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Deformation bands and fractures in porous rocks

Rocks respond to stress in the brittle regime by forming extension fractures and shear fractures (slip surfaces). Such fractures are sharp and mechanically weak discontinuities, and thus prone to reactivation during renewed stress build-up. At least this is how non-porous and low-porosity rocks respond. In highly porous rocks and sediments, brittle deformation is expressed by related, although different, deformation structures referred to as deformation bands.

Deformation bands are mm-thick zones of localised compaction, shear and/or dilation in deformed porous rocks. Figure above  shows how deformation bands kinematically relate to fractures in non-porous and low-porosity rocks, but there are good reasons why deformation bands should be distinguished from ordinary fractures. One is that they are thicker and at the same time exhibit smaller shear displacements than regular slip surfaces of comparable length (Figure (a) below). This has led to the term tabular discontinuities, as opposed to sharp discontinuities for fractures. Another is that, while cohesion is lost or reduced across regular fractures, most deformation bands maintain or even show increased cohesion. Furthermore, there is a strong tendency for deformation bands to represent low permeability tabular objects in otherwise highly permeable rocks. This permeability reduction is related to collapse of pore space, as seen in the band from Sinai portrayed in Figure (b) below. In contrast, most regular fractures increase permeability, particularly in low-permeability and impermeable rocks. This distinction is particularly important to petroleum geologists and hydrogeologists concerned with fluid flow in reservoir rocks. The strain hardening that occurs during the formation of many deformation bands also makes them different from fractures, which are associated with softening.

The difference among brittle fracturing of nonporous and porous rocks lies inside the reality that porous rocks have a pore quantity that can be utilised all through grain reorganisation. The pore space allows for effective rolling and sliding of grains. Even if grains are crushed, grain fragments may be organised into nearby pore area.

The kinematic freedom related to pore space lets in the special magnificence of structures referred to as deformation bands to shape.

What is a deformation band?

Types of deformation bands

How do deformation bands vary from regular fractures in non-porous rocks? Here are some traits of deformation bands:

  • Deformation bands are restricted to highly porous granular media, notably porous sandstone.
  • A shear deformation band is a wider zone of deformation than regular shear fractures of comparable displacement.
  • Deformation bands do not develop large offsets. Even 100 m long deformation bands seldom have offsets in excess of a few centimetres, while shear fractures of the same length tend to show meter-scale displacement.
  • Deformation bands occur as single structures, as clusters, or in zones associated with slip surfaces (faulted deformation bands). This is related to the way that faults form in porous rocks by faulting of deformation band zones.

Similar to fractures, deformation bands can be classified in a kinematic framework, where shear (deformation)bands, dilation bands and compaction bands form the end members (1st Figure). It is also of interest to identify the mechanisms operative during the formation of deformation bands. Deformation mechanisms depend on internal and external conditions such as mineralogy, grain size, grain shape, grain sorting, cementation, porosity, state of stress etc., and different mechanisms produce bands with different petrophysical properties. Thus, a classification of deformation bands based on deformation processes is particularly useful if permeability and fluid flow is an issue. The most important mechanisms are:

The different kinds of deformation bands, prominent by means of dominant deformation mechanism.

  • Granular flow (grain boundary sliding and grain rotation)
  • Cataclasis (grain fracturing)
  • Phyllosilicate smearing
  • Dissolution and cementation

Deformation bands are named after their characteristic deformation mechanism, as shown in Figure above.

And sediments, even as cataclastic float happens for the duration of deformation of nicely-consolidated sedimentary rocks and non-porous rocks.

Disaggregation bands develop by shear-related disaggregation of grains by means of grain rolling, grain boundary sliding and breaking of grain bonding cements; the process that we called particulate or granular flow (Figure above a). Disaggregation bands are commonly found in sand and poorly consolidated sandstones and form the “faults” produced in most sandbox experiments. Disaggregation bands can be almost invisible in clean sandstones, but may be detected where they cross and offset laminae (Figure below). Their true offsets are typically a few centimeters and their thickness varies with grain size. Fine-grained sand(stones) develop up to 1 mm thick bands, whereas coarser-grained sand (stones) host single bands that may be at least 5 mm thick.

Macroscopically, disaggregation bands are ductile shear zones where sand laminae can be traced continuously through the band. Most pure and well-sorted quartz-sand deposits are already compacted to the extent that the initial stage of shearing involves some dilation (dilation bands), although continued shear-related grain reorganization may reduce the porosity at a later point.

