1,721,059 research outputs found
Outcrop fracture network characterization for unraveling deformation sequence, geomechanical properties distribution, and slope stability in a flysch sequence (Monte Venere Formation, Northern Apennines, Italy)
No full textA detailed characterization of outcrop fracture networks in the turbiditic flysch sequence of the Monte Venere Formation (Northern Apennines) together with in situ measurements of rock strength using the Schmidt hammer provided important insights into the sequence of deformation and the slope stability conditions. The inferred sequence of structure formation from oldest to youngest is bedding-parallel cleavage, veins and normal faults, joints, and strike-slip faults (sheared joints). Alteration halos around fractures (joints, splay joints, strike-slip faults, and some normal faults) point out that these structures were conductive to meteoric water during uplift and erosion in the Holocene. Calcite-filled veins without alteration halos are considered local barriers to fluid flow and diffusion. Bedding thickness controls rock fracturing characterization parameters in the Monte Venere Formation. Reactivation in shear of pre-existing structures, however, causes the formation of splay joint clusters that locally increase fracture density contributing to degrade the mechanical strength of the rock. These localized clusters are apparent in detailed outcrop maps but they are usually not detected by the rock fracturing characterization parameters. Our data also imply that the presence of bedding-parallel cleavage is more important than layer thickness in controlling the rock compressive strength and ultimately the peak shear strength along a potentially sliding surface. This study takes closer look at landslide formation in a sloped flysch sequence under Mediterranean climate conditions and allowed to consider a conceptual model for landslide occurrence in which structural discontinuities and meteoric water flow through fracture networks are main triggering factors
Structural control on the occurrence of hydrocarbons in the central Roman Quarry near Lettomanopello, Majella Mountain
No full textCharacterization of sub-seismic resolution structural diagenetic heterogeneities in porous sandstones: Combining ground-penetrating radar profiles with geomechanical and petrophysical in situ measurements (Northern Apennines, Italy)
No full textDeformation bands and structurally-related diagenetic heterogeneities, here named Structural Diagenetic Heterogeneities (SDH), have been recognized to affect subsurface fluid flow on a range of scales and potentially promoting reservoir compartmentalization, influencing flow buffering, and sealing during production. Their impact on reservoir hydraulic properties depends on many factors, such as their permeability contrast with respect to the undeformed reservoir rock, their anisotropy, thickness, geometry as well as their physical connectivity and arrangement in the subsurface. We used the Ground Penetrating Radar (GPR) for detection and analysis of the assemblage of deformation bands – carbonate nodules, in high-porosity arkose sandstone in Northern Apennines of Italy. 2D GPR surveys allowed the description of the SDH spatial organization, their geometry, and their continuity in the subsurface. Petrophysical (air-permeability) and mechanical (uniaxial compressive strength) properties of host rock, deformation bands, and calcite-cement nodules were evaluated along a 30-m thick stratigraphic log to characterize the permeability and strength variations of those features. The assemblage “deformation bands – nodules” degrade porosity and permeability and produce a strengthening effect of the rock volume, imparting a strong mechanical and petrophysical heterogeneity to the pristine rock. Different textural, petrophysical, and geomechanical properties between deformation bands, nodules, and host rock result in different GPR response (permittivity). Thus, GPR response could be used to extend outcrop data (petrophysical and geomechanical) of SDH to 3D subsurface volumes in a way to reconstruct realistic and detailed outcrop analogs of faulted aquifers and reservoirs in porous sandstones
From Fractures to Flow, a Field-based Quantitative Analysis of an Outcropping Reservoir
No full textA large scientific interest has risen in the last decade on the characterization of carbonates’ mechanical and
petrophysical properties, as well as of the deformation mechanisms they are subjected to at shallow crustal levels.
On this regard, we take advantage of an exposed hydrocarbon-bearing, fractured carbonate reservoir to address
the role played by fractures present within fault damage zones on fluid flow. The study area is located along the
northern termination of the Majella anticline, in central Italy, within a quarry originally excavated by the ancient
Romans. There, hydrocarbons in the form of tar residues are present within two high-angle, oblique normal faults
and in the porous carbonate beds flanking the faults.
