This study investigates how local stress state governs permeability magnitude in fractured carbonate aquifers. By using outcrop-constrained Discrete Fracture Network (DFN) modelling from Mt. Viggiano of the southern Apennines, Italy, we investigate the control exerted by 500 m-depth tri-axial local stress state on computed horizontal permeability anisotropy. Fractured carbonate systems commonly exhibit strong permeability anisotropies that evolve with depth as fractures respond to changes in both normal and shear stresses. Accurately capturing this behaviour remains challenging due to the combined effects of fracture geometry and connectivity, as well as primary depositional architecture and stress-dependent aperture modification.Field-derived fracture datasets from four carbonate outcrops representing two contrasting paleo depositional settings are used to construct three-dimensional DFN models at the bed-package scale. Two DFN-based modelling workflows are employed to explore how different representations of fracture connectivity and flow influence predicted permeability. One approach estimates bulk permeability from fracture population statistics within distinct geocellular volumes. Differently, the other one explicitly simulates steady-state fluid flow through hydraulically connected fracture networks within a fully meshed computational domain. This integrated strategy allows evaluation of how modelling assumptions related to connectivity, aperture scaling, and flow representation affect permeability predictions without implying a preferred modelling tool.The results of this study show that increasing normal stress generally reduces horizontal permeability anisotropy, although local increases in permeability occur where favourably oriented fractures undergo shear induced dilation. Result are also consistent with the permeability response varying systematically with depositional architectures: (i) massive, high-energy carbonates dominated by non-strata bound fractures exhibit vertically persistent but weakly connected networks; (ii) on the contrary, layered, low-energy carbonates containing abundant strata-bound fractures display enhanced lateral connectivity and higher hydraulic effective transmissivity.The main outcomes of this work demonstrate that permeability anisotropy in fractured carbonates evolves with stress through its interaction with fracture orientation, connectivity, and stratigraphic architecture. Incorporating stress dependent behaviour and explicit connectivity into DFN workflows therefore improves predictions of subsurface fluid flow relevant to groundwater resources, CO₂ storage, and geothermal systems.

The results of a multiscale study of fault and fracture geometry, distribution, density, and intensity are reported for Mesozoic platform carbonates cropping out along the axial zones of the southern Apennines fold-and-thrust belt, Italy. By integrating field structural observations with digital outcrop analysis, the study focuses on Cretaceous limestone rocks exposed along natural creeks and artificial trails of the Castelsaraceno area, Raparo Mt., southern Italy. There, the limestone beds are bounded by mm- to cm-thick marly–clayey interbeds, forming a well-layered succession made up of a few m-thick bed packages bounded by several cm-thick clayish interlayers. The carbonate multilayer was first affected by thrust tectonics, with the formation of low-angle intra-carbonate thrust faults and fault bend-folding. Then, the multilayer was crosscut by extensional–transtensional high-angle faults, which displaced the previously formed contractional structural elements, and allowed carbonate exhumation from shallow crustal depths. At outcrop scales, thrust-related deformation was solved by low-angle joints and veins, rare high-angle stylolites, and low-angle sheared fractures displaying reverse kinematics. Quantitative analyses of fracture density (P20) and intensity (P21) conducted on selected portions of the thrust fault zones indicate that the low-angle joints and veins attain their highest values in the vicinity of the main slip surfaces, whereas they are almost absent in the surrounding carbonate host rocks. Plio-Quaternary transtensional deformation was solved by NW–SE- and NE–SW striking faults. The latter fault set, nicely exposed along the flanks of the Raganello Creek, was characterized by right-lateral components of slip. Incipient faults, with ca. 1 cm throw, are made up of vertically discontinuous slip surfaces, which crosscut single bed packages and abut against clayish interlayers. The slip surfaces form conjugate geometries, and are associated to high-angle fractures and veins striking NE–SW, dissecting the bed packages. The fault core is virtually absent, whereas the damage zones are very discontinuous along dip. The P20 values computed for the high-angle fractures and veins increase toward the slip surfaces, whereas the P21 values remain nearly constant. These data are interpreted as being due to fault nucleation processes associated with fracture nucleation within the limestone rocks. NE–SW striking small faults displaying throws between 10 and 60 cm are comprised of through-going main slip surfaces crosscutting multiple bed packages, and poorly developed, discontinuous fault cores flanked by m-thick damage zones. The damage zones include sub-parallel high-angle shear fractures, fractures and veins showing a positive correlation between P20 and P21, whose values increase in the vicinity of the main slip surfaces. Such a positive correlation is interpreted as due to fault growth by linkage and coalescence of pre-existing high-angle fractures, and formation of fault-related joints and veins at the extensional quadrants of single shear fractures. Similarly, large-scale NE–SW striking mature faults with throws on the order of tens of meters, made up of a m-thick fault core and 10 s of m-thick damage zones including sub-parallel fractures and veins, also show a positive P20 and P21 correlation. The main outputs of this work are synthesized into a conceptual model illustrating the transition from thrust-related deformation to extensional–transtensional faulting, documenting the evolution of fracture networks from incipient-to-small-to-mature faults.