Right-dipping compaction bands overprinting left-dipping soft-sedimentary disaggregation bands (almost invisible). The sandstone is very porous except for thin layers, where compaction bands are absent. Hence, the compaction bands only formed in very high porosity sandstone. Thin section photo shows that the compaction is assisted by dissolution and some grain fracture. Navajo Sandstone, southern Utah.

Phyllosilicate bands(also called framework phyllosilicate bands) form in sand(stone) where the content of platy minerals exceeds about 10–15%. They can be considered as a special type of disaggregation band where platy

minerals sell grain sliding. Clay minerals tend to mix with other mineral grains inside the band while coarser phyllosilicate grains align to form a neighborhood cloth in the bands due to shear-caused rotation. Phyllosilicate bands are smooth to detect, because the aligned phyllosilicates provide the band a distinct colour or cloth that may be harking back to phyllosilicate-wealthy laminae in the host rock.

If the phyllosilicate content of the rock changes across bedding or lamina interfaces, a deformation band may additionally alternate from an almost invisible disaggregation band to a phyllosilicate band. Where clay is the dominant platy mineral, the band is a ?Ne-grained, low-porosity region that could acquire offsets that exceed the few centimeters exhibited with the aid of different kinds of deformation bands. This is associated with the smearing impact of the platy minerals along phyllosilicate bands that apparentlycounteracts any pressure hardening attributable to interlocking of grains.

If the clay content of the host rock is high enough (more than 40%), the deformation band turns into a clay smear. Clay smears typically show striations and classify as slip surfaces rather than deformation bands. Examples of deformation bands turning into clay smears as they leave sandstone layers are common.

Cataclastic bands form where mechanical grain breaking is significant (Figure b). These are the classic deformation bands first described by Atilla Aydin from the Colorado Plateau in the western USA. He noted that many cataclastic bands consist of a central cataclastic core contained within a mantle of (usually) compacted or gently fractured grains. The core is most obvious and is characterized by grain size reduction, angular grains and significant pore space collapse (Figure b). The crushing of grains results in extensive grain interlocking, which promotes strain hardening. Strain hardening may explain the small shear displacements observed on cataclastic deformation bands (3–4 cm). Some cataclastic bands are pure compaction bands (Figure above), while most are shear bands with some compaction across them.

Cataclastic bands arise most regularly in sandstones which have been deformed at depths of about 1.5?3 km, although evidence of cataclasis is also stated from deformation bands deformed at shallower depths. Comparison indicates that shallowly fashioned cataclastic deformation bands display much less in depth cataclasis than the ones formed at 1.5?3 km intensity.

Cementation and dissolution of quartz and other minerals may occur preferentially in deformation bands where diagenetic minerals grow on the fresh surfaces formed during grain crushing and/or grain boundary sliding. Such preferential growth of quartz is generally seen in deformation bands in sandstones buried to more than 2–3 km depth (>90 C) and can occur long after the formation of the bands.

In?Uence on ?Uid ?Ow

Very dense cluster of cataclastic deformation bands within the Entrada Sandstone, Utah.

Deformation bands shape a commonplace constituent of porous oil, gasoline and water reservoirs, where they arise as unmarried bands, cluster zones or in fault harm zones. Although they are not going to shape seals that can keep signi?Cant hydrocarbon columns over geologic time, they are able to in?Uence ?Uid ?Ow in a few cases. Their potential to accomplish that relies upon on their inner permeability shape and thickness or frequency. Clearly, the sector of cataclastic deformation bands shown in Figure above can have a miles more in?Uence on ?Uid ?Ow than the unmarried cataclastic band proven in Figure a or b on the pinnacle.

Cataclastic deformation bands show the maximum signi?Cant permeability reductions.

Deformation band permeability is ruled by using the deformation mechanisms operative throughout their formation, which again relies upon on a number of lithological and bodily elements. In widespread, disaggregation bands show little porosity and permeability discount, while phyllosilicate and, specifically, cataclastic bands show permeability discounts up to numerous orders of magnitude. Deformation bands are thin, so the quantity of deformation bands (their cumulative thickness) is vital whilst their function in a petroleum reservoir is to be evaluated.

Also critical are their continuity, version in porosity/permeability and orientation. Many show signi?Cant variations in permeability alongside strike and dip due to variations in amount of cataclasis, compaction or phyllosilicate smearing. Deformation bands generally tend to de?Ne sets with preferred orientation (Figure above), for example in damage zones, and this anisotropy can in?Uence the ?Uid ?Ow in a petroleum reservoir, as an example at some stage in water injection. All of these elements make it dif?Cult to evaluate the effect of deformation bands in reservoirs, and each reservoir must be evaluated personally in line with neighborhood parameters including time and depth of deformation, burial and cementation history, mineralogy, sedimentary facies and extra.

