The impact of faults on fluid flow in porous carbonate rocks

Faults may impact subsurface fluid flow over production and geological timescales. Predicting fault permeability is crucial for understanding subsurface fluid flow, which is required for a number of subsurface industries. Fault permeability is determined by the micro-fabric and distribution of fault rocks, which are influenced by the deformation mechanisms exhibited during faulting, fault-related diagenesis, and fault zone architectural evolution. Non-uniform strain distributions along fault zones lead to heterogeneous fault cores, therefore, predicting fault permeability is difficult. Empirical relationships linking lithological parameters to the petrophysical properties of different fault rocks are commonly utilised to predict fault permeability in clay-bearing sequences. However, no such relationships exist for carbonate rocks. A better understanding of these controls, in addition to the characterisation of fault rock permeability is required to establish predictive relationships for fault rock permeability. This thesis combines structural, microstructural and petrophysical data from a series of carbonate-hosted fault zones in Malta. Thereby enabling an understanding of the fault zone permeability structures in various lithofacies, whilst highlighting the heterogeneity on all scales of carbonate-hosted fault zones. The continuity of fault rock is shown to increase with displacement, and displacement thresholds for both a continuous fault core and cataclasite are established for fault zones in Malta. Only fault rocks derived from high porosity host rocks have the potential to retard fluid flow over reservoir scales. Lithological heterogeneity is retained within fault cores, whereby the heterogeneity of the faulted lithofacies is linked to the variability in fault rock petrophysical properties. Novel methods of upscaling fault permeability and implementing fault rock continuity relationships to fault property modelling are presented. The results provide an example of how porosity of the host carbonate can impact fault rock permeability under low stresses. Combined with similar studies from different lithofacies and structural settings, this thesis contributes towards a generalised understanding of the controls on fault permeability in porous carbonate rocks.