Faults and Fractures in Carbonates


In Figure 4D, we show data concerning the permeability structure computed for individual fault zones crosscutting basinal and slope-to-platform carbonates of central and southern Italy. All fault zones are characterised by high values of Fa indices, which means that the fault damage zone are thicker than the relative fault cores, suggesting a ‘distributed conduit’ permeability structure. Quantitative data of porosity (φ) and Klinkemberg permeability (k) measured for samples collected across large normal fault crosscutting tight platform carbonate rocks (φ <1%; k = 10-3 millidarcy [mD]) show that both values increase within the fault damage zone (up to φ = 3% and k = 10-1 mD) whereas they reach host rock values within the fault core (φ <1%, k = 10-4 mD).21


Recent papers14,20,22 demonstrated that the general validity of this


deformational-hydraulic characterisation of fault zones is influenced by several factors, among which the most relevant are:


• amount of fault displacement; • 3D fault geometry; • fault propagation mode; •


• pre-existing deformation; • •


Figure 4: Fault Volume Characteristics A


B Distributed conduit % Damage zone High % core


Low High


High Damage zone Fault core Protolith


100 10 1


% Damage zone Combined conduit-barrier


CD Ks increases


F4 F2


0.1 0.5 F1


F3 F5


1


100 150 200 250 300


50 0


0


0.5 Fa


A: Fault zone components; B: permeability structure; C: hydraulic properties of conduit-type faults (modified from Caine et al.,19964


Italy (from Cello et al., 200116 shear sense and fault kinematics; mechanical properties of fault rocks; and environmental conditions (i.e. overburden and pore fluid pressure).


As an example, the number of fractures away from the main slip surfaces varies in a different manner across normal and strike-slip faults that formed at the same time, but in different lithologies (see Figure 5B).


); D: permeability structure of the main fault zones in basinal carbonates outcropping in central ). FA = damage zone width/fault zone width; Ks = Klinkemberg permeability.


The impact of the aforementioned factors on the final architecture of fault zones implies that current predictive models of fault permeability are characterised by significant uncertainties. The reduction of these uncertainties is a primary target in the exploitation and management of the available geofluid resources. Most of the contributions included in a recent special issue of the Journal of Structural Geology15


provide new tools to better assess the fault architecture and related permeability structure in the subsurface. n


1 Localised barrier


Permeability structure


Low Low Low


% core High


Localised conduit


aim to


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Fault zone width (m)


Ks increases Fault zone width (m)


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