Fracture and fault networks are characterised by complex spatial and dimensional properties that might affect the flow and accumulation of subsurface fluids. In geological applications, DFN models are commonly employed to compute the multiscale properties of fractured rock volumes in terms of porosity and equivalent permeability. In the present contribution, we focus on an outcrop-to-reservoir investigation of Mesozoic shallow-water carbonates exposed along the axial zone of the southern Apennines fold-and-thrust belt, FTB, Italy. The carbonates were originally deposited in lagoon-to-proximal ramp settings of the Paleo Apenninic Platform during Lower Jurassic–Upper Cretaceous times and include well-layered and massive associations of bed packages several m-thick. By integrating the results of both field and digital structural analyses, we build multiple DFNs to assess the hydraulic behaviour of geocellular volumes representative of the different scales of observation, and stochastically populated with high-angle fractures, small, and medium faults. Fractures are either strata-bound, SB, or non-strata-bound, NSB, and results compartmentalized within single-bed packages. Small faults crosscut multiple bed-packages and show a few cm-to-m throws, whereas medium faults displace multiple bed-package associations and have throws > 1m. Both small and medium faults exhibit high peaks of fracture density, P20, and intensity, P21, in correspondence with the releasing jogs disrupting mechanical interfaces such as bed package boundaries and pre-existing low-angle thrust faults. As data input for DFN modeling, the aperture values of the stochastic fractures are set as proportional to either fracture length (most favourable flow conditions) or to the square root of fracture length (least favourable conditions). At the outcrop scale, 5m-side DFN models show the highest values of fracture porosity among those considered in this work. These results are therefore consistent with both SB and NSB fractures forming the main repository for fluid accumulation. At larger scales, the 50m-side and 500m-side DFN models including small and medium faults, are characterized by higher values of equivalent permeability, which range between 10-2 and 10-1 mD. Considering the computed Kxx and Kyy values for the single geocellular volumes, near-isotropic horizontal conditions are assessed across all scales of investigation. Accordingly, altogether, high-angle SB and NSB fractures, small and medium faults form a system of well-connected network through the shallow-water carbonates. Interpreting these data in light of published values for platform carbonates in Italy, we interpret this multiscale horizontal permeability isotropy due to the severe exhumation (~ 4 to 5km) the studied carbonates went through during the Quaternary downfaulting of the southern Apennines FTB.

Calcite is a common syn-kinematic precipitate in upper crustal fault zones coating slickensides, forming slickenfibers, and infilling veins. Structural and geochemical analyses of fault-related calcites can be used to unravel the source, distribution, and mixing of parental fluids in association with past fault activity. Identifying deeply sourced fluids through syn-kinematic calcites is of paramount importance, as the correlation of the ascent of deep sourced fluids with strong earthquakes, may allow using hydrogeochemical modifications in groundwater as potential seismic precursors. In this study, we investigate the origin of syn-kinematic paleo-fluids that circulated along the Val d’Agri faults, in southern Italy. These faults bound an intermontane basin topping the largest onshore oil field in Western Europe. Since the Val d’Agri Basin is affected by natural seismicity and low magnitude oil production induced earthquakes, it is necessary to assess the potential threats of hydrocarbon fault leakage at shallow crustal levels. With this aim, we collected about 350 syn-tectonic calcites along high-angle extensional-transtensional fault zones. By combining macro- and micro- scale structural observations with carbonate isotopes (C, O, clumped, and Sr) and rare earth elements and yttrium (REY) geochemistry, we identified 5 fluid sources: (1) meteoric waters in geochemical and thermal disequilibrium with the host rocks, which interacted with superficial soil; (2) meteoric waters in geochemical disequilibrium and thermal equilibrium with the host rocks, which had limited interaction with the host rocks; (3) buffered fluids in geochemical and thermal equilibrium with the host rocks; (4) high temperature fluids in geochemical equilibrium and thermal disequilibrium with the host rocks, which ascended from the carbonate hydrocarbon reservoir; (5) hot meteoric waters in thermal and geochemical disequilibrium with the host rocks, which mixed with the deeply sourced fluids. The presence of multiple fluids is consistent with an open fault-related circulation system, which allowed mixing of shallow and deep fluids through the high-angle extensional-transtensional Val d’Agri faults. Given the societal and economic issues of this area, the recognized involvement of deep fluids during past fault activity is crucial for the context of oil exploration and production as well as for environmental monitoring. Furthermore, this suggests that the Val d’Agri Basin is an ideal region to explore fluid-fault relationships throughout the entire seismic cycle through local seismicity records and continuous groundwater monitoring.

