--> The Permeability Structure Of Sandstones Adjacent To A Reverse-Reactivated Fault
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AAPG Asia Pacific Region GTW, Pore Pressure & Geomechanics: From Exploration to Abandonment

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The Permeability Structure Of Sandstones Adjacent To A Reverse-Reactivated Previous HitFaultNext Hit

Abstract

Brittle Previous HitfaultNext Hit zones form primary controls on the mechanics and fluid flow properties of the Earth’s upper crust. Previous HitFaultNext Hit zones are characterised by Previous HitfaultNext Hit 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 Previous HitfaultNext Hit zones can have profound implications for petroleum exploration and development. Previous investigations on Previous HitfaultNext Hit 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 Previous HitFaultNext Hit 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 Previous HitFaultNext Hit is a northeast to southwest striking Previous HitfaultNext Hit with a strike length of approximately 30 km. The Previous HitfaultNext Hit was initiated as a normal Previous HitfaultNext Hit during the late Cretaceous and was subsequently reactivated as a reverse Previous HitfaultNext Hit 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 Previous HitFaultNext Hit plane. From the sample blocks, core plugs were drilled in three orientations with respect to the Previous HitfaultNext Hit plane; normal to the Previous HitfaultNext Hit plane (x), along Previous HitfaultNext Hit strike (y), and parallel to Previous HitfaultNext Hit 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 Previous HitFaultNext Hit 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 Previous HitfaultNext Hit plane is approached. There is also a progressive increase in pore throat size and pore connectivity closer to the Previous HitfaultNext Hit plane. However, grain size decreases adjacent to the Previous HitfaultNext Hit 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 Previous HitFaultNext Hit. Enhancement of the permeability structure of the sandstones adjacent to the Castle Cove Previous HitFaultTop 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.