After having documented the faulting processes and the mechanisms of carbonate deformation (Agosta et al.,
2009), in this presentation we focus on the relation between the individual fracture characteristics (typology,
length, spacing, aperture, orientation, connectivity, and distance from slip surfaces) and tar distribution. This is
accomplished by analyzing data obtained from 1D scan lines measurements carried out along the quarry’s vertical
outcrops. Within the fault damage zones, most scan lines were oriented perpendicular to the main fracture sets,
while a few others perpendicular to smaller faults.
We measured the attitude, length, distance from arbitrary points of the scan lines, aperture, content, and abutting
relations of the tips of 1.148 fractures, 620 of which were tar-free whereas 528 contained tar residues. Data
obtained from scan lines oriented perpendicular to the main fracture sets are used to investigate the relation
between individual fracture characteristics (spacing, length, aperture, and orientation) and tar infilling. Data
gathered from scan lines oriented perpendicular to small faults allow us to study the relation percentage of
hydrocarbons in fractures with respect to fracture distance from slip surfaces. This is done separately for fractures
in the fault hanging walls and fault footwalls.
After data computation, we discuss the individual relations among the different fracture characteristics and tar
distribution in terms of fracture anisotropy within the deformed carbonates, and assess the control exerted by the
local stress state on fracture porosity and, hence, on hydrocarbon migration. Then, we characterize the cumulative
length and spacing distribution laws of individual fractures and sheared fractures present in either undeformed
carbonate host rocks (background deformation) or faulted carbonates (fault-related deformation), and infer the
local stress state during deformation.
Our data and observations are consistent with the following conclusions: (i) at a large scale, hydrocarbons are
present primarily within releasing jogs of left-stepping normal faults characterized by minor component of
left-lateral slip (ii) at smaller scales, the structural position of small and medium faults crosscutting the carbonate
damage zones plays a role for hydrocarbon migration as well; (iii) fracture length and spacing data do not show
any relation between with tar distribution; (iv) fracture aperture data suggest the possible control exerted by the
stress state conditions on fracture aperture and, hence, on hydrocarbon flow; and (v) the fracture arrays showing a
pronounced anisotropy seem to focus hydrocarbon migration
Chert nodules in pelagic limestones as paleo-stress indicators: A 3D geomechanical analysis
No full textThe objective of this study is to explain the occurrence and paleo-stress significance of 3D joint clustering in chert nodules (inclusions) within a layered pelagic limestone sequence. The difference in stiffness between chert and limestone is about one order of magnitude. Field observations show that fracture localization occurs mostly in chert nodules as opposed to the limestone matrix. We show with a novel three-dimensional geomechanical modelling analysis how the inclusion (ellipsoid) axes ratio influences fracture intensity and propagation within and outside the chert nodules and how the nodules record different deformation phases under different remote stress conditions. From field observations, we recognize two joint sets in the chert nodules: joints parallel and normal to the plane containing the two major axes of the nodule (bedding plane). In the nodules, the normal-to-bedding joints are interpreted to be younger than the parallel-to-bedding ones. The modelling of the 3D Eshelby solution for the stress field inside the chert nodule and in the surrounding matrix is consistent with our field observations and it suggests a strong differential stress during deformation (σminr/σmaxr < 0.3). Chert nodules in a deformed carbonate sequence, therefore, can provide important clues on the paleo-stress conditions, the temporal sequence of events, and fracture distribution heterogeneity
Evolution of strike-slip faults in basinal carbonate deposits within a thrust front sheet: A case study from the Maiella mountain, central Italy
No full textStructural control on karst water circulation and speleogenesis in a lithological contact zone: The Bossea cave system (Western Alps, Italy)
No full textGeological surveys, structural mapping, and thin section analysis helped to unravel the speleogenesis and water
circulation in the Bossea cave (Northern Italy), an unconventional karst systemat the contact between carbonates
and underlying insoluble rocks. The surveys within the cave and in the surrounding surface outcrops revealed a
complex structural setting caused by the coupling of lithological-mechanical heterogeneities with two major regional
