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Deformation style and hydraulic properties of tight carbonates within Apulian platform and slope-to-basin succession (southern Italy)
No full textThe 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.\ud
1. Structural features of platform carbonates in the Apeninnic foreland.\ud
- Structural properties of fractured and faulted Cretaceous platform carbonates, Murge Plateau (southern Italy).\ud
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.\ud
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.\ud
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.\ud
- Bedding-parallel stylolites in shallow-water limestone successions of the Apulian Carbonate Platform (central-southern Italy).\ud
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.\ud
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.\ud
Laboratory measurements revealed that the permeability of the studied Cretaceous limestones is very low (<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.\ud
2. Structural features of slope-to-basin carbonates in the Apeninnic foreland.\ud
- Structural features of mass-transport deposits within carbonates from southern Italy.\ud
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.\ud
The Late Jurassic–Early 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.\ud
- Tectonically- and gravity-driven deformation structures in slope-to-basin carbonates, the Gargano Promontory (Italy).\ud
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.\ud
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.\ud
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.\ud
3. Discrete fracture network modelling of fractured and faulted carbonates.\ud
- 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.\ud
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.\ud
- Characterization of the permeability anisotropy of Cretaceous platform carbonates by using 3D fracture modelling: the case study of Agri Valley fault zones (southern Italy).\ud
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\ud
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.\ud
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
Permeability anisotropy of fractured and faulted tight Cretaceous platform carbonates by use of 3D fracture modelling (Agri Valley, southern Italy)
No full textIn 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
Deformation mechanisms and petrophysical properties of chert and limestone fault rocks within slope-to-basin succession (Gargano Promontory, Southern Italy)
No full textPERMEABILITY STRUCTURES OF FAULT ZONES CROSSCUTTING TIGHT CRETACEOUS PLATFORM CARBONATES (VAL D’AGRI, SOUTHERN ITALY)
No full textThe high-angle faults system that affects the Appeninic platform carbonates cropping out in the Val d’Agri area is composed of a main WNW oriented left-lateral strike-slip fault and several associated secondary faults: N020°±N030° trending right-lateral/transtensional, N090°±N110° trending left-lateral transtensional and N130°±N150° trending left-lateral transpressional. Even if these faults are characterized by different dimensional parameters (i.e. length and displacement) and architecture (i.e. fault core and damage zone thickness), qualitative classifications relative to the hydraulic properties of fault zones suggest that both faults (strike-slip and normal) could act as conduits for subsurface fluid flow.
In this paper, with the aim of compute the relative permeability between the host-rock and fault zones as well as to evaluate the hydraulic properties of both strike-slip and normal faults, a Discrete Fracture Network (DFN) model, was constructed using the software MOVE.. The DFN model of the rock mass including two fault zones and the host-rock proves that the total permeability is higher for the fault zones and between the faults is higher for the normal fault. Directional variations of the permeability are also shown and relate on the preferential orientation of the fractures together with theirinterconnectivity and length
Spatial and dimensional variations of the faults and fractures attributes, and their influence on the permeability of the Cretaceous platform carbonates in Val d'Agri, southern Italy
No full textIn the Agri Valley, high-angle faults crosscut platform carbonates that are analogues of the lithological units that host the deep-seated Val d’Agri field, which is among the largest onshore oil reservoirs in Western 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
Mass-transport deposits within basinal carbonates from southern Italy
No full textA 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. The Late Jurassic-Early 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 intervals of lithologically similar, but chaotic strata that are characterized by significant internal distortion. The stratigraphic thickness of these deformed units, 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, together with 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 directly overlying the MTDs. Structural features within MTD’s, provide information about the direction of the mass movement, and hence the orientation of the paleoslope. Measurements in the eastern and north-eastern part of Gargano Promontory suggest flow is directed towards the E and N respectively. 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
Discrete Fractures Network (DFN) of the faulted Upper Cretaceous platform carbonates outcropping in the Murge area (Altamura Formation, Murge, Italy)
No full textDiscrete Fractures Networks (DFN) of representative volumes corresponding to faulted Upper Cretaceous platform carbonates exposed in Murge were modeled. The obtained DFN models allowed us to compute fracture porosity, and relative permeability distribution of the faulted carbonates, with the aim to decipher the contribution of fault-related deformation on reservoir-scale fluid flow.
The study carbonates crop out in Pontrelli quarry at Murge area, Italy, and correspond to an excellent analogue of the hydrocarbon reservoirs currently exploited in southern Italy. The outcrops expose a well-layered limestone affected by both background (diffuse) and fault-related (localized) deformation. Background deformation is made up of two orthogonal sets of joints striking N30 and N120, respectively. The fault-related deformation is represented by two strike-slip fault zones trending N120 (dextral) and N160 (sinistral), which solve an apparent vertical displacements above seismic resolution (about 20m). Minor faults pertaining to the two sets are present within the fault damage zones and surrounding rock volumes.
In order to construct the DFN model we elaborated the cumulative frequency distributions and the scaling relationships of spatial and dimensional parameters of both background and fault-related fractures (orientation, length, intensity, and aperture) by mean of scan line and scan area surveys conducted in the field. Finally, the overall permeability tensor of the representative volume was obtained using the software MOVE, which is based on Oda method (1985)
Structural properties, distribution and internal architecture of synsedimentary submarine mass-transport deposits in southern Italy
No full textA detailed characterization of Mass-Transport Deposits or MTDs, in terms of emplacement processes, depositional products, structural properties and internal architecture, is crucial to define the hydraulic properties of slope-to-basinal deposits. In fact, MTDs are distinct mechanical units and significant heterogeneities within sedimentary succession. They may show different fractures parameters, including intensity and orientation of fractures, when compared to underlying and overlying undeformed strata. MTDs can therefore play a significant role in hydrocarbon exploration and production, inasmuch as they may act as barriers or conduits for fluid flow in the subsurface.
