Characterization of a fractured carbonate reservoir analogue in the southern Apennines (Italy)

In carbonate reservoirs characterized by low matrix porosity, fracture networks represent the main factor controlling fluid migration and consequently reservoir quality. As fracture distribution in buried reservoirs is difficult to study in detail, outcrop analogues represent the most relevant source to predict the fracture network characteristics in three dimensions and to constrain their evolution through geological time. In this study, joints and veins have been analyzed at various scales in a carbonate succession cropping out in the southern Apennines. This succession is considered as a geological analogue of subsurface carbonate reservoirs hosting the Val D’Agri and Tempa Rossa oil fields in the Basilicata region. The studied succession, located at Mt. Chianello (Campania region), comprises a 1200 m thick sequence of Cretaceous shallow water carbonates characterized by an alternation of dolomitic and calcareous beds, with variable textures and crystal sizes. The first phase of this work is focused on the structural analysis of this kilometre-sized outcrop of allochthonous carbonate units by means of the study of faults and fractures and their relationships with folds, as well as their crosscutting relationships. Structural and paleostress analyses allowed the writer to unravel a superposed deformation pattern within a general framework of convergent continental margin evolution, following the stages of Mesozoic extension. The reconstructed tectonic evolution involves: (i) early extensional faulting and fracturing associated with bending of the foreland lithosphere during forebulge and foredeep stages (including the development of both ‘tangential’ and ‘radial’ normal faults and tensile fractures; Early-Middle Miocene); (ii) large-scale thrusting and folding (Late Miocene); (iii) transcurrent faulting (including two distinct sub-stages characterized by different remote stress fields; Pliocene-Early Pleistocene), and (iv) extensional faulting (late Quaternary). Stage (i) normal faults – generally occurring as conjugate sets – and related fractures and veins are variably deformed and overprinted by later horizontal shortening. Despite having experienced such a long and complex structural history, the studied carbonates are characterized by a ‘background’ fracture network – including two joint/vein sets orthogonal to each other and to bedding – that appears to be associated with the early fault sets that formed during the first (foredeep/forebulge-related) deformation stage. Therefore, away from younger (Late Miocene to Quaternary) fault zones, the permeability structure of the studied carbonates appears to be essentially controlled by the early, inherited fracture network. As a similar fracture network is likely to characterize also the buried Apulian Platform carbonates, representing the reservoir units for major oil fields in southern Italy, the results also bear possible implications for a better understanding of fluid flow in the subsurface and related hydrocarbon production. The next part of this work is mainly aimed at analyzing the dependence of different rock parameters such as lithology, bed thickness, crystal size and matrix porosity controlling the spatial and statistical distribution of stratabound and non-stratabound fracture systems away from major fault damage zones. Fracture analysis carried out on Aptian-Albian limestone and dolomite adjacent mechanical layers points out that, although the studied carbonates have been subjected to a uniform regional stress field, the deformation style may vary within the different mechanical beds. Micro-scale studies, performed on thin sections and acetate peels representative of adjacent mechanical layers, showed that the longitudinal strain in carbonate rocks is accommodated by joints, veins (also at the outcrop scale), mechanical twins, and micro-cracks in dolomite crystals. The results of the statistical analysis of non-stratabound micro-fracture density indicate that the role of crystal size largely overcomes that of lithology in controlling micro-scale rock strain, as fine-grained dolomites behave similarly to limestones with a comparable particle size. In particular, dolomites exhibit an inverse relationship between crystal size and fracture density (excluding micro-cracks), well described by a power law. On the other hand, crystal size also represents the main factor controlling the amount of finite strain related to the ratio between micro-fractures and micro-cracks in dolomites. Generally this ratio tends to increase with the increasing crystal size. Although the micro-cracks are ubiquitous in coarse dolomites, their role in the fluid circulation within the rock mass is negligible, being non-connected and sealed by mineralization. For all of these reasons, crystal size influences significantly the hydraulic behavior of non-stratabound fractures systems. On the other hand, the analysis of fractures from single-bed scan lines provides confirms the well-known notion that bed thickness is the dominant parameter controlling stratabound fracture distribution, whereas lithology and textures do not play a major role. The third and final section of this thesis includes a structural and diagenetic investigation performed on microstructures (veins and stylolites) hosted in the Aptian-Albian part of the analyzed succession. This study documents the different episodes of vein (fracture) formation, interpreted in relation to the kinematic evolution of the Mt. Chianello ridge and inferred paleostress conditions. Based on previously described tectonic history, five different vein sets have been recognized matching the macro- and micro-scale observations with cathodoluminescence analysis. The isotopic composition of cements filling the different fracture sets revealed a limited interaction with the host rock. The densely spaced early veins, forming together with two tensile joint sets the planar structure network affecting the whole succession, record multiple fracturing events. Early fractures, veins and normal faults formed as a consequence of the extension related to foreland bending. Subsequently during the incorporation of the Mt. Chianello carbonates in the Apennine accretionary wedge, an early layer parallel shortening caused the formation of bedding-parallel veins and bedding-normal stylolites. Finally, during the following uplift and final emersion stages, the latest calcite cement generations were precipitated from meteoric fluids circulating along fractures associated with strike-slip fault systems and late extensional faults. Despite the numerous episodes of cementation, obliterating most of the small fractures, a considerable number of larger fractures (joints) showing on average aperture higher than 0.5 mm are still open and appear to control fluid flow in the surface analogue. The fracture analysis carried out on the Mt. Chianello carbonates represents an integrated approach that can be applied to fractured layered carbonate analogues. The results of different parts of this thesis can provide useful parameters for the characterization, modelling and simulation of carbonate reservoirs and in general and of reservoirs for the hydrocarbons of the Basilicata region specifically.