The influence of reverse-reactivated normal faults on porosity and permeability in sandstones: a case study at Castle Cove, Otway Basin

Abstract

An understanding of fault zone structure and transmissibility can have significant implications for reservoir appraisal and development within petroleum systems. Studies tend to focus on low porosity host rocks that have experienced simple tectonic histories while the influence of complex fault systems (that have undergone multiple phases of deformation) on porous rocks within fault damage zones have not been investigated. We present results from a detailed mineralogical and geomechanical investigation of the Castle Cove Fault within the Otway Basin at Castle Cove, southeast Australia. Castle Cove provides excellent exposures of the Lower Cretaceous Eumeralla Formation, which is a fine-grained volcanogenic sandstone with moderate to high porosity (up to 27%) and low permeability (mostly <1 mD). The Castle Cove Fault originated as a normal fault during the late Cretaceous and was reverse-reactivated during Miocene–Pliocene compression. Core plugs were sampled at distances between 0.5 to 225 m from the fault and were orientated with respect to the fault plane. We show that closer to the fault (within 75 m), porosity increases by nearly 10% (from approximately 17% to 24%) and permeability increases by two orders of magnitude (from 0.04 mD to 2.92 mD). Microstructural investigations from thin sections show an increase in microfracture intensities closer to the fault. Also observed is a change in the morphology of pore-lining chlorite, from well-structured away from the fault to broken up and disaggregated adjacent to the fault. This study highlights the importance of detailed mineralogical and petrophysical analyses when attempting to understand the reservoir properties of high porosity and low permeability sandstones.