Diagenetic and tectonic processes taking place in platform carbonates produce significant textural and mineralogical modifications through time, controlling the pore space in terms of dimension, geometry, shape, and connectivity of single pores, and influencing both total and effective porosity. Focusing on the different types of solution surfaces, this study is conducted on Lower Jurassic, Cretaceous, and Eocene, carbonates exposed at the Viggiano Mt. and Raparo Mt., southern Apennines, Italy. Microscale analyses show that the primary porosity of these rocks was occluded by pervasive blocky cements, which precipitated during burial diagenesis of the carbonates. We aim to assess the role exerted by the roughness of bed-parallel and low-angle to bedding solution surfaces, on the pore properties and permeability values of a variety of carbonate lithofacies such as mudstones, packstones, grainstones and rudstones. Specifically, we show the results of Nuclear Magnetic Resonance (NMR), gas-porosimetry and water-permeability tests conducted on plugs cored either orthogonal or parallel to bedding interfaces. All the study plugs show an amount of effective porosity lower than 5%, with mean values of ca. 3%. Excluding larger microfractures, and sporadic intrafossil and intercrystal molds, among the various types of solution surfaces we document that the rough, seismogram-type stylolites localize secondary porosity, while smooth, wave-type stylolites do not. The seimogram type stylolites, due to the non-selective carbonate’s dissolution, form a poorly connected vuggy porosity, and the NMR results the pores are subspherical to tubular (r

Understanding the impact of local stress states on computed permeability for fractured carbonates or any other lithotype is crucial to better assess the modalities of fluid flow in the subsurface. Considering Mesozoic fractured carbonates exposed along the flanks of the Viggiano Mt. of southern Italy, we investigate the control exerted by the local fracture networks on the output of DFN modelling of geocellular volumes whose dimensions are like those of the studied outcrops. Specifically, the following four sedimentary units are considered:Scarrone La Macchia II, (SLM II), well-layered, Sinemurian–Pleinsbachian carbonate succession of wackestone-packstones to grainstones arranged in discrete bed packages originally deposited in a low-energy, open lagoon environment. Scarrone la Macchia I, (SLM I), Toarcian oolithic carbonates characterized by bed amalgamation originally formed in a ramp setting rimmed by oolithic sand shoals. Piana del Buon Cuore (PBC), Lower Cretaceous - Upper Jurassic limestones whose clasts consist of oolites, oncolites and intraclasts deposited in a high-energy platform margin environment. The Il Monte (ILM), massive, amalgamated, Cretaceous carbonate rudstones and grainstones originally deposited along the paleo-slope of the carbonate platform. By employing existing field data (Abdallah et al., 2023, 2024), we carried out Discrete Fracture Networks (DFN) modelling of 5 m-sided geocellular volumes including internal sub-volumes representative of single carbonate beds. This work was conducted by means of high-resolution computational meshes provided by the dfnWork ® code, which is capable of non-reactive solute transport simulation, and constrain of imposed depth-equivalent stresses to assessing effective horizontal permeability (effkxx, effkyy).Focusing on the results achieved for the Viggiano Mt. aquifer, we simulated depth conditions of 500m coupled with principal stress axes of ~13MPa (sv), 10 MPa (shmax, NW-SE), and 7.58 MPa (shmin, NE-SW). The theoretical aperture data were hence modulated by the local stress state conditions. SLM I, exhibits an increase in permeability anisotropy ratio as a function of the stresses, hence, leading to flow channelling within the network. Lagrangian solute transport simulation supports the afore-mentioned results by marked changes in primary flow path, increase in path tortuosity, as a function of stress, and delay in breakthrough time. Similar results were achieved for PBC and ILM units. Differently, SLM II undergoes the opposite effect, where the permeability ratio seems to reduce drastically at a depth of 500m and stabilizes after that. We infer this behaviour as due to non-linear-fluid-flow behaviour as a function of aperture closure or dilation, as seen in highly connected systems of fractures including both stratabound and non-stratabound elements. Accordingly, SLM II is characterized by efficient mechanical units made up of bed interfaces, which were able to compartmentalize the vertical growth of high-angle fractures.This research highlights the complex behaviour of permeability anisotropy in fractured carbonate rocks, in response to depth-equivalent stresses, and the importance of building realistic geomechanically coupled-DFN models to estimate fluid-flow and storage properties of fractured rocks at depth.

