Deformation Bands In Sandstones Research Articles

The use of network topology to assess connectivity of deformation bands in sandstone: A quantitative approach based on digital outcrop models

Clusters of deformation bands associated with fault damage zones modify the petrophysical properties of rocks and influence fluid migration. Many works discuss the influence of deformation bands regarding changes in porosity and permeability, but only a few address connectivity as a control of the fluid flow in porous media. We used digital outcrop models to perform a quantitative assessment of connectivity using a UAV-based system at the Melancias outcrop, Rio do Peixe Basin, Brazil. Topology determined the frequency, intensity and dimensionless intensity of deformation bands in three circular windows visually analyzed and interpreted. The geometry of the bands defines the proportion of node types, influences the distribution of I-, Y- and X-nodes, and controls the connectivity of fractures along the outcrop. Results provided by geometric and topological analyses corroborate connection per line as the more reliable connectivity indicator. Furthermore, they were crucial to defining the northeast sector as a corridor to fluid flow, whereas the other two sectors tend to be permeability barriers. We also analyzed the influence of trace lengths on the band network's overall orientation and geometric arrangement. For modeling and simulations purposes, the spatial relationships of nodes and branches help in the understand of potential hurdles to fluid migration and the variability of reservoirs and aquifers.

Quantifying occurrence of deformation bands in sandstone as a function of structural and petrophysical factors and their impact on reservoir quality: an example from outcrop analog of Productive Series (Pliocene), South Caspian Basin

Deformation bands (DB) are known to influence porosity and permeability in sandstones. This study aims to predict the occurrence of DB and to quantify their impact on reservoir properties based on field measurements in the steeply dipping limb of a kilometer-scale fold in Yasamal Valley, western South Caspian Basin. An integrated approach of characterizing bands and their effect on reservoir properties included measurements of natural gamma radioactivity and permeability using portable tools, along with bed dip and the count of DB across distinct facies. A set of core analyses was performed on outcrop plugs with and without bands to estimate the alteration of rock properties at the pore scale. Interpretation of outcrop gamma-ray data indicates the absence of bands in Balakhany sandstones containing shale volume greater than 18% for unconsolidated and 32% for calcite-rich facies. A high amount of calcite cement appears to increase the number of DB. A poor, positive trend between bed dip and DB concentration was identified. We show that net to gross, defined as the thickness fraction of sandstone bound by mudstones, is among the parameters controlling the occurrence of bands. Samples containing a single DB show a 33% and 3% decrease in permeability and porosity, respectively, relative to the host rock. We reveal a new set of lithological and petrophysical factors influencing DB occurrence. This study offers a direct tool that can be applied in subsurface reservoir analogs to predict the occurrence and concentration of DB and estimate their influence on rock properties.

Regular spacing of deformation bands in sandstone: Layer-thickness control or constitutive instability?

We examine whether bedding thickness or constitutive properties correlate with the spacing of equally spaced shear-enhanced compaction bands and compactional shear bands in Miocene sandstones of the Whakataki Formation, North Island, New Zealand. Quantitative spacing analysis reveals a positive correlation between bedding thickness and the spacing of strata-bound deformation bands, as is commonly observed for veins, joints, and normal faults. The observed spacing to thickness ratio (STR) ranges from 0.02 to 0.14 and is considerably smaller than observed for veins, joints, and normal faults (usually ≥ 1). Modelling compaction bands as stiff tabular inclusions in a layered sequence, simple elastostatic numerical simulations show that they act as stress amplifiers, which may explain STR ≪ 1. Additionally, we obtained laboratory estimates of rock permeability and strength to test if the constant band spacing can be explained with a constitutive instability proposed for compaction bands: the stationary cnoidal wave. The results from this study indicate that this model fails to explain the observed spacing if we use the material properties obtained in the laboratory. This discrepancy likely arises from the following: [1] laboratory estimates of host-rock properties are not the same as those at depth; [2] the used semi-analytical solution for the stationary cnoidal wave employs an incorrect value (m = 3) of the stress exponent; and [3] the equation does not account for coupled layered media. While we cannot conclusively falsify the cnoidal-wave model for compaction band spacing per se, it appears likely that the existing solution should not be used for predicting band spacing in layered porous sandstones.

