Fabrizio Agosta

Sedimentologic and diagenetic controls on pore-network characteristics of Oligocene–Miocene ramp carbonates (Majella Mountain, central Italy)

This article addresses the controls exerted by sedimentologic and diagenetic factors on the preservation and modification of pore-network characteristics (porosity, pore types, sizes, shapes, and distribution) of carbonates belonging to the Bolognano Formation. This formation, exposed at the Majella Mountain, Italy, is composed of Oligocene–Miocene carbonates deposited in middle- to outer-ramp settings. The carbonates consist of (1) grainstones predominantly composed of either larger benthic foraminifera, especially Lepidocyclina, or bryozoans; (2) grainstones to packstones with abundant echinoid plates and spines; and (3) marly wackestones to mudstones with planktonic foraminifera. The results of this field- and laboratory-based study are consistent with skeletal grain assemblages, grain sizes, sorting, and shapes, all representing the sedimentologic factors responsible for high values of connected primary macroporosity in grainstones deposited on the high-energy, middle to proximal outer ramp. Cementation, responsible for porosity reduction and overall macropore shape and distribution in grainstones to packstones deposited on the intermediate outer ramp, was mainly dependent on the following factors: (1) amount of echinoid plates and spines, (2) grain size, (3) grain sorting and shapes, and (4) clay amount. Differently, in the wackestones to mudstones, laid down on the low-energy, distal outer ramp, matrix is the key sedimentologic factor responsible for low values of scattered macroporosity and dominance of microporosity. The aforementioned results may be useful to improve the prediction of reservoir quality by means of mapping, simulating, and assessing individual carbonate facies with peculiar pore-network characteristics.

Hydraulic properties of fault zones in porous carbonates, examples from central and southern Italy

We present the results of in situ permeability measurements performed, using a portable field permeameter, on normal and strikeslip fault zones that crosscut high-porosity carbonate grainstones. The measurement sites expose in the Cretaceous Orfento Formation of the Majella Mountain (Abruzzo, Italy), and the Lower Pleistocene deposits of the Favignana Island (Sicily, Italy). Nine small-displacement, compactive shear banding-based fault zones have been tested in the field. The fault offset ranges between 10 and 200 centimeters. The acquired permeability data indicate a two orders of magnitude decrease of porosity and permeability from the host rock to the cataclastic fault cores. A clear dependence of the fluid circulation paths through porous carbonates is therefore inferred at depth due to orientation, density and connectivity of the fault zones. Moreover, this study indicates the key role played by the pore network characteristics (pore dimensions above all) of undeformed host rocks on determining extremely different permeability values of the faulted porous carbonate grainstones. Accordingly, the results presented in this study may be helpful in applications such as geofluids management for improving the forecasting of carbonate reservoir quality and understanding the extent of reservoir compartmentalization.

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 (<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.

Structural architecture and Discrete Fracture Network modelling of layered fractured carbonates (Altamura Fm., Italy)

This work focuses on the spacing and height distributions of bed-perpendicular fracture sets present within a carbonate multilayer, and on their effect on the volumetric (P32 and fracture porosity) and hydraulic (correspondent permeability) properties of the rock mass computed after DFN modelling. Results of integrated field and laboratory analyses are consistent with an uneven spacing distribution of Strata-Bound (SB) fractures. The oldest SB fractures formed within individual limestone beds bounded by bed interfaces, which likely acted as mechanical interfaces. Differently, the youngest SB fractures formed within smaller rock volumes bounded by the pre-existing fractures. Non Strata-Bound (NSB) fractures consist on sheared bed-perpendicular fractures, originally compartmentalized within individual beds, which now form incipient strike-slip faults. DFN modelling of representative rock volumes show that both volumetric and hydraulic properties are strongly affected by the two main sets of NSB fractures, which form a conjugate system of faults producing the principal structural anisotropy in the fractured carbonate multi-layer.

Fracture properties analysis and discrete fracture network modelling of faulted tight limestones, Murge Plateau, Italy

The modelling of natural fracture in reservoirs requires, as input data, the results of previous detailed and accurate analysis of the 3D fracture network. These 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, allow the construction of more representative numerical models of natural fractured reservoirs. This paper focuses on 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. The studied 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 these models 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.

