--> The Permeability Structure Of Sandstones Adjacent To A Reverse-Reactivated Fault

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The Permeability Structure Of Sandstones Adjacent To A Reverse-Reactivated Fault

Abstract

Brittle fault zones form primary controls on the mechanics and fluid flow properties of the Earth’s upper crust. Fault zones are characterised by fault cores which are often surrounded by a peripheral zone of fracturing and faulting, known as the damage zone. The permeability structure of the damage zone can control fluid flow at a range of scales, from grain scale micro-structures and pore fabrics to outcrop scale structures. An understanding of the permeability structure and transmissibility of fault zones can have profound implications for petroleum exploration and development. Previous investigations on fault zone permeability structure often focus on low porosity (less than 5%) host rocks, while studies on porous sedimentary host rocks, which commonly form reservoir rocks, are limited. We present mineralogical and geomechanical data from porous sandstones (Eumeralla Formation) collected at the Castle Cove Fault at Castle Cove in the Otway Basin, southeast Australia. The Eumeralla Formation is a fine-grained volcanogenic sandstone deposited as syn-rift sediments during the mid-Cretaceous. The Castle Cove Fault is a northeast to southwest striking fault with a strike length of approximately 30 km. The fault was initiated as a normal fault during the late Cretaceous and was subsequently reactivated as a reverse fault during the Neogene. At Castle Cove, a total of ten orientated sample blocks were collected in the hanging wall at distances between 0.5 to 225 m from the Castle Cove Fault plane. From the sample blocks, core plugs were drilled in three orientations with respect to the fault plane; normal to the fault plane (x), along fault strike (y), and parallel to fault dip (z). Thin sections were also prepared in each orientation for detailed mineralogical and microstructural analysis. The orientated core plugs were used to measure changes in porosity, permeability, pore throat size, and pore connectivity relative to the Castle Cove Fault plane. Porosity increases by approximately 10% (from 17 to 24%) and permeability increases by two orders of magnitude (from 0.04–2.92 mD) as the fault plane is approached. There is also a progressive increase in pore throat size and pore connectivity closer to the fault plane. However, grain size decreases adjacent to the fault plane. High-resolution thin section analyses reveal an increase in microfractures within quartz and K-feldspar grains and an increase in deformation of authigenic clays adjacent to the Castle Cove Fault. Enhancement of the permeability structure of the sandstones adjacent to the Castle Cove Fault is attributed to the formation of these grain-scale structures and the change in clay morphology as a result of faulting. The results from this study have important implications for understanding the reservoir properties of high porosity, low permeability, and clay-rich sandstones.