Extreme capillary heterogeneities and in situ fluid compartmentalization due to clusters of deformation bands in sandstones

Previous work has shown that individual deformation bands act like capillary barriers and influence fluid saturation. More common in nature, however, are clusters of deformation bands that form complex three dimensional geometries. The aim of this study is to analyze the extent and mechanisms of fluid compartmentalization due to clustered bands. Drainage multiphase fluid flow experiments were performed on a Navajo sandstone core sample characterized by diversely oriented clusters of deformation bands, that sub-divide the host rock into several compartments. Medical X-ray CT images were acquired while nitrogen was injected at progressively higher flow rates into a water-saturated core during transient and steady-state conditions. Spatial and temporal analyses of the non-wetting phase plume migration suggest that deformation bands act like capillary barriers and contribute to the development of an extremely tortuous saturation front. Differential pressure behavior across the core is linked to the breakthrough of N2 into the individual compartments, resulting in highly variable N2 saturation throughout the experiment. Migration into downstream compartments occurs via the exceedance of capillary entry pressure in portions of the bands. Simulation models of simplified systems demonstrate that capillary end effects and discontinuities in the deformation bands impact fluid saturation. The experiments and models presented here show that clusters of deformation bands have the potential to strongly compartmentalize a sandstone reservoir. Hence, prior analysis of the geometry of deformation band structures in a reservoir could significantly reduce the risk of overestimating reservoir capacity, and improve predictions of fluid mobility.

Deformation bands in chalk, examples from the Shetland Group of the Oseberg Field, North Sea, Norway

Deformation bands are described in detail for the first time in carbonate rock from the subsurface and in chalk from the North Sea. The samples are from 2200 to 2300 m below sea level, in upper Maastrichtian to Danian chalk in the Oseberg Field. The deformation bands were investigated using thin-section analysis, SEM and computed tomography (CT). There is a reduction in porosity from 30 to 40% in the matrix to ca. 10% or less inside the deformation bands. They have apparent thicknesses ranging from less than 0.05–0.5 mm and have previously often been referred to as hairline fractures. Their narrowness is probably the reason why these features have not previously been recognised as deformation bands. The deformation bands in chalk are very thin compared to deformation bands in sandstone and carbonate grainstones which have mm to cm widths. This is suggested to be due to the fine grain size of the chalk matrix (2–10 μm), and it appears to be a positive correlation between grain-size and width of deformation bands. The deformation bands are suggested to have been formed as compactional shear bands during mechanical compaction, and also related to faulting.

Propagation energies inferred from deformation bands in sandstone

Propagation energies for five examples of deformationbandsinporoussandstonesfromfourfield sites in Nevada, Utah, and France were calculated by using the J-integral approach. Pure compaction bands that accommodate primarily closing displace- ments from the Valley of Fire (Nevada) site have a closing-mode band propagation energy of JI = 5.5 ± 1.6kJ/m 2 . Shear-enhanced compaction bands having subequal amounts of normal (closing) and shear strains across them from the same site have propagation energies of JI = 6.11 ± 1.8kJ/m 2 and JII = 1.52 ± 0.5kJ/m 2 for closing and shearing modes respectively. Closing- and shearing-mode band propagation energies for shear-enhanced compaction bands from the Buckskin Gulch (Utah) site, 5.63 ±1.7 and 1.91 ± 0.57kJ/m 2 and the Uchaux (France) site, 11.0 ± 3.3and2.35 ± 0.7kJ/m 2 respectivelyarecom- parabletothevaluesforsimilarstructuresfromtheVal- ley of Fire site. Cataclastic deformation bands at the Orange (France) site accommodate significantly larger values of shear offset than do shear-enhanced compac- tion bands, with a ratio of shear to closing offset that exceeds 7. The calculated closing- and shearing-mode propagation energies for these shear bands, 17.8 ±5.4 and 3.3 ± 1.0kJ/m 2 respectively are comparable to those for pure or shear-enhanced compaction bands deformed at approximately the same depth of bur- ial. Strain softening of cataclastic compactional shear bands implies unstable shearing, whereas stable and perhaps strain-hardening shearing may be inferred for shear-enhanced compaction bands. Displacement- lengthscalingrelationsforshear-enhancedcompaction bands of the form D ∝ √ L are consistent with the