The role of mechanical stratigraphy on normal fault growth across a Cretaceous carbonate multi-layer, central Texas (USA)

We present the results of an integrated field and laboratory investigation of the deformation mechanisms associated with nucleation and development of normal faults across heterogeneously layered Cretaceous carbonates. These results shed a new light on the role of partitioning of failure structures in the fault growth across such a carbonate multi-layer. Structural data show that the oldest structural elements consist of low-angle to bedding pressure solution seams, which formed during burial diagenesis of limestone rocks, such as wackestones, containing a large amount of carbonate mud.Subsequently, we envision that normal fault nucleated due to shearing of these burial-related structures. Continuous slip along the sheared low-angle pressure solution seams localized fault-related dilation in the surrounding stiffer carbonate beds, which mainly consisted of packestones and dolostones. In particular, dilation localized within releasing jogs, as shown by the high frequency of high-angle to bedding splay joints. The splay joints were eventuallysheared during fault development due to fault-related bed tilting.Linkage of the sheared elements originally compartmentalized within individual beds, which were characterized by variable cut-off angles, determined the observed scalloped vertical trace of the through-going normal faults across the carbonate multi-layer.

Characterisation of the permeability anisotropy of Cretaceous platform carbonates by using 3D fracture modeling: 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 kinematics; 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.

Shearing of syn-sedimentary carbonate breccia along strike-slip faults, Altamura Fm., Southern Italy

Fault breccia present along strike-slip faults was studied by mean of an integrated field and laboratory investigation. In particular, samples of sheared pre-existing, syn-sedimentary gravitative breccia were detailed studied in order to decipher the deformation mechanisms associated to strike-slip faulting. The sheared gravitative breccia is made up of matrix-supported, monomictic, unsorted intraformational fragments embedded within a reddish hematiterich, clayish matrix. Microsparry calcite cement is also present around the limestone fragments. In contrast, tectonic breccia consists of comminuted limestone fragments embedded in a calcite-rich matrix also including small amounts of reddish matrix. Particle shape analysis of representative images of the study hand specimens shows a variability of the box-counting dimension (D0-value) between the two different types of breccias, which can be explained in terms of peculiar strain localization within a threefold aggregate that characterizes the gravitative breccia, which is made up of calcite fragments, clayish matrix and very minute oxides.

Dynamic weakening along incipient low-angle normal faults in pelagic limestones (Southern Apennines, Italy)

Slip along low-angle normal faults is a mechanical paradox requiring activation of strain weakening mechanisms. Microstructures present in the slip zones of incipient low-angle normal faults cutting carbonates in the Southern Apennines of Italy show that slip was promoted by two weakening mechanisms producing a reduction of the friction coefficient: (1) high pore fluid pressures; (2) dynamic weakening related to thermal decomposition indicated by decarbonation microstructures and concomitant localized dynamic calcite recrystallization. Furthermore, as a consequence of thermal decomposition, nanoparticles occur as infilling of injection veins, suggesting that powder lubrication processes are active along the slip surface during seismic slip. Supplementary materials: A geological sketch of the study area, detailed field photographs of the studied faults and detailed micrographs are available at http://www.geolsoc.org.uk/SUP18806.

New insights on the structural setting of the Monte Alpi area, Basilicata, Italy

The Monte Alpi is a key area to decipher the structural setting of the southern Apennines fold-and-thrust belt. There, high-angle faults juxtapose Mesozoic carbonates of the inner Apulian Platform, and their terrigenous Messinian sedimentary cover, against the allochthon terranes. In the recent past, two main tectonic models related to the evolution of the Monte Alpi area ascribed the significant exhumation of the Apulian carbonates to two different mechanisms. The first one, highlighted the role of high-angle faults, which affect and cross-cut both carbonates and allochthon terranes. The second model, inferred the significant exhumation of the Apulian Platform carbonates as due to low-angle extensional faulting. In light of the aforementioned discrepancies, the present work shows the results of original field and laboratory analyses aimed at reconstructing the structural setting of both Apulian carbonates and allochton terranes. In particular, two different folding stages and the geometry of the thrust sheets are assessed, by means of detailed field and micro-structural analyses, for the allochton terranes. In the Apulian carbonates, both attitude and kinematics of the syn-sedimentary high-angle faults bounding the Upper Messinian deposits are documented. Furthermore, the tectonic structures associated to the contractional, strike-slip and extensional stages are distinguished based on their abutting and crosscutting relationships. Finally, the results of such a work are discussed in terms of the time-space evolution of deformation in the Monte Alpi area; five main tectonic stages are deciphered for the Pre-Pliocene to Holocene times.