Permeability of carbonate fault rocks: a case study from Malta

Abstract

The inherent heterogeneity of carbonate rocks suggests that carbonate-hosted fault zones are also likely to be heterogeneous. Coupled with a lack of host–fault petrophysical relationships, this makes the hydraulic behaviour of carbonate-hosted fault zones difficult to predict. Here we investigate the link between host rock and fault rock porosity, permeability and texture, by presenting data from series of host rock, damage zone and fault rock samples from normally faulted, shallowly buried limestones from Malta. Core plug X-ray tomography indicates that texturally heterogeneous host rocks lead to greater variability in the porosity and permeability of fault rocks. Fault rocks derived from moderate- to high-porosity (>20%) formations experience permeability reductions of up to six orders of magnitude relative to the host; >30% of these fault rocks could act as baffles or barriers to fluid flow over production timescales. Fault rocks derived from lower-porosity (<20%) algal packstones have permeabilities that are lower than their hosts by up to three orders of magnitude, which is unlikely to impact fluid flow on production timescales. The variability of fault rock permeability is controlled by a number of factors, including the initial host rock texture and porosity, the magnitude of strain localization, and the extent of post-deformation diagenetic alteration. Fault displacement has no obvious control over fault rock permeability. The results enable better predictions of fault rock permeability in similar lithotypes and tectonic regimes. This may enable predictions of across-fault fluid flow potential when combined with data on fault zone architecture.

Key controls on the hydraulic properties of fault rocks in carbonates

“…This has been observed and documented previously in both siliciclastic (e.g. Shipton and Cowie 2003) and carbonate rocks (Cooke et al, 2020), showing the control of porosity on deformation style. Moreover, it is easier to reduce the permeability of a high permeability rock than one with an initial low permeability.…”

“…However, there is surprisingly little data on the porosity and permeability of carbonate fault rocks (e.g. Agosta et al, 2007;Bastesen et al, 2009;Haines et al, 2016;Michie and Haines 2016;Tondi et al, 2016;Cooke et al, 2020;Kaminskaite et al, 2020). By the time of this publication, authors were aware of only one publicly available documented study where petrophysical data has been used in a predictive sense for calculation of carbonate fault rock permeability and transmissibility multipliers in a cellular model (Michie et al, 2018).…”

Internal Structure Characteristics and Formation Mechanism of Reverse Fault in the Carbonate Rock, A Case Study of Outcrops in Xike’er Area, Tarim Basin, Northwest China

“…From the geological map and stratigraphic relationship on both sides of the outcrop fault, it can be seen that the Xike'er fault is a NW trending reverse fault with a certain strike-slip property, and the fault dip angle is about 83 °(Figure 2). The fault core is the most concentrated part of the rock mass displacement on both sides of the fault, it absorbs most of the displacement of the fault and develops sliding surfaces and fault rocks (Caine Saul et al, 1996;Cooke et al, 2019). The xike'er fault has a typical dual structure, and its fault core is characterized by multi-layer structure, which is filled with calcite vein, fault breccia and fault gouge along the sliding surface (Figure 3A).…”