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.

Epigenetic karstic systems in carbonate rocks commonly result from progressive dissolution by acidic meteoric waters over thousands to millions of years. The generation of secondary porosity and permeability improvement due to dissolution in carbonate reservoirs of geofluids (e.g., groundwater, hydrocarbons, and CO2) can profoundly impact reservoir storage capacity and subsurface fluid flow. This study investigates the control of structural discontinuities such as stylolites, fractures, and primary sedimentary discontinuities on the generation of multiscale karst dissolution cavities by epigenetic fluid percolation in a Late Cretaceous carbonate sequence (Jandaíra Formation) in the Potiguar Basin, Northeastern Brazil. The study relies on micro- and macroscale analyses such as stratigraphic logs, field structural investigations, rock strength data collected in the field (Schmidt hammer), microtomographic and drone images, thin section analyses, porosity and permeability laboratory measurements. The results show that bed-perpendicular stratabound and non-stratabound stylolites and fractures can be enlarged due to meteoric water percolation until they merge and form a single channel system that crosscuts all sedimentary multilayers. Bed-parallel stylolites are ubiquitous in carbonate sequences overprinting bed interfaces and layers. Where not dissolved, bed-parallel stylolites have low porosity and permeability and thus can act as barriers to vertical fluid flow. Where dissolved, such stylolites can contribute to horizontal fluid flow and form channel porosity. The results of this study led to a formulation of a conceptual model of rock dissolution along structural and sedimentary discontinuities that affects carbonate rock successions in the subsurface.

Bed-perpendicular, opening-mode diffuse fractures are common features in carbonate rocks because they become brittle during the first stages of diagenesis. Early-fractures could be independent of tectonics and form a background structural network from sub-millimeter to 10's of centimetres scale. This study focuses on the outcrop-to-micro scales structural, stratigraphic, and petrographic characterization of Lower Cretaceous, shallow-water, tight limestones of the Inner Apulian Platform exposed along the axial sector of the southern Apennines fold-and-thrust belt, Italy. This work aims at understanding the formation of early diagenetic fractures during the first stages of sediment lithification, and on their impact on subsequent deformation mechanisms associated to polyphase tectonics. The results of field structural surveys, of petrographic and cathodoluminescence analyses of representative samples unravel the main structural-diagenetic processes in shallow-water carbonates. We document that early brittle fractures were intrinsically related to the host rock pore type and cementation processes, and summarize the overall time-dependant evolution of the structural assemblages in a conceptual model. This model includes all abutting and crosscutting relationships among the various structural elements from field to thin-section observation, and form the now days fracture network.

This work assesses the impact of the stratigraphic and structural heterogeneities on the fluid distribution and migration within a deformed carbonate reservoir analogue. The study was carried out in the abandoned Roman Valley quarry (Majella Mountain, central Italy), well-known for its historical bitumen extraction. The studied rocks consist of ramp carbonates of the lower member of the Bolognano Formation (Oligocene-Miocene in age) mainly composed of grainstones, and secondary packstones and wackestones. In the quarry, the exposed rocks are crosscut by two high-angle oblique-slip faults striking WNW-ESE with up to 40 m of throw. With the aim of deciphering the contribution of both matrix and fracture pore space on the fluid storage and migration, both laboratory measurements and field-based Discrete Fracture Network (DFN) models (cubes with 4-m per side) have been carried out. The obtained dual-porosity/permeability model, which includes the lithological and the structural heterogeneities observed at the location, was used to run flow simulations under different scenarios of well locations. The fluid distribution in the matrix, resulting from these flow simulations, is consistent with field observations where bitumen localizes within the most permeable lithofacies (grainstones). In the fault zones, the fracture network causes an important fluid flow anisotropy, enhancing the flow in a direction subparallel to the faults. The flow orthogonal to the fault zones is controlled by type and lateral continuity of fault rocks, where fault breccias represent conduits and cataclasites localized barriers.