The in?Uence of deformation bands on petroleum or groundwater production depends on the permeability evaluation, cumulative thickness, orientations, continuity and connectivity.

Given the numerous varieties of deformation bands and their one-of-a-kind outcomes on ?Uid ?Ow, it is vital to recognize the underlying situations that control when and where they form. A wide variety of factors are in?Uential, together with burial intensity, tectonic environment (state of strain) and host rock homes, which includes degree of lithi?Cation, mineralogy, grain size, sorting and grain form. Some of these elements, particularly mineralogy, grain size, rounding, grain shape and sorting, are more or much less constant for a given sedimentary rock layer. They may, however, range from layer to layer, that's why fast modifications in deformation band development may be visible from one layer to the next.

Other factors, which include porosity, permeability, con?Ning pressure, stress kingdom and cementation, are in all likelihood to exchange with time. The end result may be that early deformation bands are distinct from the ones formed at later degrees within the identical porous rock layer, for example at deeper burial depths. Hence, the collection of deformation systems in a given rock layer re?Ects the physical changes that the sediment has experienced throughout its history of burial, lithi?Cation and uplift.

To illustrate a standard structural development of sedimentary rocks that undergo burial after which uplift, we use the diagram and upload characteristic structures (Figure above). The earliest forming deformation bands in sandstones are normally disaggregation bands or phyllosilicate bands. Such structures shape at low con?Ning pressures (shallow burial) when forces across grain contact surfaces are low and grain bindings are weak, and are therefore indicated at shallow degrees in Figures above and determine on the cease. Many early disaggregation bands are related to neighborhood, gravity-controlled deformation such as neighborhood shale diapirism, underlying salt motion, gravitational sliding and glaciotectonics.

Cataclastic deformation bands can arise in poorly lithi?Ed layers of pure sand at shallow burial depths, but are a good deal extra common in sandstones deformed at 1?3 km intensity. Factors promoting shallow-burial cataclasis consist of small grain touch regions, i.E. Properly sorting and properly-rounded grains, the presence of feldspar or

different non-platy minerals with cleavage and decrease hardness than quartz, and vulnerable lithic fragments. Quartz, for instance, seldom develops transgranular fractures underneath low con?Ning strain, however may fracture by way of ?Aking or spalling. At deeper depths, considerable cataclasis is promoted with the aid of excessive grain contact stresses. Abundant examples of cataclastic deformation bands are observed inside the Jurassic sandstones of the Colorado Plateau, in which the age relation among early disaggregation bands and later cataclastic bands is very consistent (Figure above).

When a sandstone becomes cohesive and loses porosity at some point of lithi?Cation (left facet of Figure above), deformation occurs with the aid of crack propagation rather than pore area disintegrate, and slip surfaces, joints and mineral-?Lled fractures form directly without any precursory formation of deformation bands. This is why late, overprinting structures are almost forever slip surfaces, joints and mineral-?Lled fractures. Slip surfaces also can form by way of faulting of low-porosity deformation band zones at any burial depth.

Joints and veins generally postdate both disaggregation bands and cataclastic bands in sandstones. The transition from deformation banding to jointing may also occur as porosity is reduced, extensively via quartz dissolution and precipitation. Since the effect of such diagenetically managed strengthening may additionally range locally, deformation bands and joints might also develop simultaneously in unique components of a sandstone layer, however the wellknown pattern is deformation bands ?Rst, then faulted deformation bands (slip floor formation) and ?Nally joints (tensile fractures in Figure above) and possibly faulted joints.

The modern-day fractures in uplifted sandstones have a tendency to form sizable and regionally mappable joint units generated or at the least in?Uenced via elimination of overburden and cooling for the duration of nearby uplift. Such joints are said in which sandstones had been uplifted and uncovered, which include at the Colorado Plateau, but are not likely to be developed in subsurface petroleum reservoirs unexposed to signi?Cant uplift. It consequently appears that knowing the burial/uplift history of a basin with regards to the timing of deformation activities may be very beneficial while considering the form of structures found in, say, a sandstone reservoir. Conversely, exam of the sort of deformation structure present also gives data approximately deformation depth and different situations at the time of deformation.

Credits: Haakon Fossen (Structural Geology)

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