Deformation style and hydraulic properties of tight carbonates within Apulian platform and slope-to-basin succession (southern Italy)

The present Ph.D. Thesis dissertation includes the results of research projects aimed at focussing on the three following topics: 1) structural features of platform carbonates in the Apenninic foreland; 2) structural features of slope-to-basin carbonates in the Apeninnic foreland; 3) discrete fracture network modelling of fractured and faulted carbonates. 1. Structural features of platform carbonates in the Apeninnic foreland. - Structural properties of fractured and faulted Cretaceous platform carbonates, Murge Plateau (southern Italy). Upper Cretaceous platform carbonates, which represent good analogues of the fractured and faulted carbonate oil reservoirs of southern Italy, are widely exposed in the Murge Plateau (Apulian Region). Here, a detailed field analysis focused on defining the structural architecture of fault and fracture networks. The study outcrops are represented by large and inactive quarries located in three main council areas: Altamura, Gioia Del Colle and Minervino. Within these quarries, the exposed well -bedded carbonates are crosscut by a set of bed-parallel stylolites and two sets of bed-perpendicular cross-orthogonal joints/veins. These structural elements were likely formed under vertical loading during burial diagenesis and flexure of the Apulian foreland of the Southern Apennines fold-and-thrust belt. Bed-parallel stylolites and bed-perpendicular cross-orthogonal joints/veins represent the background deformation that was overprinted by the fault-related localized deformation. The fault sets documented in the study area are arranged in two kinematically-compatible fault networks. The first one is made up of WNW-ESE and NNW-SSE oriented strike-slip faults, right- and left-lateral, respectively, and NW-SE oriented normal faults. The second fault network consists of WNW-ESE oriented left-lateral strike-slip faults, and NE-SW oriented normal faults. First, both architecture and dimensional parameters of the fault and fracture networks have been characterized and computed by means of statistical analysis. Then, the permeability structures associated to the aforementioned networks have been assessed in order to determine the role exerted by fault architecture and dissolution/cementation processes on the fluid storage and migration pathways within the studied platform carbonates. Network 1 faults shows a quite variable fluid behavior, in which the fluid flow is strongly affected by inherited structural elements and karst dissolution, whereas network 2 faults show a more uniform, fluid conduit behavior. - Bedding-parallel stylolites in shallow-water limestone successions of the Apulian Carbonate Platform (central-southern Italy). Bedding-parallel stylolites typically represent the product of chemical compaction (overburden weight-induced pressure solution) experienced by carbonate successions during their burial history, when bedding is still horizontal. Due to their common occurrence in carbonate rocks, with lateral extents that can exceed 1 km, bedding-parallel stylolites are of special interest for the hydrocarbon industry because they may affect the regional fluid flow in the subsurface. Aimed at assessing the development and distribution of bedding-parallel stylolites in shallow-water, platform limestone successions, field and laboratory studies were carried out on Cretaceous limestones originally pertaining to the Apulian Carbonate Platform realm and now exposed in three distinct Italian locations: Maiella Mountain, Gargano Promontory and Murge Plateau. Results point to a prominent role played by the geological characteristics of limestones on development and localization of bedding-parallel stylolites within shallow-water, platform limestone successions. In particular, bedding-parallel lamination and fine rock grain size, co-occurring in stromatolitic limestones, determined there laterally more extensive and closely spaced stylolites than in the associated calcilutites and calcarenites. Large fenestral pores, which are ubiquitous in stromatolitic limestones, represent rock heterogeneities able to influence the roughness of individual stylolites. Laboratory measurements revealed that the permeability of the studied Cretaceous limestones is very low ( minus 10 μD). Pilot tests suggest that bedding-parallel stylolites in stromatolitic layers are not barrier to fluid flow but may represent pathways through low-permeability, platform limestone successions in the subsurface. 2. Structural features of slope-to-basin carbonates in the Apeninnic foreland. - Structural features of mass-transport deposits within carbonates from southern Italy. A detailed characterisation of submarine mass-transport deposits (MTDs), in terms of both emplacement processes and internal architecture of depositional products, is crucial to define the hydraulic properties of slope-to-basinal deposits. When compared to underlying and overlying undeformed strata, MTDs form distinct mechanical units and may represent significant heterogeneities within sedimentary successions and may act as barriers or conduits for fluid flow in the subsurface. The Late JurassicEarly Cretaceous basinal Maiolica Formation exposed in the Gargano Promontory (southern Italy) represents an ideal natural laboratory to study the complex stratigraphic architecture of ancient MTDs. This formation consists of undisturbed intervals of flat-lying, thin-bedded, cherty micritic limestone interbedded with chaotic intervals of lithologically similar strata characterized by significant internal distortion. The stratigraphic thickness of these deformed beds, which are interpreted to represent several types of mass movements (e.g., slumps and, to a lesser