The Late Jurassic–Early Cretaceous basinal Maiolica Formation exposed in the Gargano Promontory (southern Italy) represents an ideal natural laboratory to study the stratigraphic architecture of ancient MTDs. This formation consists of undisturbed intervals of flat-lying, light coloured, thin-bedded, cherty micritic limestone interbedded with chaotic intervals characterized by extensively folded and contorted strata of similar facies. The stratigraphic thickness of these deformed strata, which are interpreted to be slump/slide deposits and, to a lesser extent, debris-flow deposits, varies from several decimetres to tens of metres. The internal deformation comprises down-slope verging soft-sediment folds, and both normal and reverse faults. Normal faults are most common in the updip portion of the MTDs, whereas reverse faults are best developed toward their frontal margins, where they often form imbricates of multiple thrusts. 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.
The structural properties and the distribution of studied MTDs are discussed in the context of the Cretaceous tectonics of the Gargano Promontory, as well as on the characteristics of the paleoslope of the Apulian Platform
Discrete fracture network modelling of faulted and fractured Apulian platform carbonates (Altamura Formation, southern Italy)
No full textHydraulic properties of representative volumes of fractured/faulted Upper Cretaceous Apulian platform carbonates were modeled using a stochastic approach (DFN), with the aim to decipher the contribution of background and localized deformation on fluid flow in tight platform carbonates at outcrop-scale.
The study rocks crop out in quarries located at Murge area, Italy, and correspond to excellent analogues of the hydrocarbon reservoirs currently exploited in southern Italy. The outcrops expose a well-layered limestone affected by both diffuse (background) and fault-related (localized) deformation.
Background deformation consists of bed-parallel and bed-perpendicular stylolites, four sets of stratabound joints and two sets of non-stratabound fractures.
The fault-related deformation is represented in the small scale by sub-vertical sheared fractures with few-cm offset and mostly characterized by the absence of fault core and discontinuous damage zones. In a larger scale, the fault related deformation is localized in strike-slip fault zones striking N120° (right-lateral) and N160° (left-lateral), which are characterized by a vertical separation above seismic resolution (> 20m), high fractured damage zone (about 30m thick) and well-developed fault core of about 1m of thickness.
The field work consisted of obtaining both spatial and dimensional properties of individual fracture sets (length, intensity, aspect ratio and aperture) pertaining to either background or localized deformation. Posterior analysis consisted of finding useful relationships, such as, dependency of fracture aperture on both orientation and length, fracture intensity variations along the fault zones, fracture length relationship with orientation.
Discrete fracture network (DFN) models for host rock (at different scales) and damage zones were obtained with a stochastic approach by using MOVETM software and the outputs of individual DFN models consist on volumetric fracture intensity (P32) fracture porosity and 3D hydraulic conductivity (Kx, Ky, Kz) upscaled using the method of Oda (1985).
The studied carbonates show an high variability of these properties at various scales and among different zones (host rock, damage zone and fault core) highlighting the importance of both sub-seismic and seismic resolution strike-slip fault networks on flow during well production at a reservoir-scale
Control exerted by rock anisotropy on chemical compaction localization in Bahamian-type peritidal limestones of the Apulian Platform (Italy)
No full textIn this contribution we present the results of a work aimed at assessing the geological, petrophysical and mechanical factors affecting chemical compaction localization (overburden-inducedpressure solution) in tight, Bahamian-type platform limestones. The products of such compaction include bed-parallel stylolites, which represent structural features that may affect the permeability of the rock due to the presence of clayish insoluble residue along them. By combining field and laboratory studies
(sedimentological, structural, petrographic and statistical analyses), we have investigated Upper Jurassic and Cretaceous peritidal limestones originally pertaining to the Apulian Platform realm and now exposed in three distinct Italian locations: Maiella Mountain, Gargano Promontory and Murge Plateau. All these limestones are crosscut by bed-parallel stylolites that are more abundant (up to 10 times longer and less spaced from each other) in the laminated microbial bindstones than in the other,
more isotropic carbonate facies (mostly structureless lime mudstones to grainstones). A first-order control of bed thickness on spacing and, hence, localization of bed parallel stylolites is excluded. By contrast, lithological factors clearly played a major role on localize stylolites within the microbial bindstones. One prominent factor is likely the ubiquitous occurrence in bindstones of bed-parallel laminae. Accordingly, petrographic thin sections display stylolites localized at laminae boundaries. However, bedparallel laminae are ubiquitous also in some grainstones where stylolites are only a little more abundant than in the nonlaminated counterparts. So, other factors peculiar of microbial
bindstones may have dramatically influenced stylolite localization as well. A key factor is probably the ubiquitous occurrence of former fenestral voids that, to date, are mostly filled by coarsely sparry calcite cement. Similar distributions of stylolites in structureless limestones (mudstones to grainstones) are consistent with rock grain size having probably played only a minor role in their localization (stylolites slightly enhanced in muddy limestones). By contrast, clay amount of the rock (0-4%
by volume) seem have not played any significant role in enhancing stylolite localization. To sum up, our results point to a major role played by the lithological characteristics of limestones on the development and localization of bed-parallel stylolites. Additional ongoing laboratory experiments including petrophysical and geomechanical analyses may help to improve these results and to better understand the role played by stylolite localization on the overall rock permeability
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