1,721,267 research outputs found
Microstructural, petrophysical, and mechanical properties of compactive shear bands associated to calcite cement concretions in arkose sandstone
No full textDiagenetic concretions are common in sandstones, but little is known about a possible structural control on such features, and even less so in arkose. We studied compactive shear bands (CSB) with cataclasis in high-porosity arkose associated with spatially heterogeneous carbonate cementation. We investigated the mineralogy, microstructures, grain-size and grain-shape, porosity and pore-size, for both host rock and CSBs. We documented the relationship between CSBs and concretions and quantified the effect of both deformation and cementation on petrophysical and mechanical properties of the assemblage bands-nodule with respect to the pristine rock. Microstructural observations on this particular mineralogy allowed us to identify different fracturing mechanism that affect feldspar (intragranular fracturing) and quartz (spalling/flaking of edges). Cleavage-facilitated fracturing of feldspar results in i) preferential cataclasis of feldspar with respect to quartz, and ii) cleavage control on cataclasis, grain-shape and grain-organization within the CSB. The spatial association between concretions and CSBs suggests that CSBs played a role in cement precipitation. The combined effect of deformation and cementation on mechanical properties is reflected on the rock-strength measurements that show a rock strengthening in the assemblage bands-nodule. Our findings represent new insights into the deformation mechanism in arkose, the relationship between structures and diagenesis as well as their effect on petrophysical and geomechanic characteristics
Polygonal deformation bands
No full textWe report for the first time the occurrence of polygonal faults in sandstone, which is compelling given that layer-bound polygonal fault systems have been observed so far only in fine-grained sediments such as clay and chalk. The polygonal faults are shear deformation bands that developed under shallow burial conditions via strain hardening in dm-wide zones. The edges of the polygons are 1-5 m long. The shear deformation bands are organized as conjugate faults along each edge of the polygon and form characteristic horst-like structures. The individual deformation bands have slip magnitudes ranging from a few mm to 1.5 cm; the cumulative average slip magnitude in a zone is up to 10 cm. The deformation bands heaves, in aggregate form, accommodate a small isotropic horizontal extension (strain <0.005). The individual shear deformation bands show abutting T-junctions, veering, curving, and merging where they mechanically interact. Crosscutting relationships are rare. The interactions of the deformation bands are similar to those of mode I opening fractures. The documented fault networks have important implications for evaluating the geometry of km-scale polygonal fault systems in the subsurface, top seal integrity, as well as constraining paleo-tectonic stress regimes
Water and (bio)chemical cycling in gravel pit lakes: A review and outlook
Full text linkThe world produces 1.7 × 108 metric tons of gravel and sand per year (USGS, 2015) creating many gravel pit lakes that change the morphology and drainage pattern of catchments. Gravel pit lakes abruptly intersect the geologic layering creating an environment where surface and groundwater will interact and where elaborate food webs can develop. Here we preview previous work on gravel pit lakes and compiled a comprehensive hydrochemical database to compare the chemistry of gravel pit lake water with other types of surface and groundwater. Water budget calculations confirm that gravel pit lakes cause freshwater loss in temperate and Mediterranean climates where surface water evaporation is larger than the actual evapotranspiration of vegetated land that was replaced by the gravel pit lakes. Groundwater fed gravel pit lakes where evaporated water is replaced by groundwater are especially sensitive to climate change.The gravel pit lakes included in this review have a relatively low acidity and high alkalinity most likely caused by weathering and leaching of carbonates in the catchment. The inflow of groundwater is a key process in gravel pit lakes with important consequences. The creation or presence of the gravel pit lakes may induce fluctuation of the up-stream water table which enhances groundwater flow and redox reactions in the soil. Groundwater rich in dissolved elements typically meets more alkaline water in gravel pit lakes enhancing the precipitation of metal oxides, calcite and other composite minerals including phosphorus (P), calcium (Ca) and carbon (C). Gravel pit lakes provide many different ecological habitats increasing the biodiversity in typically an agricultural or urban setting. Plant and animal species observed in gravel pit lakes consists of phytoplankton, zooplankton, micro plankton, macrophytes, fish and birds similar to natural lakes but the fact that gravel pit lakes may be only groundwater fed, or instead in open contact with rivers causes large variations between the ecosystem of different lakes. Plants and animal species take part in the chemical cycling of gravel pit lakes by, among others, uptake of atmospheric carbon dioxide (CO2) and nitrogen (N2), of dissolved compounds including bicarbonate (HCO3), iron (Fe) and manganese (Mn); of elements including phosphate (P) and Fe from lake sediments, and carbon mineralization and burial. Gravel pit lakes may contribute to denitrification of groundwater as N is consumed by plankton, but they may also enhance the mobilization of soil-bound compounds like potentially toxic (trace) metals released from aquifer sediments. The creation of gravel pit lakes provides more available sites for carbon burial but once deposited on the lake bottom, metals and other elements may be released again due to redox cycling, influenced by climatic or land use change. Gravel pit lakes are water bodies of recent formation and so far only a few different settings have been studied in detail compared to other types of natural- and man-made lakes. From this review it is evident that gravel pit lakes are hydrochemically most similar to so called 'marl lakes' or 'nutrient rich' lakes. Key areas for further research include the study of gravel pit lakes in other settings to better separate the similarities and differences between natural and gravel pit lakes. Also the feedback mechanisms between change in land use and climate, ground- and lake water chemistry ecological functioning and use of the gravel pit lakes need to be addressed
