Along-strike permeability variation in carbonate-hosted fault zones

Abstract

Variation in permeability along-strike of 10 faults, from differing lithofacies, host porosities and juxtaposition types, has been examined to improve our ability to understand controls on fault core development in carbonates, which in turn will help to predict the likely impact of faults on fluid flow. A large range in the permeability values has been recorded, up to 7 orders of magnitude variation in one example, created by significant fault core microstructural heterogeneity. A number of factors influence fault core development, and hence also control along-strike variation in microstructures and permeability. Juxtaposition of lithofacies is one key factor that appears to dictate the degree of heterogeneity along the fault core, where an increased microstructural heterogeneity is observed with juxtaposition of differing lithofacies, due to a variety of mechanisms active. This leads to a fault core with highly variable permeability values. Conversely, an increased microstructural homogeneity is observed in fault cores where the same or similar lithofacies are juxtaposed, with limited mechanisms active. However, a wide range in permeability has also been documented in these scenarios, but often far lower than scenarios of juxtaposition of different lithofacies: up to a maximum of 4 orders of magnitude has been recorded. However, the permeability range is usually much narrower, at around 2 orders of magnitude, with fractured outliers causing an increase to the spread of permeability values. Although the range of mechanisms active are not varied, the intensity of deformation and diagenetic processes does vary. Further, any later-stage processes such as fracturing and/or dissolution overprints the original textures. Both factors act to increase the permeability heterogeneity of the fault core.