Understanding the origin and distribution of damage within carbonate-hosted fault zones is crucial, yet it remains a complex challenge, which hampers the overall assessment of their mechanical and hydraulic structure. In carbonate-hosted fault zones, shattered to intensely brecciated non-cohesive rocks have been reported. Although their origin has been related to the propagation of multiple seismic ruptures, great uncertainties persist regarding their interpretation and distribution. The NW-SE striking, approximately 15 km long Roccapreturo Fault, in the central Apennines of Italy, is an intriguing case study where non-cohesive fault rock domains occur within its footwall damage zone. These domains elongate in a NE-SW direction for ~200 meters from the main slip surface. We employed a multiscale approach to better understand the distribution and origin of the non-cohesive fault rocks. The fault geometry and throw distribution along the main fault segments were characterized through fault-perpendicular geological cross-sections. Virtual outcrop models of key exposures, located in and around an abandoned quarry, were constructed using Structure from Motion-Multiview Stereo photogrammetry. These models utilized photos taken with a Mavic Mini 2 drone. The interpretation of virtual outcrop models, combined with classical fieldwork, allowed us to map the damage and minor fault strands. The Roccapreturo Fault displaces Cretaceous rocks originally deposited in various depositional environments. Along its strike, from NW to SE, the fault intersects rocks from internal or restricted carbonate platform, margin, and proximal slope to basin depositional environments. Notably, non-cohesive fault rocks are exposed between the margin and proximal slope rocks. This area coincides with the maximum throw of the fault, which is ca. 600 meters, and with the intersection with a system of pre-existing NE-SW-striking steeply dipping faults. At the outcrop scale, faults exhibit two preferred orientations, parallel and perpendicular to the main slip surfaces of the Roccapreturo Fault, respectively. The former ones show predominant dip-slip kinematics, while the latter ones show both dip-slip and strike-slip kinematics. We interpret the distribution of non-cohesive fault rocks along the Roccapreturo Fault as influenced by its intersection with the NE-SW fault system, where most of the slip accumulated. Accordingly, the pre-existing NE-SW faults accommodated transtensional slip during latest extensional deformation and coeval rock exhumation from depth. The transition of the Cretaceous depositional environments, which was accommodated by the NE-SW-striking faults, therefore highlights the pivotal role of pre-existing anisotropies in dictating the distribution of damage, particularly of non-cohesive fault rocks, in carbonate hosted faults.

Shallow-water carbonates include a variety of heterogeneities such as bed interfaces, laminations, stylolites, and pressure solution seams forming rock multilayers crosscut by high-angle strata-bound fractures. At a larger scale, bed package interfaces and other primary stratigraphic contacts exert a similar control compartmentalizing high-angle faults within discrete sedimentary units. The aforementioned heterogeneities commonly form within the depositional environments, and/or at specific diagenetic conditions under given burial depths. Mechanical compaction, dissolution, cement precipitation, and other physical/chemical processes hence alter the original framework of the carbonates, and contribute to the acquirement of their long lasting mechanical properties. To further investigate this topic, in other words to assess the time-dependent fracture stratigraphy of shallow-water carbonates, this presentation focuses on the influence exerted by tectonics on the mechanical layering of the Mesozoic platform carbonates of southern Italy. By analyzing outcrops lying along the axial zone of the southern Apennines fold-and-thrust belt, and within its forebulge area, published data are discussed altogether to decipher the control exerted by thrusting tectonics on the formation of mechanical interfaces within Lower Jurassic to Upper Cretaceous carbonates. Rocks exposed in the foreland domain include a number of high-angle fractures. These fractures are mainly bounded by bed interfaces, and their spacing values vary proportional to the bed thickness. Such a proportionality is exhibited by both mud- and grain-supported carbonate lithofacies, which show saturated to oversaturate conditions. Differently, carbonates lying in the axial zone of the southern Apennines belt are characterized by values of fracture density and intensity that do not vary proportionally with the bed thickness. In order to investigate the significance of the latter data, detailed microstructural analyses aimed at assessing the timing of pressure solution processes with respect to the diagenetic history and tectonic evolution of the carbonates are considered. Besides the effects of early embrittlement of the carbonate grainstone lithofacies, which occurred due to cement precipitation in phreatic marine environment that prevented the effects of localized dissolution at the grain-to-grain contacts, two main phases of pressure solution characterized the carbonates. The first one took place during Meso-Cenozoic sedimentary burial with formation of wave-like, bed-parallel surfaces. In cat, the continuous burial of the carbonates, down to depths of ca. 1.5 km, promoted the development of solution surfaces along the bed interfaces, and also within the single beds. Small, isolated, wave-like surfaces formed as isolated elements within the single carbonate beds. The second phase occurred during Upper Miocene thrusting tectonics, at depths of ca. 4 km, with formation of seismogram-like surfaces at low-angle to bedding. The latter surfaces consisted of both stylolites and slickolites with sub-vertical teeth, which cut across the pervasive blocky cements of the carbonates, dissolved the pre-existing veins, and formed laterally persistent surfaces throughout the carbonates. Accordingly, the combination of both pure shear (stylolites) and sub-simple shear (slickolites) strain caused formation of new mechanical interfaces in the carbonate beds, and therefore modified the thickness of single mechanical units throughout the Mesozoic carbonates.

