. A sensitivity analysis is carried out in order to test the seeding procedure of the software, and the input fracture aperture values. Results of DFN modelling are consistent with the small faults forming the main repository for underground geofluids. Differently, the fluid migration paths through the fault damage zone are mainly controlled by opening-mode-related fractures, which mainly form clusters at the extensional quadrants of sheared structural elements. Due to their attitude and distribution, these features mainly enhance fault-parallel fluid migration paths. In fact, the computed values of horizontal equivalent permeability obtained along this direction are about one order of magnitude higher than those along the orthogonal direction. The approach and methodologies applied in this work, together with the results of the multiple DFN model configurations show the relative influence and role of the different structural elements on fluid storage and migration properties of limestone-hosted, fault damage zones.