Caracterização meso e microscópica de bandas de deformação em arenitos porosos: um exemplo nas tectonossequências Paleozoica, Pré- e Sin-rifte da Bacia do Araripe, Nordeste do Brasil

Deformation bands are narrow tabular volumes developed in porous sandstones. Although these structures are a product of brittle deformation, they may have internally a continuous displacement gradient. When granular cataclasis is the dominant deformation mechanism, the initial properties of their host rocks (i.e., porosity and permeability) can change significantly. The deformation bands in sandstones from the pre- and syn-rift of the Araripe Basin were studied in meso and microscale in order to classify them and to understand the deformation mechanisms involved during their nucleation and development. Their geometric-spatial, kinematic, and rheological criteria allowed establishing relations between the origin of deformation bands and lithification of their host rocks. Additionally, some inferences on their influence to the fluid flow in the reservoir-scale were outlined. Moreover, the study of deformation bands contributed to the understanding of the tectonic evolution of the studied basin. Accordingly, the study of deformation bands can support research on local and regional aspects of the tectonosedimentary evolution of sedimentary basins.

Deformation bands in sandstone: a review

Deformation bands are the most common strain localization feature found in deformed porous sandstones and sediments, including Quaternary deposits, soft gravity slides and tectonically affected sandstones in hydrocarbon reservoirs and aquifers. They occur as various types of tabular deformation zones where grain reorganization occurs by grain sliding, rotation and/or fracture during overall dilation, shearing, and/or compaction. Deformation bands with a component of shear are most common and typically accommodate shear offsets of millimetres to centimetres. They can occur as single structures or cluster zones, and are the main deformation element of fault damage zones in porous rocks. Factors such as porosity, mineralogy, grain size and shape, lithification, state of stress and burial depth control the type of deformation band formed. Of the different types, phyllosilicate bands and most notably cataclastic deformation bands show the largest reduction in permeability, and thus have the greatest potential to influence fluid flow. Disaggregation bands, where non-cataclastic, granular flow is the dominant mechanism, show little influence on fluid flow unless assisted by chemical compaction or cementation.

Structural framework and Mesozoic–Cenozoic evolution of Ponta Grossa Arch, Paraná Basin, southern Brazil

The integration of structural analyses of outcrops, aerial photographs, satellite images, aeromagnetometric data, and digital terrain models can establish the structural framework and paleostress trends related to the evolution of Ponta Grossa Arch, one of the most important structures of the Paraná Basin in southern Brazil. In the study area, the central-northern region of Paraná State, Brazil, the arch crosses outcropping areas of the Pirambóia, Botucatu, and Serra Geral Formations (São Bento Group, Mesozoic). The Pirambóia and Botucatu Formations are composed of quartz sandstones and subordinated siltstones. The Serra Geral Formation comprises tholeiitic basalt lava flows and associated intrusive rocks. Descriptive and kinematic structural analyses reveal the imprint of two brittle deformation phases: D1, controlled by the activation of an extensional system of regional faults that represent a progressive deformation that generated discontinuous brittle structures and dike swarm emplacement along a NW–SE trend, and D2, which was controlled by a strike-slip (transtensional) deformation system, probably of Late Cretaceous–Tertiary age, responsible for important fault reactivation along dykes and deformation bands in sandstones.

Chemical bleaching indicates episodes of fluid flow in deformation bands in sandstone

Jurassic sandstones on the Colorado Plateau have been variably bleached through interaction with hydrocarbon-bearing solutions or other reducing agents. Deformation bands in the Navajo Sandstone have a variety of colors in comparison with the host rock color that indicate the timing of bleaching relative to deformation-band formation. White deformation bands in red sandstone indicate that deformation bands were likely permeable at an early dilatant stage in their development history. Field characteristics, petrography, bulk rock chemistry, clay mineralogy, and geochemical modeling show that bleached deformation bands experienced an episode of chemical reduction where fluids removed some iron and left the remaining iron as pyrite and magnetite. Mass-balance calculations show that as much as 10 kg of chemically reducing fluid per 100 g of rock (1500 pore volumes of fluid) are necessary to remove 0.1 wt.% iron from a deformation band. These large pore volumes suggest that moving, reducing solutions regionally bleached the sandstone white, and bleached deformation bands resulted where deformation bands provided localized fluid access to unbleached, red sandstone during an initial dilatant stage. Alternatively, access of reducing soil solutions may be provided by gravity-driven, unsaturated flow in arid to semiarid vadose zones. Color and chemical composition is a valuable index to the pathway and timing of hydrocarbon movement through both host rocks and deformation bands.