left-lateral ESE-WNWstrike-slip faults. Transpression associated with the faults led to a disharmonic deformation,
accommodated by a detachment surface, between the buckled basement meta-volcanics and the
disharmonically-folded-marble sequence laying on top. Low-angle shear zones, recumbent or drag folds, and
small stacks of duplexes are localized next to this detachment. The preferential pathways for water circulation
inside the vadose zone are the folded bedding interfaces in the meta-carbonate sequence. Fracture clusters at
slipping bed terminations, on the other hand, provide through-sequence connectivity. Once underground stream
water flow reached the cataclastic and highly deformed meta-volcanics of the detachment core and damage
zones, erosion started, removing large volumes of rocks. The downward erosion of the Permian basement
rocks caused gravitational instability and subsequent roof collapses, aided by structural control in the damage
zone above the detachment surface (slip surfaces in between duplexes or drag folds). This gravitational instability
ultimately led to the formation of the giant halls that are characteristic of the downstreampart of the Bossea cave
From fractures to flow: A field-based quantitative analysis of an outcropping carbonate reservoir
No full textFractures can play an important role in the fluid storage –migration properties of fault damage zones. In this present contribution, we document the role exerted by fractures on fluid flow in carbonate damage zones of hydrocarbon-bearing, km-long, oblique-slip normal faults with 10's of m-throw. The carbonate fault damage zones were analysed by mean of scan line surveys conducted in both tar-free and tar-rich outcrops. In this way, the relationships among the individ ual fracture characteristics (length, spacing, aperture, orientation,
connectivity and distance from slip surfaces pertaining to small faults of the fault damage zones) and hydrocarbons have been established. Data obtained by scan line surveys were also used to compute the
amount of fracture porosity, the degree of fracture connectivity and, based upon simple assumptions, the orientation of the local σhmax at times of faulting. Additionally, scan line surveys were also carried out along outcrops exposing unfaulted carbonate host rocks. The results of our computation are consistent with a carbonate host rock made up of a quite isotropic fracture array comprised of isolated and coupled fractures, in which individual fracture sets have negative exponential spacing distributions. In terms of fluid fl ow, the
fracture array of the carbonate host rock enhances the fluid storage. Conversely, the fracture array of the fault damage zones is characterized by a pronounced anisotropy due to interconnected fractures, which enhance the fluid migration. Fractures in the fault damage zones include those inherited from background
deformation and others related to the faulting processes. The latter fracture sets are characterized by power law spacing distributions. In conclusion, counter-intuitively, both fracture length and fracture spacing do not have any correlation with hydrocarbons in the fault damage zones. On the contrary, fracture anisotropy, fracture spread and fracture orientation are positively correlated with hydrocarbons
Effects of outcrop-scale structural and diagenetic heterogeneities on flow and mass transport in a porous sandstone aquifer
Full text linkWe apply outcrop-based structural and in-situ petrophysical properties measurements for the construction of flow and mass transport calibrated numerical models in a porous sandstone aquifer. The hydraulic conductivity in this aquifer is influenced by the presence of deformation bands and related carbonate nodules. These heterogeneities are shown to decrease the hydraulic conductivity of the host rock by 2-3 orders of magnitude. The result obtained is robust, given that the models were calibrated with hydrologic field data. Our upscaling methodology for hydraulic conductivity allows inclusion of outcrop-scale structures and diagenetic features by means of inversion of the advective velocity for conservative particles. This approach can be used for easily implementing field data in aquifers or other geofluids reservoir simulators. Our experiments show that the use of an equivalent isotropic hydraulic conductivity approach fails to correctly account for mass transport in porous sandstone aquifers and we recommend implementing, as much as possible, the local heterogeneities and anisotropies in hydraulic conductivity within the model to be able to have a more realistic and conservative estimate of advection and dispersion. Our findings should be helpful to those scientists dealing with geofluids modeling and groundwater pollution
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