Faults and Fractures in Carbonates

Worldwide, about 50 % of natural geofluids (i.e. mineral and hydrothermal waters, geothermal fluids, oil and gas) are hosted in carbonate reservoirs.1 Unlike other types of rocks, carbonates consist of a great variety of lithotypes. Based upon the nature and organisation/shape of the constituent elements (grains, pores, cement, minerals, etc.), which are strongly related to the depositional setting and in relation to their diagenetic evolution, carbonate rocks are characterised by a wide range of porosity and permeability.2,3 Besides the aforementioned primary controls on the petrophysical properties of carbonate rocks, the containment and migration of geofluids in carbonate rocks are strongly influenced by fault zones and fractures. In the last few years, numerous researches were aimed at understanding the faulting and fracturing processes in carbonate rocks, as well as the quantification of their spatial and dimensional distributions in relation to the physical-chemical properties of the various types of carbonates. Following these lines of research, the Faults and Fractures in Carbonates (F&FC) project of the University of Camerino, currently sponsored by a consortium of oil companies, is aimed at significantly improving current knowledge on the role of faults and fractures in the fluid flow properties of carbonate rocks. In several geological contexts, carbonate rocks are characterised by both diffused and localised strain, which can be represented by background deformation and fault zones, respectively. The combination of diffused and localised deformation may form a discontinuity network that affects the hydraulic rock properties. With regards to background deformation, the primary control on fracture types, spacing and connectivity is provided by the skeletal grain assemblage, heterogeneities such as bedding or inclusions and lateral/vertical variations in cementation and porosity. For what concerns the fault zones, in low-porosity carbonate rocks they form preferential sites for fluid migration or accumulation, 16,17 whereas they represent barriers to fluid migration in high-porosity carbonates.10,11,14 In the present contribution, we group the results of our research activities into three main subjects: (i) background deformation, in which we report an examples of studies aimed at investigating the control exerted by host rock heterogeneity on the development of background structures in layered carbonates; (ii) faulting of high-porosity carbonates, characterised by a faulting mechanism which determines both volumetric and shear strain localisation into narrow tabular bands that constitute seals for fluid flow;10,11 and (iii) faulting of low-porosity carbonates, in which we report a synthesis of our studies focusing on the deformation mechanisms, architecture, multiscale and petrophysical properties of various types of fault zones.