Chemical bleaching indicates episodes of fluid flow in deformation bands in sandstone

Chemical bleaching indicates episodes of fluid flow in deformation bands in sandstone

The damage zone-fault core transition in carbonate rocks: implications for fault growth, structure and permeability

We studied the nucleation and growth of cataclastic fault cores from fractured damage zones in extensional and strike-slip fault zones in carbonate rocks. Analysed fault zones have similar protolith lithology and sedimentary fabric, but different geometry, kinematics, size, tectonic environment and deformation history. Orthorhombic rock lithons, a few decimetres in size, characterise the structural fabric of damage zones. Lithons derive from the intersection of a dominant fracture/cleavage set with bedding and/or joints. At the damage zone–fault core transition, orthorhombic lithons reduce in size and approach an isometric shape. Their cross-sectional aspect ratio has an average value of 1.4. Analysed fault cores have similar rock textures, sorting and comminution degree. Particle-size distributions of fault core rocks show linear trends in log-log graphs and average fractal dimension of 2.5. Our results on rock fabrics suggest that fault core development initiates from rock masses in damage zones, where the shape anisotropy of orthorhombic lithons favours additional fracturing at high angle to their long axes. Eventually, smaller, nearly isometric lithons generate from repeated fracturing of orthorhombic lithons. When the aspect ratio of these lithons approaches the threshold value of about 1.4, particle rotation is favoured and cataclastic flow starts. Owing to the granular nature of the damage zone-fault core transitions in carbonate rocks, analogies with the nucleation of deformation bands in sandstones can be established. Our results may be of use to the industry for quantitative characterisation of fault zone permeability. According to the proposed model, radical changes on the permeability properties are expected during the growth of fault cores.

High resolution petrophysical measurements of deformation bands in sandstones

Sub-seismic scale deformation bands have an impact on production from hydrocarbon reservoirs since they have dramatically reduced porosity and permeability. These changes are commonly quantified using relatively low-resolution techniques, which invite host rock bias in measurement and inevitably underestimate the potential of these structures as fluid barriers. High-resolution petrophysical measurement within these discontinuities and the characterization of their variable microstructures are therefore critical in their incorporation into advanced reservoir simulation models. These have been carried out using pressure decay probe permeametry (PDPK) of slabbed sandstones containing a range of deformation band types combined with image analysis porosimetry of optical thin sections containing deformation bands. The higher resolution techniques show that much greater differences between host rock and deformation band petrophysical properties exist than when measured using conventional techniques. Cataclastic deformation bands in clean, highly porous sandstones have experienced the greatest reductions in sandstones have experienced the greatest reductions in sorting relative to their host. There is less contrast in these properties between clay-rich deformation bands and clay-rich host sandstones. This is due to an already low depositional porosity hence giving little opportunity to reduce the porosity further. The influence of lithology upon the formation of deformation bands does not apply to the same extent to cemented deformation bands, the cement of which is affective in modifying sandstones with high depositional porosity and permeability.

Spatial relationships within fault damage zones in sandstone

Orientation analyses of deformation bands in sandstones from the Gullfaks region, northern North Sea, show that different sets of bands are a common feature within damage zones associated with faults with displacement in the order of several tens of metres or more. The different sets may coexist within the same depth intervals, or may occur in cluster zones located at different depths. The strike of the deformation bands is generally subparallel to the main faults, but may deviate from this in complex areas where larger-scale faults with different orientations coalesce. Analyses of smaller-scale faults (less than a few tens of metres displacement) do not display the same clear “conjugate” sets of deformation bands. A possible explanation is that deformation bands in the early stages of fault development are subparallel to the main faults. As the displacement along the smaller faults increases, the bands may intersect and form a single slip surface with a more complex geometry. At this stage, a second set of deformation bands may develop in order to accommodate the different stresses that exist along and near the fault plane. During slip along a main fault, smaller splay faults may develop in the footwall or hanging wall. The splay faults will be associated with abundant deformation bands with orientations that may differ from that of the “conjugate” sets, depending on the orientation of the splay faults with respect to the main faults.