Compactive shear bands
No full textHigh-porosity arkose sandstones of Loiano (LS; Northern Apennines, Italy) are medium to coarse-grained and composed by quartz, feldspar, rock fragments, a few carbonate clasts, and minor accessories. LS are an example of structural-controlled diagenesis, because of the presence of calcite-cement nodules associated to the deformation bands (DBs). A DB consists of a cataclastic and low-porosity tabular zone with a thickness from ≤1 mm up to 2 mm. Slip along single DBs ranges from 4-5 mm up to 2-4 cm. Zone of bands thickness ranges between 0.8 and 60 cm and accommodate offset up to 0.5 m. Lithology (clay content) and rock properties (porosity, sorting, grain-size) are essential factors in determining fault zone characteristics. Different fracture mechanisms characterize quartz (flaking, spalling) and feldspar (intragranular fracturing along cleavage). These different fracture mechanisms control DB’s fabric and grain size distribution. Brittle conditions in the DB core are also demonstrated by bookshelf-like micro-fracturing. DBs are characterized by compaction and cataclasis that result in extensive grain interlocking promoting strain hardening. Porosity reduction produces a corresponding decrease in permeability down to 3 orders of magnitude with respect to the host rock. Cementation increase the cohesion and the reduction of porosity/permeability caused by deformation. Nodules along zone of bands are the most cohesive and weathering-resistant features in the outcrop
Structural control on epigenic gypsum caves: Evidences from Messinian evaporites (Northern Apennines, Italy)
No full textWe documented the relations among fractures, bedding, geological structures and the development of epigenic
gypsumcaves in the Bologna sector of the Northern Apennines, Italy. Different methods such as surface and subsurface
surveys, aerial photo interpretation, structural analyses and hydrogeological observations were integrated
to unravel different types of structurally-controlled epigenic gypsum karst speleogenesis. Data collected
highlight a close relationship between the main directions of caves development and structural trends in the
Northern Apennines (NW-SE and NE-SW). The results show that large offset faults (N10 m), bedding interfaces
and anticline axes control the development of underground drainage basins and compartmentalize flow
among adjacent areas. Furthermore, collapse hall morphology is mostly controlled by high-angle joints, bedding,
and splay joints associated with bedding plane parallel faults, while the main structural weaknesses focus the
surface flow, leading to a concentrated pattern of underground drainage and the formation of dominant conduits
Combining Ground-Penetrating Radar profiles with geomechanical and petrophysical in situ measurements to characterize sub-seismic resolution structural and diagenetic heterogeneities in porous sandstones (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, altering flow paths, 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. Deformation bands offsets (from a few mm to 20-40 mm) and diagenetic heterogeneities (carbonate nodules) dimensions (from 0.2 to 15 m in length; from 0.1 to 1.0 m in thickness) make them SDH below seismic resolution.
We used Ground Penetrating Radar (GPR) for detection and analysis of the assemblage “deformation bands - carbonate nodules”, in high-porosity arkose sandstone of the Northern Apennines (Italy). Petrophysical (air-permeability) and mechanical (uniaxial compressive strength) properties of host rock, deformation bands, and calcite-cement nodules were evaluated along a 30-meters thick stratigraphic log to characterize the permeability and strength variations of those features. 2D GPR surveys allowed the description of the SDH spatial organization, geometry, and continuity in the subsurface. The assemblage “deformation bands – nodules” decreases porosity and permeability and produces a strengthening effect of the rock volume, inducing a strong mechanical and petrophysical heterogeneity to the pristine rock. Different textural, petrophysical, and geomechanical properties of deformation bands, nodules, and host rock result in different GPR response (dielectric permittivity; instantaneous attributes). We show that GPR can be useful to characterize variations in petrophysical and geomechanical properties other than characterize the geometry and spatial distribution of flow baffles and small-scale flow barriers in the subsurface such as deformation bands and cement-nodules. GPR showed its worth as a high-resolution and non-invasive tool to extend outcrop information (petrophysical and geomechanical data) to 3D subsurface volumes in a way to reconstruct realistic and detailed outcrop analogues. Such potential could be critical in assisting and improving the characterization of SDH networks in the study of faulted aquifers and reservoirs in porous sandstones
In–situ quantification of mechanical and permeability properties on outcrop analogues of offshore fractured and weathered crystalline basement: Examples from the Rolvsnes granodiorite, Bømlo, Norway
Full text linkFractured and weathered crystalline basement units below erosional unconformities potentially represent unconventional
reservoirs for georesources (oil, mineral and water). The reservoir properties and characteristics
strongly depend on secondary processes connected to the local structural and alteration/weathering history.