This work focuses on the role played by depositional/diagenetic heterogeneities on the vertical growth of subseismic, high-angle structural elements such as joints, sheared joints, small faults, and well-developed faults in Lower Cretaceous tight carbonates exposed along the 100's m-high, km-long, western cliff of the Monte Alpi massif. The study carbonates pertain to the Inner Apulian Platform, and form a surface structural analogue of the fractured carbonate reservoirs currently exploited for hydrocarbon production in nearby areas of southern Italy. By combining traditional field stratigraphic and structural methods with photogrammetric analysis of a large portion of the aforementioned cliff, we assess the abutting and crosscutting relationships among the sub-seismic structural elements and different types of flooding surfaces. At smaller scale, bed interfaces extend laterally for a few tens-of-metres, bound tens-of-centimetres-thick beds characterized by amalgamation and bifurcation geometries, and form sequence boundaries of the 4th-5th order. At larger scale, transgressive surfaces extend laterally for hundreds-of-metres, include mm-thick terrigenous material, bound multiple metre-thick bed packages made up of multiple beds, and form sequence boundaries of the 3rd order. Field evidences are consistent with joints and sheared joints often forming stratabound and non-stratabound elements, respectively. At a reservoir scale, prominent transgressive surfaces are laterally continuous for kilometres, include up to a few centimetres-thick, clay-rich levels and collapsed breccia, bound tens-of-metres-thick bed package associations, and form sequence boundaries of the 2nd order. There, small faults with up to 10's of cm-offset mainly localize within individual bed package associations, whereas well-developed faults with up to a few m-offset crosscut multiple prominent transgressive surfaces. Results after DFN modelling of representative geo-cellular models highlight the control exerted by the study structural elements on the fluid accumulation and flow properties of the fractured carbonates. These data are useful to fill the gap between core and seismic data, as they provide information on the sub-seismic domain (i.e. below seismic resolution).

The complex fluid saturation distribution and influence of microscale and mesoscale fractures on the fluid accumulation and flow properties of carbonates are still interesting challenges for petroleum geologists. For this reason, in order to know the relative role played by the aforementioned features on the storage and migration properties of tight limestones, the present work focuses on a surface analogue cropping out in southern Italy. By first applying a deterministic Discrete Fracture Network (DFN) modelling to a 1 m(3) geocellular volume, an amount of 0.3% of fracture porosity and a fracture connectivity configuration above the percolation threshold are computed. In addition to mesoscale fracture porosity, in order to gather information on matrix porosity and microscale fractures, we investigate the pore type, geometry, and textural anisotropy of selected rock plugs by mean of integrated petrophysical, ultrasonic, and optical microscopy analyses experiments. Results show values of connected porosity ranging between 1% and 6%, presence of vugs localized along pre-existing structural heterogeneities, and the predominance of stiff pores within the carbonate matrix. The rare microfractures are mainly oriented orthogonal to bedding. The estimated crack density (0.19) shows that the contribution of fracture porosity at microscale (phi = 0.078%) is very low if compared to that of matrix porosity and also a structural configuration above the percolation. The present study therefore documents that the carbonate matrix forms an isotropic medium, which profoundly affects the storage capability of the study limestones. In fact, the amount of storage provided by carbonate matrix and microscale fractures is greater than that due to mesoscale fractures. Moreover, we document that the matrix contribution on porosity is much more significant than the contribution provided by micro fractures. Finally, the present work shows the importance of integrating different methodologies on the assessment of fracture porosity at different scales of observation. In fact, a great benefit for development and production operations can be obtained by performing studies of surface analogs, which allow the detailed investigation of rock masses below seismic resolution.

Summary Recent geological and structural analyses of the Monte Alpi carbonate massif focused on the dimensional properties and growth mechanisms of large-scale, high-angle fault zones. In this work, we focus on the role played by primary heterogeneities on the vertical growth of high-angle structural elements such as joints, sheared joints, persistent fracture zones, and small faults. By combining traditional field structural analyses with 3D photogrammetric analysis, we are able to document the attitude and height distribution of individual structural elements. Data are statistically elaborated in order to assess their multi-scale dimensional properties, which are discussed in light of the abutting and crosscutting relationships with respect to the primary heterogeneities. Results of this work fill the gap that exists between the in situ and seismic resolution analyses, and provide new insights into the mechanical stratigraphy of tight, platform-related carbonates of the Inner Apulian Platform.