Distinct element modeling of deformation bands in sandstone

We have conducted numerical experiments with the distinct element method to study factors that control the development of deformation bands in sandstone. These experiments show how sorting and initial porosity of the host rock control the development and the mode of deformation in the area of strain localization. The results of the numerical experiments are in qualitative agreement with field and microstructural observations of deformation bands at Arches National Park (Utah). In our numerical experiments sand grains are modeled as cylindrical elements that move in response to externally applied boundary conditions. Systems of elements that have a large variability in radius and/or loose packing deform at lower applied stresses than systems of elements that have a uniform radius and/or tight packing. The mode of deformation in the first kind of aggregate is particulate flow, where elements of different sizes move easily with respect to each other due to a low degree of interlocking. The mode of deformation in the second kind of aggregates is localized failure on small deformation bands. Shear bands in our numerical experiments nucleate as a zone of dilatancy and propagate via organization of dilatant zones into discrete faults. The presence of a flaw in the form of a ‘weak’ grain promotes the nucleation and propagation of shear bands.

Field Aided Characterization of a Sandstone Reservoir: Arroyo Grande Oil Field, California, USA: ABSTRACT

The Arroyo Grande Oil Field in Central California has been productive since 1905 from the miopliocene Edna member of the Pismo formation. The Edna member is a massive poorly consolidated sandstone unit with an average porosity of 0.2 and a permeability of 1000-5000 md; the producing levels are shallow, 100 to 500 m from the ground surface. Excellent surface exposures of the same formation along road cuts across the field and above the reservoir provide an opportunity to study reservoir rocks at the surface and to relate fracture and permeability distribution obtained from cores to folds and faults observed in outcrops. We mapped in outcrops the major structures of the oil field and determine the statistical distribution and orientation of small faults (deformation bands) that have been observed both in cores and outcrop. The relation between deformation bands and major structures has also been characterized with detailed mapping. By using synthetic logs it is possible to determine the log signature of structural heterogeneities such as deformation bands in sandstone; these faults cause a neutron porosity drop respect to the host rock in the order of 1-4%. Image analysis has been used to determine the petrophysical properties of the sandstone inmore » outcrop and in cores; permeability is three orders of magnitude lower in faults than in the host rock and capillary pressure is 1-2 orders of magnitude larger in faults than in the host rock. Faults with tens of meters offsets are associated with an high density of deformation bands (10 to 250 m{sup -1}) and with zones of cement precipitation up to 30 m from the fault. By combining well and field data, we propose a structural model for the oil field in which high angle reverse faults with localized deformation bands control the distribution of the hydrocarbons on the limb of a syncline, thereby explaining the seemingly unexpected direction of slope of the top surface of the reservoir which was inferred by well data only.« less

Small faults formed as deformation bands in sandstone

Small faults with displacements of a few millimeters or centimeters are abundant in the Entrada and Navajo Sandstones, in the San Rafael Desert, Utah, where they are important primary structures, preceding the development of large faults with displacements of several meters or tens of meters. The small faults contain no surfaces of discontinuity, rather they occur as deformation bands about one millimeter and tens or hundreds of meters long, and across which the displacements are distributed. Two zones with different modes of deformation can be distinguished within a deformation band: an outer zone where the matrix, including pores and matrix material, deforms; and an inner zone, about 0.5 mm thick, where the sand grains fracture and further consolidation takes place. Fracturing of the grains is controlled by contact geometry; the grains tend to split into subgrains along lines connecting contact points between the grains. Then the angular subgrains, which are readily fractured, are further granulated and mixed with the matrix. The final product is the deformation band, with much smaller grain size, poorer sorting, and lower porosity than the original parent sandstone. The sandstone on either side of a deformation band is almost undisturbed — fractures are rare there — so that deformation is highly localized within the band. The material within a deformation band apparently strain hardens as a result of the deformation; perhaps this is why the shear displacement across a deformation band is at most a few centimeters.