Focusing on the Mesozoic and Eocene carbonate samples from the Viggiano Mt. and Raparo Mt., Southern Apennines of Italy, we examine the role of solution surfaces on the present-day porosity and permeability. Carbonate lithofacies include mudstones, packstones, grainstones, and rudstones and gas-porosimetry measurements show effective porosity up to 5 %. The value is mainly due to the secondary porosity localized along microfractures and within the rough bed-parallel and rough bed-oblique solution surfaces, contrary to what was found in the smooth bed-parallel ones, showing that roughness can control pore localization. NMR (Nuclear Magnetic Resonance) results indicate that the former group of solution surfaces include vugs characterized by subspherical to tubular shapes (pore throat r < 3 mu m), and low aspect ratios (r < 2), forming pores with low sensitivity to compression. The microfractures form capillary porosity (r approximate to 1 mu m) and are characterized by high aspect ratios (r > 2), typical of pores with high sensitivity to compression. Permeability measurements at room pressure reveal that samples with visible microfractures are characterized by values up 2 order magnitude greater than of those with only visible solution surfaces, showing that the pore connectivity is controlled by opening-mode, sub-mm scale microfractures. At confining pressures greater than 25 MPa, both fracture- and stylolite dominated samples show similar values of permeability, indicating that at depths larger than ca. 1 km, the rough stylolite localize effective porosity that may enhance the along-solution surface fluid flow in carbonates.

Fault zones in carbonates control the storage and transport fluid properties. Their physical state can be very complex due to the interplay of diagenetic processes and multiple deformation episodes, leading to complex pore spaces given by overlapping voids space and crack damage. Aiming to assess key physical properties such as porosity and permeability for fault zones in the Araxos Promontory (Greece), we present the results of microstructural and laboratory measurements, including density, porosity, VP, VS and electrical resistivity for 54 blocks of Mesozoic carbonate host rocks and fault breccias originating from high-angle extensional and strike-slip fault zones. The host rocks consist of carbonate mudstones, wackestones, packstones, and sedimentary breccias from the Senonian and Vigla formations (Ionian zone) and include vugs due to selective dissolution. These rocks exhibit average density values, low porosity values, and medium-to-high P- and S-wave velocities. In contrast, fault breccia samples feature microfractures and display a wider range of density and porosity values, up to 5–10 times higher than those of the host rock, and a higher degree of anisotropy. To assess rock permeability and porosity-permeability relations, three different workflows were employed: (i) using Effective Medium Theory, inverting ultrasonic measurements; (ii) modelling a constant crack aperture of 1 µm; and (iii) using crack density values obtained from 2D image analysis. Selected samples were also tested in pressure vessels with confining pressure up to 80 MPa to reproduce in situ conditions. The application of the first two workflows showed a systematic variation of permeability with porosity. In contrast, the results of the third workflow, based solely on the outcomes of the digital image analysis, did not exhibit systematic variation. This behaviour is interpreted as a result of the non-selective dissolution of the outcropping carbonates, which causes a wide range of measured fracture aperture values and a lack of a clear poro–perm trend for host rocks due to the presence of stiff, sub-rounded pores and small vugs. On the contrary, the fault breccia exhibits a linear increase in permeability with porosity due to a connected pore network including microfractures.

The Val d’Agri Basin is a Quaternary sedimentary basin topping multiple tectonic units of the southern Apennines fold-and-thrust belt and a giant oilfield within deeper Apulian Platform carbonates. This basin is bounded by the seismically active East Agri (EAFS) and Monti della Maddalena (MMFS) extensional fault systems. The reservoir rocks are sealed and separated from shallower thrust sheets by a clay-rich and overpressured mélange. The role of this mélange during fault evolution at shallow crustal levels is widely debated and perhaps underestimated. Here, through multi-scale structural analyses and U–Pb dating of syn-tectonic calcite mineralizations, we gain new insights into the Val d’Agri fault system architecture, their structural maturity, and their relations with both natural and induced seismicity. Consistent with present-day NE-SW crustal stretching, the macro-scale structural architecture of both EAFS and MMFS is controlled by NW-SE and NE-SW fault sets, which displaced and in part re-sheared inherited pre- and syn-orogenic structures. The lack of evident clustering of meso-scale faults and the radial pattern of related slickenlines suggest that polygonal-like faulting occurred, particularly along the EAFS, due to lateral spreading of the Irpinia mélange in the subsurface. Structural data show that the MMFS is characterized by a higher structural maturity (slip longevity), with calcite U–Pb ages indicating the onset of long-lasting extensional tectonics in Early-Middle Miocene time. The original results are discussed in terms of seismotectonic setting of the study area, emphasizing the role played by both the thickness and spatial distribution of plastic mélange in modulating fluid pressure and seismic faulting.