SummaryBed-perpendicular diffuse fractures are common features in carbonates that could be affected by early embrittlement process, which may enhance vertically persistent, opening-mode fractures during the first stages of diagenesis. Early-developed fractures could be independent of tectonics and form a background structural network at different scale. This study focuses on the structural, stratigraphic and petrographic characterization, from outcrop- to micro-scale of Lower Cretaceous, shallow-water, tight limestones pertaining to the Inner Apulian Platform paleogeographic/tectonic domain. These carbonates consist of a wide spectrum of inner platform-platform margin calcareous facies, namely mudstones, wackestones, packstones, grainstones, bindstones, floatstones and rudstones. The presented work is addressed on understanding the development of the early diagenetic features which occurred during the first stages of the sediment lithification processes and the description of the formation, geometry and distribution of the contemporaneous and subsequent structural elements. Field analyses in concert with laboratory analyses unrevealed the main structural-diagenetic events which affected the studied shallow-water carbonates. Petrographic analyses highlight early fractures are intrinsically related to the host rock pore type. Finally, evolution of the fracture network is summarized in a conceptual model by taking into account the several tectonic phases which affect the Monte Alpi sector

T204.Friends of Hoth: Episode II-Icy Bodies of the

The Upper Jurassic to Lower Cretaceous platform‐slope to basinal carbonate strata cropping out in Gargano Promontory (southern Italy) are partly dolomitized. Fieldwork and laboratory analyses (petrographic, petrophysical and geochemical) allowed the characterization of the dolomite bodies with respect to their distribution within the carbonate succession, their dimensions, geometries, textural variability, chemical stability, age, porosity, genetic mechanisms and relation with tectonics. The dolomite bodies range from metres to kilometres in size, are fault‐related and fracture‐related, and probably formed during the Early Cretaceous at <500 m burial depths and temperatures <50°C. The proposed dolomitization model relies on mobilization of Early Cretaceous seawater that flowed, downward and then upward, along faults and fractures and was modified in its isotopic composition moving through Triassic and Jurassic strata that underlie the studied dolomitized succession. Despite the numerous cases reported in literature, this study demonstrates that hydrothermal and/or high‐temperature fluids are not necessarily required for fault‐controlled dolomitization. Distribution and geometries of dolomite bodies can be used for palaeotectonic reconstructions, as they partly record the characteristics (size, attitude and kinematics) of the palaeo‐faults, even if not preserved, that controlled dolomitization. In Gargano Promontory, dolomites record Early Cretaceous palaeo‐faults from metres to kilometres long, striking north‐west/south‐east to east/west and characterized by normal to strike‐slip kinematics. Dolomitization increases the matrix porosity by up to 7% and, therefore, can improve the geofluid storage capacity of tight, platform‐slope to basinal limestones. The results have a great significance for characterization of geofluid (for example, hydrocarbons) reservoirs hosted in similar dolomitized carbonate successions. Distribution, size and shapes of reservoir rocks (i.e. dolomite bodies) could be broadly predictable if the characteristics of the palaeo‐fault system present at the time of dolomitization are known.

The work tackles the control exerted by a sub-seismic fracture network on both secondary porosity and correspondent permeability of outcropping tight carbonates. Taking advantage of excellent 3D exposures, located in the Murge Plateau of southern Italy, the fracture network is investigated at different scales of observation. The rock multi-layer is made up of 10's of cm-thick, sub-horizontal, laterally continuous limestone beds crosscut by stratabound fractures, non-stratabound fractures, and small faults named as persistent fracture zones with low amounts of offset. Stratabound fractures consist of bed-perpendicular joints and sheared joints, non-stratabound fractures of incipient, cm-offset, sub-vertical faults, whereas the 10's of cm-offset persistent fracture zones are made up of 10's of m-high, m-thick fractured damage zones. The aforementioned structural elements localize within discrete carbonate units bounded by primary features such as bed surfaces, prominent surfaces and sedimentary breccia horizons. Such interfaces therefore affected the fracture stratigraphy of the limestone rock, and thus impact the fluid flow properties of the carbonate multilayer by compartmentalizing deformation.In the field, the fracture network is investigated by means of scanline and scan area methodologies to document the orientation, intensity, height distribution, mechanical aperture and roughness of individual fractures exposed along vertical outcrops and pavements of abandoned quarries. Then, Discrete Fracture Network (DFN) models of representative geocellular volumes are built, according to the different scales of analysis, to compute both fracture porosity and correspondent permeability (Kxx, Kyy, Kzz). Results of such a work show that the most prominent non-stratabound fracture set forms the major control on fluid storage and migration at the scales of single beds and bed-packages. At a larger scale, we document that fluid migration mainly occurs along the persistent fracture zones, which enhance the fault-parallel flow. As a whole, the persistent fracture zones form localized fluid conduits embedded within carbonate matrices that show isotropic fluid flow properties.

