Spatial distribution of micrometre‐scale porosity and permeability across the damage zone of a reverse‐reactivated normal fault in a tight sandstone: Insights from the Otway Basin, SE Australia

Abstract

Knowledge of the permeability structure of fault‐bearing reservoir rocks is fundamental for developing robust hydrocarbon exploration and fluid monitoring strategies. Studies often describe the permeability structure of low porosity host rocks that have experienced simple tectonic histories, while investigations of the influence of faults with multiple‐slip histories on the permeability structure of porous clastic rocks are limited. We present results from an integrated petrophysical, microstructural, and mineralogical investigation of the Eumeralla Formation (a tight volcanogenic sandstone) within the hanging wall of the Castle Cove Fault which strikes 30 km NE–SW in the Otway Basin, southeast Australia. This late Jurassic to Cenozoic‐age basin has experienced multiple phases of extension and compression. Core plugs and thin sections oriented relative to the fault plane were sampled from the hanging wall at distances of up to 225 m from the Castle Cove Fault plane. As the fault plane is approached, connected porosities increase by ca. 10% (17% at 225 m to 24% at 0.5 m) and permeabilities increase by two orders of magnitude (from 0.04 mD at 225 m to 1.26 mD at 0.5 m). Backscattered Scanning Electron Microscope analysis shows that microstructural changes due to faulting have enhanced the micrometre‐scale permeability structure of the Eumeralla Formation. These microstructural changes have been attributed to the formation of microfractures and destruction of original pore‐lining chlorite morphology as a result of fault deformation. Complex deformation, that is, formation of macrofractures, variably oriented microfractures, and a hanging wall anticline, associated with normal faulting and subsequent reverse faulting, has significantly influenced the off‐fault fluid flow properties of the protolith. However, despite enhancement of the host rock permeability structure, the Eumeralla Formation at Castle Cove is still considered a tight sandstone. Our study shows that high‐resolution integrated analyses of the host rock are critical for describing the micrometre‐scale permeability structure of reservoir rocks with high porosities, low permeabilities, and abundant clays that have experienced complex deformation.

The Effect of Authigenic Clays on Fault Zone Permeability

“…Microstructural results from this study have shown that: (a) Clay assemblages typical of both kaolinite and chlorite have been progressively deformed and disaggregated with decreasing distance to the fault surfaces in both the Scapa sandstone and the Eumeralla sandstone; and (b) crystal diameters of both authigenic kaolinite and authigenic chlorite crystals have been reduced close the fault surfaces (Figure 7). Relationships observed between diagenetic clays and tectonic fractures indicate that precipitation of clays occurred pre‐ and syn‐deformation, as clays were not present in large transgranular fractures observed in BSE images (Debenham et al., 2019; Farrell & Healy, 2017) and–in the case of kaolinite precipitation–were related to fracturing and dissolution of K‐feldspar. Based on this deformation history, crystal size reduction is interpreted as being caused by comminution via cataclasis or crushing rather than nucleation of smaller, poorly crystallized grains in previously faulted rocks.…”

“…The Eumeralla Formation sandstone is noted for its thick pore‐lining authigenic chlorite (Debenham et al., 2019; Duddy, 2003) which constricts/narrows pore throats and, even in this high porosity sandstone, greatly limits the magnitude of permeability. This study has also shown how fault deformation of authigenic chlorite has altered the distribution of aggregate forms by destroying pore residing chlorite garlands and disaggregating euhedral rosettes of pore lining chlorite (Figure 7).…”

“…Grain size distributions were quantified for Eumeralla sandstones using laser particle analysis. Results showed a weak to moderate linear relationship of reducing grain size with increasing proximity to the fault surface (Section 4.5. and Figure 8 of Debenham et al, 2019). Smaller grains would be packed more tightly and decrease pore throat sizes so would not explain the observed patterns of increasing pore throat sizes with increasing proximity to the fault surface.…”

“…In the North Scapa Fault (Orkney, UK) damage zone sandstones contain authigenic kaolinite, while at the Castle Cove Fault in Victoria (Australia), damage zone sandstones contain authigenic chlorite. Previous analysis of these two samples suites have shown relationships between fault rock pore fabric and anisotropic permeability (Farrell & Healy, 2017) and increasing porosity and permeability (Debenham et al., 2019). However, although these papers both noted changes in authigenic clay distributions with increased fault related deformation, they did not document or quantify the profound effect that even minor amounts of pore and throat filling authigenic clay can have on sandstone properties (Wilson & Pittman, 1977).…”

“…Pore size distributions show a visibly bimodal pattern of large intergranular pores (up to 500 μm diameter) with irregular perimeters and smaller intergranular pores encircled by pore lining chlorite. Extensive fine, intragranular microfractures are observed in unfaulted and faulted samples and contribute to a fraction of the porosity but show no common alignment and no change in the intensity/density of the microfractures with proximity to the fault (Figure 4b and Supporting Information S1) (Debenham et al., 2019).…”