Report studying the implications of Deformation Bands and their ability to compartmentalise subsurface reservoirs and restrict fluid flow. Paper references recent scientific studies.

1. 1
The role of deformation bands in modifying reservoir performance in
porous sandstones
Tom Adamson
Department of Geology and Geophysics, University of Aberdeen
The impact of deformation bands on
reservoir porosity
In porous sandstones, deformation bands
are mm-thick zones associated with slip
surfaces of localized shear and
compaction. They are compaction
structures that lead to porosity loss
depending on the amount of cataclasis
which generates a “mechanically strong
and stiff inter cataclastic rock (Fossen et
al., 2017). The most common are shear
bands. However, cataclastic shear bands
typically involve a change in the porosity
of 25% in sandstone to 10-15% within the
band, an observation exemplified in figure
1. There are significant losses of porosity
caused by deformation bands in both fine
and coarse sandstones (Griffiths, Faulkner,
Edwards and Worden, 2016). Fossen et al
(2011) put forward ideas that minimum
porosity for deformation band
development and propagation is lowered
by the addition of shear to compaction.
Additionally, the addition of quartz cement
lowers the porosity within the deformation
band core, and “further increases the
likelihood of reservoir
compartmentalization.” (Griffiths,
Faulkner, Edwards and Worden, 2016).
Deformation bands are fault like structures
that act as flow barriers and trap
Figure 1: Porosity calculations for deformation band and host rock.
(Griffiths, Faulkner, Edwards and Worden, 2016)

2. 2
hydrocarbons in deformed porous
sandstone reservoirs and represent
permeability and porosity tabular elements
with compaction. They vary depending on
porosity at the time of deformation, but
“introduce a permeability anisotropy to the
reservoir,” (Ballas, Fossen and Soliva,
2015) and do not have sealing properties.
The relationship of deformation bands
to faults and their significance
Within porous sandstone reservoirs,
deformation bands with mm-thick
displacements are developed instead of
slicken-lined fault surfaces and can reduce
permeability by up to three orders of
magnitude (Antonellini and Aydin 1994),
making them important barriers to fluid
flow during production. Information about
distribution and orientation are useful
factors to build models of sub-seismic
structures such as faults. A study of
deformation bands in Utah by Antonellini
and Aydin showed they occur as (a)
isolated structures, (b) linked systems, (c)
interconnected zones and (d) in a zone at
either side of faults (Figure 2). Aydin and
Johnson (1983) suggest this timeline
represents the development of a single
deformation band to an ordinary fault.
During this process, displacement
increases from mm at stage (a), to several
metres or more at stage (d). There is an
evolution of strain hardening that causes
“sequential formation of new deformation
bands during deformation,” (Fossen and
Hesthammer, 1998) resulting in isolated
zones of clustered bands. This evolution is
different for faults, where slip accumulates
along more slip surfaces. Unlike faults,
slip surfaces only occur in deformation
bands on a micro-scale, such as offsetting
Figure 2: Development of a fault from
deformation bands. Principal sketch from
Aydin and Johnson (1983).

3. 3
grain boundary. The significance lies in
the interpretation of seismic data, where
the scaling relationship of displacement-
length between bands and faults must be
adjusted depending on whether
deformation bands dominate or not (figure
3). Finally, deformation bands with an
offset of a few centimeters should not be
neglected due to their likely length of
several hundred meters.
The impact of patterns they form on
fluid flow in the subsurface
Deformation bands are unlikely to form
seals capable of holding hydrocarbon
columns, but they can influence fluid flow.
This depends on their thickness or
frequency and internal permeability
structure. Their permeability “is governed
by the deformation mechanisms operative
during their formation” (Fossen, 2010), but
their cumulative thickness is important
within petroleum reservoirs. Low-pressure
phyllosilicate bands form in the sand
where the content of platy minerals
exceeds 15%, while cataclastic bands form
with significant mechanical grain
breaking. These show permeability
reductions of several orders of magnitude.
Rotevatn et al. (2013) concluded that
Figure 3: Displacement-length data for deformation bands and faults in porous Jurassic
sandstones, San Rafael Swell area, Utah. Plotted together with data from other sources. From
Fossen and Hesthammer, 1998)