Abstract. Aiming at assessing the porosity and permeability properties, we present the results of microstructural and laboratory measurements, including density, porosity, VP, VS, and electrical resistivity. These measurements were performed in dry and in saturated conditions on 54 blocks of Mesozoic carbonate host rocks and fault breccias collected in Greece. The host rocks consist of carbonate mudstones, wackestones, packstones, and sedimentary breccias from the Senonian and Vigla formations. These rocks exhibits average density values, low porosity values, and medium-to-high P- and S-wave velocities. Fault breccias originate from high-angle extensional and strike-slip fault zones, displaying a wider range of density, porosity values up to 5–10 times higher than host rock, along with ultrasonic velocities. Regardless of lithology, the carbonate host rocks might include vugs due to selective dissolution. Conversely, the fault breccia samples feature microfractures. Slight textural anisotropy is documented in the carbonate host rocks, while a higher degree of anisotropy characterizes the fault breccias. Selected samples were also tested in pressure vessels with confining pressure up to 80 MPa, revealing that transport properties along microcracks in fault breccias can significantly increase with increasing depth. To assess rock permeability and porosity-permeability relations, three different protocols were employed. Two of them were based on the Effective Medium Theory, where permeability was computed by inverting ultrasonic measurements, assuming an array of penny-shaped cracks embedded in an impermeable host matrix. The aspect ratio and crack width were obtained by the seismic measurements, modeling either by assuming all cracks as isolated or unconnected or all cracks connected into the network. The application of these two protocols showed a systematic variation of permeability with porosity. In contrast, the results of the third protocol, based on the digital image analysis outcomes only, did not exhibit systematic variation. This behavior was interpreted as a result of the not-selective dissolution of the outcropping carbonates causing a wide range of measured fracture aperture values. This study found that carbonate host rocks lacked a clear poro-perm trend due to the presence of stiff, sub-rounded pores and small vugs. On the contrary, fault breccia exhibited a linear increase in permeability with porosity due to a connected pore network including microfractures.

We investigate the multiscale geometrical and dimensional properties of fracture and fault networks crosscutting Lower Jurassic to Cretaceous shallow-water carbonate rocks. The carbonates are exposed along the flanks of the Viggiano Mt., on the eastern side of the High Agri Valley Basin of southern Italy. Furthermore, we employ the results of field and digital structural analyses to derive the input parameters for subsequent Discrete Fracture Network modelling (DFN) of geocellular volumes whose dimension and architecture resemble those of the investigated sites. DFN modelling is carried out for geocellular volumes representing the studied bed packages (5m-side volumes), outcrops (50m-side volumes), and reservoir-scale carbonate cliffs (500m-side volumes). Notably, modelling is obtained considering the minimum mechanical aperture value obtained in the field for porosity computations and theoretical values of fracture hydraulic aperture for permeability computations. This is because the mechanical aperture values and roughness profiles collected in the field along single fractures are unreliable due to weathering and localized karst-related dissolution. Our findings reveal that a scale-dependent geometry characterizes the Cretaceous carbonates over three orders of magnitude. These results support previously published data, and contrast with those obtained for the Lower Jurassic carbonates, which suffer of some bias due to the quality of the exposures. After DFN modelling, we show that the amount of computed fracture porosity is mainly due to the SB and NSB fractures, and that equivalent permeability is greater within faulted rock volumes. In terms of horizontal permeability, which is the most reliable result after DFN modelling, we document near isotropic horizontal permeability ellipses at all scales of observations. At individual outcrops, we note that the permeability ellipses are elongated parallel to the dominant fault sets that denotes how localized strain affects the directionality of fluid flow. Finally, we summarize the original data in a conceptual model illustrating the modalities of meteoric fluid infiltration through the vadose zone, and then subsequent horizontal flow in the phreatic zone present within the fracture carbonates at shallow depths.

