Textural Controls on the Permeability and Structure of Fault Zones in Shallow Burial Limestones, Malta

Abstract

Summary The lack of a predictive tool for calculating fault permeability in carbonate reservoirs has led to an increasing amount of research towards the permeability structure of carbonate-hosted fault zones. However, a better understanding of fault rock distributions and their potential petrophysical properties is required to predict the impact of faults in carbonate reservoirs. This research combines structural, microstructural and petrophysical data from a series of carbonate-hosted fault zones in Malta, enabling an understanding of the fault zone permeability structures in various lithofacies, whilst highlighting the heterogeneity on all scales of carbonate-hosted fault zones. From the studied exposures, fault displacements of 30 m are required for a continuous fault core, but 100–200 m displacement is required for a continuous cataclasite veneer. Fault rocks have reduced macro-scale heterogeneity compared to host rocks, whilst the outcrop scale heterogeneity is increased. Fault rock permeability measurements show that permeability is reduced relative to the host rock for all faulted lithofacies. Cataclasite exhibits the lowest permeability (geometric average = 10−3 mD). These reductions are large enough to have a noticeable impact on fluid flow over production scales for the c.60% of high porosity fault rocks, however less than 10% of fault rocks derived from low porosity host rocks exhibit these permeability reductions. 30% of fault rocks derived from high porosity host rocks exhibit permeability reductions sufficient to behave as a significant barrier to fluid flow.