4. 4
“thickness variations are of negligible
importance when considering deformation
bands on fluid flow in sandstone aquifers
and reservoirs.” This is dependent on
deformation bands not sealing the matrix.
Fluid flow more parallel to the
deformation band strike causes a greater
negative effect. Furthermore, jointed bands
provide a pathway for fluid flow, while the
volumetric flow rate through bands can
exceed or equal discharge flow rate
through an equivalent volume of
sandstone. A geometrically complex
assemblage of deformation bands may
affect the fluid flow that would require a
3D approach (Rotevatn and Fossen 2011).
Effective permeability in porous
sandstones comes down to connectivity
and configuration of deformation bands
and the permeability contrast with the host
rock.
Conclusion
The effect of deformation bands in
reservoirs is difficult to evaluate due to
changing orientation, connectivity and
porosity/permeability from changing
cataclasis and compaction. Normal fault
regimes form cataclastic shear bands that
show intense cataclasis and high
permeability reduction. Moderate burial
depth around 3km favours cataclastic
bands that show intense permeability
reduction and cataclasis. Porous sandstone
conditions are secondary factors that
influence permeability, while good host
rock properties “constitute conditions
favourable for greater permeability
contrasts recorded in cataclastic bands”
(Fossen et al., 2017). Deformation bands
are capable of redirecting flow regardless
of permeability and rock contrast, but this
must be at least 3 orders of magnitude to
effect large scale production. If the effect
is noticeable, deformation bands become
“less permeable with respect to the host
rock, allowing for a higher fluid path
tortuosity” (Zuluaga, Rotevatn,
Keilegavlen and Fossen, 2016) that yields
fewer hydrocarbons. When optimizing
strategies for sandstone reservoirs,
producers must incorporate deformation
bands when planning and predicting flow
and production.
Word Count: 855

5. 5
References
Antonellini, M. and Aydin, A. (1994)
Effect of faulting on fluid flow in porous
sandstones: petrophysical properties.
American Association of Petroleum
Geologists Bulletin 78, 355-377.
Aydin, A. and Johnson, A. M. (1983)
Analysis of faulting in porous sandstones.
Journal of Structural Geology 5, 19-31
Ballas, G., Fossen, H. and Soliva, R.,
2015. Factors controlling permeability of
cataclastic deformation bands and faults in
porous sandstone reservoirs. Journal of
Structural Geology, 76, pp.1-21.
Fossen, H., 2010. Structural Geology. 1st
ed. Cambridge: Cambridge University
Press, pp.141-148.
Fossen, H. and Hesthammer, J., 1998.
Deformation bands and their significance
in porous sandstone reservoirs. First
Break, 16(1), pp.21-25.
Fossen, H., Soliva, R., Ballas, G.,
Trzaskos, B., Cavalcante, C. and Schultz,
R., 2017. A review of deformation bands
in reservoir sandstones: geometries,
mechanisms and distribution. Geological
Society, London, Special Publications,
459(1), pp.9-33.
Griffiths, J., Faulkner, D., Edwards, A. and
Worden, R., 2016. Deformation band
development as a function of intrinsic
host-rock properties in Triassic Sherwood
Sandstone. Geological Society, London,
Special Publications, 435(1), pp.161-176.
Rotevatn, A., Sandve, T., Keilegavlen, E.,
Kolyukhin, D. and Fossen, H., 2013.
Deformation bands and their impact on
fluid flow in sandstone reservoirs: the role
of natural thickness variations. Geofluids,
13(3), pp.359-371.
Rotevatn A, Fossen H (2011) Simulating
the effect of subseismicfault tails and
process zones in a siliciclastic reservoir
analogue:implications for aquifer support
and trap definition. Marine andPetroleum
Geology,28, 1648–62.
Zuluaga, L., Rotevatn, A., Keilegavlen, E.
and Fossen, H., 2016. The effect of
deformation bands on simulated fluid flow
within fault-propagation fold trap types:
Lessons from the San Rafael monocline,
Utah. AAPG Bulletin, 100(10), pp.1523-
1540
.