To fully characterise the behaviour of carbonate rocks in the subsurface it is important to understand their textural heterogeneity, and how their textures may be modified by faulting. A number of fault zones were investigated in detail, firstly analysing the microstructural, petrophysical as well as mechanical properties of the host rocks. Secondly, describing the fault zone architectures by mapping fault rock distributions and fracture patterns. Lastly, correlating the deformation mechanisms forming the faults to the initial rock properties and the stress conditions during faulting. Moreover, triaxial laboratory deformation was performed on a large number of host rock samples covering all carbonate rock types, as well as the whole range of porosities (<1-52%). Deformation mechanisms that resulted in sample’s failure were studied in order to compare them with the naturally-occurring deformation. Moreover, permeability changes were investigated induced both by natural faulting and laboratory deformation. The results proved to be comparable, and showed that simplified rules may be derived in terms of predicting hydraulic properties of deformed carbonates. For instance, permeability generally seems to decrease due to deformation for carbonates with porosity >10%, and may be either increased or decreased for lower porosity samples. Higher porosity (>10%) carbonates fail due to distributed or localized cataclastic flow or focused damage around the macropores, resulting in porosity reduction. Lower porosity (<10%) carbonates fail in a brittle manner due to brecciation and transitional- or brittle-shearing, leading to porosity increase. Significant reduction in permeability, however, may only be produced by diagenetic processes, such as recrystallization and cementation, or very high-strains, which are able to create
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fine-grained cataclasites. However, even though these fault rocks gain very low permeability, they become prone to brittle deformation. Therefore, these potentially sealing fault rocks may be cut by open fractures if were subjected to further faulting or uplift, and hence, while creating permeability anisotropy in the reservoir, they may not form good seals. Nevertheless, several fault examples in this study showed fracture blunting at the surface of the fault rocks suggesting that fault sealing is possible both in highly-porous and very tight carbonates.