extent, slides and debris flows), varies from several decimetres to tens of metres. The internal deformation features comprise down-slope verging folds, and both normal and reverse faults. In several places, the studied MTDs exhibit signs of reworking, as demonstrated by reactivation of the slump-related faults resulting in deformation of beds overlying the MTDs. Moreover, the internal architecture of studied MTDs is discussed in the context of triggering mechanisms related to the characteristics of the Cretaceous paleoslope of the Apulian Platform. - Tectonically- and gravity-driven deformation structures in slope-to-basin carbonates, the Gargano Promontory (Italy). The aim of this work was to decipher and characterize the deformation along the palaeoslope of the Apulian platform margin, where a strong interaction of rooted (tectonically-induced) and superficial (gravity-driven) structures takes place. The study area is located in the Gargano Promontory, where Cretaceous slope-to-basin carbonate rocks are exposed. These sedimentary rocks are correspondent with Apulian platform margin, oriented NE-SW and NW-SE, in the southern and northern portion of the promontory, respectively. Here we documented pre-lithification tectonically- and gravity-driven deformation structures, as well as more recent faults. Cretaceous tectonic features have been arranged into two principal sets of faults: (i) NW-SE normal faults and (ii) WNW-ESE strike-slip and transtensional faults. We discussed pre-lithification structures in a context of paleogeography of the Cretaceous platform margin and tectonic history of the area. Post-Cretaceous faults are discussed in the framework of the Pliocene-Pleistocene tectonic evolution of the Apennines and its foreland. Additionally, the architecture of the faults of the different genesis and timing has been discussed in terms of their hydraulic properties. With the aim to study the mechanical and the chemical processes in the fault zone and to evaluate their effect on the fault permeability, several fault rock samples have been collected along fault zones crosscutting slope-to-basin carbonates in the Gargano Promontory. At the moment, only preliminary data concerning the relative amount and composition of matrix, cement and clasts, and 2D porosity are presented on samples collected along a normal fault in the Lower Cretaceous Casa Varfone Formation. 3. Discrete fracture network modelling of fractured and faulted carbonates. - Fracture properties analysis and discrete fracture network modelling of fault-related fractured tight limestones, Murge Plateau, Italy. The modelling natural fracture in reservoirs requires, as input data, the results of a previous detailed and accurate analysis of the 3D fracture network. This data could be derived from well logs and production tests, which however limit our understanding of the fracture geometry, intensity and distribution, and outcrop analogues. Data obtained applying scanline and scanarea methodologies on rocks exposed at the surface, in fact, often allow the construction of numerical models quite representative of natural fractured reservoirs. This paper deals with the DFN modelling of natural fractures associated to strike-slip faults crosscutting tight carbonates, which are exposed along vertical walls and pavements of an inactive quarry of the Murge area, southern Italy. Indeed, the study outcrops expose the inner structure of two conjugate fault zones striking WNW-ESE and NNW-SSE, respectively. DFN models were built according to the spatial and dimensional properties computed for the natural fracture network. The results of such a modelling show that the overall fault permeability is 3 to 4 orders of magnitude higher than the host rock permeability. The fault damage zones form the main fluid conduits, with the highest permeability values computed for fault-parallel fluid flow. Such a pronounced permeability anisotropy obtained for the fault damage zone is mainly related to the fracture dimension, both lengths and heights, and their aperture values. - Characterization of the permeability anisotropy of Cretaceous platform carbonates by using 3D fracture modelling: the case study of Agri Valley fault zones (southern Italy). In the Agri Valley, high-angle faults crosscut platform carbonates that are analogues of the lithological units that host the deep seated largest onshore oil reservoir in Europe. The main faults are W-NW oriented with a left-lateral strike-slip kinematic; additionally, three sets of related secondary faults are present: (i) N-NE oriented with right-lateral/transtensional kinematics, (ii) E-W trending left-lateral transtensional and (iii) N-NW trending left-lateral transpressional. Two of the secondary N-NE striking faults, strike-slip and transtensional, together with the adjacent host rock, were selected to build a Discrete Fracture Network model eventually used to evaluate the hydraulic properties and permeability anisotropy of these faults. The outcomes of this modelling show that the total permeability of the fault zones is higher than that one of the host rock. Moreover, the results are consistent with the transtensional fault having higher permeability values relative to the strike-slip one. The permeability anisotropy within the fault damage zone as well as in the host rock is mainly related to the fracture orientation. Overall this contribution gives new insights on how the geological setting where tight carbonates are formed affects the style of deformation of this rock and how later deformation influences structural and hydraulic properties of studied tight carbonates. The ultimate goal of this contribution is to reduce uncertainties in prediction of anisotropy imparted by faults and fractures on fluid flow within platform and slope-to-basin carbonates.