This paper describes a similar to 200 m-thick section of the Pisco Formation exposed at Cerro Colorado, an important fossiliferous site in the Ica desert. In order to properly place the fauna in its correct relative position, this study establishes the stratigraphic framework within which the different fossil-bearing intervals of this site can be compared and may prove invaluable in future high-resolution studies on the faunal change. Most of the Pisco Formation deposits exposed at Cerro Colorado consist of gently dipping fine-grained sandstones, diatomaceous siltstones and diatomites with minor ash layers and dolomites deposited within nearshore and offshore settings. To facilitate detailed stratigraphic correlations within the Pisco strata for a 30 km(2) area, eight marker beds have been defined and large-scale (1:10,000 scale) geological mapping conducted to determine fault positions, styles and offsets. The geological map shows that there are two important angular unconformities in the study area. The first one is the interformational basal unconformity of the Pisco Formation against folded, faulted, and planated Oligo-Miocene rocks of the Chilcatay Formation. The second is a low-angle intraformational erosional discontinuity of up to 4 degrees angular discordance that allows the subdivision of the Pisco stratigraphy exposed in the study area into two informal allomembers. Dating of the exposed succession by diatom biostratigraphy suggests that the age of the lower allomember is late Miocene, whereas the upper allomember is late Miocene or younger.

New knowledge on patterns of fibrous gypsum veins, their genetic mechanisms, deformation style and weathering is provided by a field- and laboratory-based study carried out on the Neogene to Quaternary Pisco Basin sedimentary strata exposed in the Ica desert, southern Peru. Gypsum veins vary considerably in dimensions, attitudes and timing, and can develop in layered and moderately fractured rocks also in the absence of evaporitic layers. Veins occur both as diffuse features, confined to certain stratigraphic levels, and localized within fault zones. Arrays formed by layer-bounded, mutually orthogonal sets of steeply dipping gypsum veins are reported for the first time. Vein length, height and spacing depend on the thickness of the bed packages in which they are confined. Within fault zones, veins are partly a product of faulting but are also inherited layer-bounded features along which faults are superimposed. Owing to the different petrophysical properties with respect to the parent rocks and their susceptibility to textural and mineralogical modifications, water dissolution and rupture, gypsum veins may have a significant role in geofluid management. Depending on their patterns and grade of physical and chemical alteration, veins may influence geofluid circulation and storage, acting as barriers to flow and possibly also as conduits.

An up to 1500m-thick clastic succession, the late Zanclean Baronia Synthem, has been analysed in detail in the Ariano wedge-top Basin (southern Apennine, Italy). In the Baronia Mountains the studied sediments are well exposed and laterally mappable due to young uplift and exhumation and rest unconformably on a complexly deformed pre-Pliocene substratum formed by Triassic to Miocene allochthonous units. The Baronia Synthem has been resolved into seven facies associations that are representative of distinct fluvial, deltaic, nearshore and offshore depositional environments and can be grouped into lowstand, transgressive, and highstand systems tracts. Using an integrated approach comprising original geological field mapping at 1:10,000 scale, conventional sedimentary facies analysis and a sequence stratigraphic approach, this paper provides a detailed description and interpretation of facies associations and new insights on the stratigraphic architecture and the geological history of this portion of the basin fill.

We present the results of in situ permeability measurements performed, using a portable field permeameter, on normal and strike slip 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.

This outcrop-based study reports diffuse joints and veins, normal to strike-slip fault zones and minor folds that developed, from Miocene to Quaternary, within the clastic to siliceous sedimentary record of the forearc Pisco Basin of southern Peru. Patterns, orientations, dimensional parameters and other outcrop-scale characteristics of the various deformation features are illustrated and their genetic mechanisms and timing of development are inferred. These new structural data and interpretations allow a better constraint of the structural style and evolution of the Pisco Basin, and can represent useful guidelines for characterizing the outcrop-scale deformation affecting similar forearc basins along the Peruvian coast.Major results of this study are that the development of the documented deformation features, their patterns, dimensional parameters and kinematics seem influenced by local perturbations of the paleo-stress field by mechanic processes partly independent of plate tectonics forces. These processes include strain localization on both pre-existing and progressively forming new structural discontinuities, and cyclic switches of the horizontal, principal stress axes sigma 2 and sigma 3. In particular, we discuss how different normal fault patterns, from sub-parallel to multidirectional/polygonal, could form in a same deformation phase in response of the local sigma 2/sigma 3 magnitude ratio, as an evolution of stratabound, mutually orthogonal vein sets. (C) 2015 Elsevier Ltd. All rights reserved.