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Host Rock Controls on the Petrophysical Properties of Carbonate-Hosted Fault Rocks: A Case Study From Malta


During faulting, the undeformed host rock is typically subject to damage within a deformation zone. Through increasing fault displacement, the magnitude of deformation along the fault plane generally increases whilst the zone of deformation widens. This results in a deformation series that produces a suite of fault rocks throughout the fault zone, each with differing textures and petrophysical properties. The deformation mechanisms through which these fault rocks are formed are dependent upon a number of factors, including the physical and textural properties of the undeformed host rock. Therefore, the deformation series and flow properties of the fault zone products are specific to the juxtaposed lithofacies. This is well understood within siliciclastic rocks, linking clay content of the host sediment, the time of faulting, and the depth of burial to the resultant fault rock type, each with a known permeability range. However, there is currently no predictive tool to link the host rock parameters, such as lithofacies, to fault rock properties within carbonate rocks. In this study, we present a detailed analysis of the microstructural and petrophysical properties of fault rocks present in Malta and how they relate to the deformation series of each studied lithofacies. Host and fault rocks were sampled from a number of Maltese fault zones hosting tens to hundreds of metres displacement. Host and fault rock microstructures are characterized using optical and SEM imagery, aiding the understanding of the deformation series present in Maltese fault zones. The petrophysical properties associated with the deformation series have been characterized using various techniques, including permeametry, porosimetry, mercury injection capillary pressure (MICP), nuclear magnetic resonance (NMR), and X-ray tomography. The links between host rock properties and the across fault flow potential within the deformation series are investigated in an effort to obtain some quantifiable relationships that may be applicable to other normal faulted shallow burial limestones. The application of these relationships to studied fault zones in similar settings help to both verify and further constrain the findings within this study. The use of these relationships aid the development of a predictive tool for across fault fluid flow in hydrocarbon reservoirs, particularly at carbonate - carbonate juxtapositions.