Understanding the factors that govern past fluid circulation in tectonically active and/or hydrocarbon-rich basins is crucial for elucidating present-day fluid-flow scenarios. We investigate the circulation of paleo-fluids in the extensional-transtensional Val d'Agri Basin (southern Italy), home to a giant oil field and significantly affected by both natural and human-induced seismicity. Our aim is to understand how faulting and the variable thickness of the clay-rich tectonic melange, which constitutes the seal of the hydrocarbon reservoir, influenced past fluid flow under different tectonic regimes. To achieve this, we combined multiscale structural observations with isotope (C, O, clumped, and Sr) and Rare Earth and Yttrium (REY) analyses of fault-related calcite mineralizations. Using analytical methodologies that allow the analysis of sub-milligram samples for carbonate clumped isotopes, we provided a detailed characterization of the variability in precipitation temperatures and composition of parental fluids in both space and time. Our results reveal five main types of parental fluids, ranging from meteoric to intraformational and deep crustal, which were differently involved in the tectonic evolution of the Val d'Agri Basin. During orogenic shortening, vertical fluid circulation was mostly limited and compartmentalized, whereas post-orogenic extensional faulting promoted the ascent of deep fluids. Our findings indicate that the sealing properties of the melange were likely enhanced locally by increased thickness but were also compromised by fault activity and associated seismic events. Fluid circulation in the study area has been influenced by the prevailing tectonic regime (compressive vs. extensional), stratigraphic-structural architecture, properties of impermeable horizons, and seismic events. The model proposed for the Val d'Agri Basin elucidates past processes that are useful for understanding current fluid circulation in the basin itself and can be applied to other basins where fluid circulation is partly manipulated by human activities.

Pressure solution processes taking place during diagenesis deeply modify the hydraulic properties of carbonates, affecting their mechanical layering and hence the dimension, distribution, and connectivity of high-angle fractures. The formation of stylolites is controlled by the texture of the host rock and therefore by the depositional environment and the diagenetic processes that involve it. This study reports the results of a multidisciplinary study carried out on a Jurassic–Cretaceous carbonate platform in southern Italy. The goal is to unravel the control exerted by single carbonate textures and specific diagenetic processes on the formation of bed-parallel stylolites. Microfacies analyses of thin sections are aimed at obtaining information regarding the composition and texture of the carbonates. Petrographic observations coupled with CL analyses are key to deciphering their diagenetic history. Results are consistent with carbonates originally deposited in a shallow-water realm in which carbonate mud is occasionally abundant. In this environment, early cementation inhibits their chemical compaction. In grain-supported facies, pressure solution is only localized at the grain contacts. During shallow burial diagenesis, precipitation of blocky calcite predates the formation of bed-parallel stylolites in the grain-supported facies. Contrarily, mud-supported facies favor chemical compaction, which results in stylolites showing a good lateral extension and thick sediment infill. A classification of different types of stylolite morphology is attempted in relation to facies texture. In detail, rougher morphology (sharp-peak) characterizes the stylolites nucleated in grain-supported facies, while smoother morphology (rectangular to wave-like) is observed in stylolites on mud-supported facies. Application of this knowledge can be helpful in constraining the diagenetic history of carbonate rocks cored from depth, and therefore predict the fracture stratigraphy properties of carbonates buried at depth.

AbstractAiming at understanding the source of the fluids that mineralizing within seismically active fault zones, we investigate the noble gas isotopes (i.e., helium (He), neon (Ne), and argon (Ar)) in the fluid inclusions (FIs) trapped in the calcite veins sampled along high‐angle fault zones of the Contursi hydrothermal basin, southern Italy. The latter basin lies in close vicinity of the MW = 6.9, 1980 Irpinia earthquake and exposes numerous fault scarps dissecting Mesozoic shallow‐water carbonates. The analyses of noble gases (He, Ne, Ar) are conducted to identify the origin of the volatiles circulating along the faults at the time of calcite precipitation. Then, outcomes of this discussions are compared with currently outgassing of deep‐sourced CO2 coupled to mantle‐derived He in that area, whose output is larger than those from some volcanic areas worldwide. The results indicate that He in FIs is dominated by a crustal radiogenic component (4He), and by an up to 20% of a mantle‐derived component (3He), with a highest isotopic signature of 1.38 Ra. This value is consistent with the highest percentage of mantle‐derived He associated to high‐flux CO2 gas emission in the investigated area (1.41 Ra). We propose that the variability of the He isotopic signature measured in primary FIs can result from early trapping of fluid inclusions or post trapping processes and seismic activity that modify the pristine He isotopic signature (i.e., derived from the crust and/or mantle) in groundwater along the faults during periods of background seismicity. Such investigations are fundamental to understand fluid migration in fault systems and the role of fluids in processes of earthquake nucleation.