Here we present the results of in–situ field quantification of mechanical (uniaxial compressive strength) and
petrophysical (permeability) properties of a fractured and weathered crystalline basement at selected outcrops
on the island of Bømlo (western Norway). The Bømlo outcrops are believed to represent an onshore analogue of
the unconventional oil reservoir hosted in the offshore Utsira High granodioritic fractured basement (northern
North Sea). The off– and onshore crystalline basements have both undergone surficial weathering during the
Mesozoic, as shown by the occurrence of a dated, variably thick saprolitic profile on top of fresh fractured
basement blocks. The Bømlo crystalline basement is characterized by a complex and highly permeable fracture
network. Fault rocks within its fault zones are characterised by an anisotropic mechanical strength and by an
average permeability that is two orders of magnitude larger than that of the host rock. The matrix permeability
and mechanical strength are significantly affected by alteration/weathering products. Analysis of the textural
and mineralogical characteristics of the weathered outcrops allowed us to constrain the variation of permeability
and mechanical strength as a function of increasing alteration and to infer their distribution in the, now eroded,
top–basement weathering profile on Bømlo. Weathering enhances permeability and drastically decreases the
mechanical strength. Nevertheless, evolved saprolitic horizons may act as low–permeability top–seal units to the
fractured and weathered crystalline basement reservoir. The obtained permeability and mechanical data are
finally used to better constrain the potential reservoir rocks, the fluid migration pathways, and to discuss their
role in the geomechanics of a conceptualised fractured and weathered crystalline basement unconventional
reservoir
Structurally controlled development of a sulfuric hypogene karst system in a fold-and-thrust belt (Majella Massif, Italy)
No full textWe documented the deformation in the southeastern domain of the Majella anticline (Central Apennines, Italy) to
highlight timing and structural characteristics of different fracture sets affecting the outcropping Cretaceous-
Miocene ramp carbonates. An isolated and inactive hypogene karst system produced by sulfuric acid (Cavallone-
Bove cave system) was studied following a multidisciplinary approach. Our findings suggest that deeprooted,
sub-vertical strike-slip fault zones reaching the H2S source rocks were the main vehicle for ascending
acidic fluid flow. Linkage and intersection of these faults by splays in extensional stepovers and pre-orogenic
normal faults permitted ascending fluids to reach multiple recharge points (feeders) near the paleo watertable.
In proximity to the oxygenated groundwater, where H2SO4 was produced, lateral dissolution focused
along bedding planes and zones of localized deformation (fracture clusters) characterizing the hinge of the
anticline. We conclude that structural position in the anticline and large-offset, vertically extended strike-slip
fault zones control the localization of efficient permeability pathways and represent first order controlling features
for fluid flow in the fold-and-thrust belt. This study provides insights into the understanding of time-space
evolution, geometry, and pattern of sulfuric hypogene karst systems in folded carbonates, whose prediction is
critical for fractured and karstified reservoirs
Freshwater resource characterization and vulnerability to climate change of the Shela aquifer in Lamu, Kenya
No full textSalinization of coastal groundwater systems
causes a severe deterioration both in amount and quality of
fresh groundwater resources. To support the sustainable
use and management of fresh groundwater, quantification
and characterization of these coastal resources are important
in view of the population growth anticipated in many
African countries. Analytical methods were used to determine:
(1) the shape and volume of the freshwater lens, (2)
the elevation of the water table, (3) the depth of the
freshwater/saltwater interface in the Shela aquifer, and (4)
the expected change of volume resulting from change of
recharge and sea level rise driven by climate change. The
results of the analytical modelling have shown that the
average hydraulic conductivity is 0.755 m/d, the average
water table elevation is 2 m above sea level and the average
depth of the freshwater/saltwater interface is -
80 m.a.s.l. The volume of the aquifer is &124 9 106 m3
when discharge from the Shela well field is factored in.
Climate change is expected to have an impact on the
recharge and ultimately the aquifer’s volume; under the
A1b conditions, the volume is expected to increase to
199 9 106 m3 whereas in the A2 scenario it is expected to
decrease to 27 9 106 m3. The saltwater intrusion indicator
M for today’s conditions (0.004) decreases to 0.5 M in the
A1b scenario by 2100 whilst it increases to 24.9 M in the
A2 scenario for the same time period, indicating an
extremely higher vulnerability to saltwater intrusion in the
latter scenario. A simple linear correlation with the
expected population growth of 1.25 million people by 2050
shows the aquifer failing as a water source by 2033
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