Structural analysis coupled with geochemical study of syn-tectonic mineralizations unraveled the role of fluids during the polyphase tectonic evolution of the Val D’Agri, a seismically-active intermontane basin located in the Southern Apennines fold and thrust belt, hosting the largest onshore oil field in western Europe. In this basin, the structural control on present-day fluid circulation is still not well constrained. For this reason, the aim of this work is to reconstruct the Val d’Agri fault system (VAF) architecture and paleo-fluid circulation during the basin tectonic evolution. The VAF evolution was caused by non-coaxial polyphase stress regimes that can be summarized in: 1) Upper Miocene-Lower Pliocene compressional regime; 2) Upper Pliocene-Lower Pleistocene late orogenic strike-slip regime 3) Early Pleistocene-Present post-orogenic extensional regime. Based on new field work, mapping, and structural analysis, we recognized that the VAF is organized in different oriented faults sets. The main sets are N-S-, NE-SW-, and ESE-WNW-striking faults, where the last one has the higher degree of maturity. U-Pb dating of calcite slickenfibres highlighted the long-term tectonic history of these fault sets, with episodes of activation and reactivation of inherited faults. In particular, we recognize: N-S-striking normal faults that reactivated inherited Upper Miocene-Lower Pliocene thrusts; Upper Pleistocene NE-SW-striking normal-lateral faults; one Miocene ESE-WNW-striking normal-lateral fault, with also evidence of reactivation in more recent time. Clumped isotopes analysis together with optical and cathodoluminescence observations of about 50 syn-tectonic calcite mineralizations allowed us to link specific fluid pathways to the different stress regimes. Indeed, bed-parallel veins and mineralizations sampled along transpressive, transtensive, and normal fault sets show that: 1) during the Upper Miocene-Lower Pliocene compressive tectonic phase, fluid circulation occurred in a closed system, characterized by host-rock buffered fluids; 2) during Upper Pliocene-Lower Pleistocene late orogenic phase, fluid circulation occurred in an open system, characterized by meteoric water with low to moderate residence time; 3) during the Lower Pleistocene-Present extensional phase, fluid circulation occurred in an open system, characterized by the mixing of meteoric water and uprising deep high temperature fluids. We highlight that the reconstruction of paleo to present-day fluid circulation is a valid tool for assessing natural and induced seismic hazard in seismically active areas where hydrocarbons are exploited and fluids are injected into the crust, such as in the Val d'Agri basin.

Karst processes may be critical for developing secondary porosity and permeability within carbonate reservoirs and aquifers. Karstification can significantly influence reservoir storage capacity and subsurface fluid flow. This study investigates the interplay between fluid-flow pathways and diagenesis in an epigenic karst setting in Cretaceous shallow-water carbonate rocks, Potiguar Basin, Brazil. The results indicate that dissolution occurred at all diagenetic stages, but the last stage is the most important because it is related to the most recent subaerial exposure. Dissolution produced during the last karst stage occurred in the vadose and phreatic environments, leading to the precipitation of isopachous, blocky and syntaxial cements. Dissolution affected the original pore network within the matrix and localized on preexisting discontinuities such as stylolites, fractures, and bedding surfaces. The resulting karst cavities are classified as vugs, channels and caverns, varying from a few millimeters to hundreds of meters in size. In epigenic karst systems, high porosity percentages are commonly due to moldic, intragrain/intrafossil and intercrystal porosities. The results of our study show how such porosities can be connected to fractures, veins and stylolites at the microscale. Karst dissolution can further increase porosity and the overall rock permeability due to aragonite and calcite cement dissolution during eodiagenesis, microfractures and stylolites dissolution during mesodiagenesis and all cements, fractures and stylolites during telodiagenesis.

This study aims to contributing to the reconstruction of the geodynamic evolution of the Southern Apennines fold-and-thrust belts (SA-ftb). The main tectonic units across the regional decollement, high-strain zone of the SA-ftb outcropping at the Monte Alpi key area are investigated by using a multi-tool approaches. The shales of Late Triassic-Jurassic Zia Santa Fm. and the Upper Messinian terrigeneous deposits were analyzed by X-ray diffraction, X-ray fluorescence, and both optical and scanning electron microscopies. The results indicate that the two study rock units reflect their dissimilar tectonic evolution until the Lower Pliocene. In fact, data show that the Upper Messinian rocks include mafic-derived grains not observed in the Zia Santa Fm. rocks, which derived from dismantling of Liguride-derived ophiolitic units. Moreover, the measured chemical indices of alteration suggest a degree of paleoweathering processes ranging from moderate (Zia Santa Fm.) to intense (Messinian), in accordance with the amount of kaolinite. The mixed-layers illite/smectite features indicate a variable prograde alteration relatable with the overall structural architecture of the high strain zone documented at both meso and microscales. In particular, the Zia Santa Fm., which pertains to a shallower structural position, shows a slightly higher degree of diagenesis than the studied Messinian shales. The major degree of diagenesis/deformation is interpreted as due to the complex tectonic history and deformation processes of the Zia Santa Fm., which was previously labelled as part of the tectonic mélange decoupling the allochthons of the SA-ftb from the autochthon Inner Apulian carbonates of the Monte Alpi Unit. In particular, the Messinian shales show a variable diagenesis/deformation depending upon their distance from the regional decollement; samples collected in its close vicinity are characterized by a higher degree of illitization of the mixed layers I/S, and by total absence of kaolinite. This data suggests that kaolinite minerals took part in the smectite illitization process, which occurred within a compressive stress field associated to the migrating SA-ftb.