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1.1 - Natural Fracture Types
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1.1.1 - Natural fracture types |
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Acknowledgements.
This section is an expanded, hypertext version of material presented in the
AAPG Geologic Atlas of Borehole Images (Lacazette, 2000), which in turn grew
out of unpublished collaborative work with Terry Engelder, Wayne Narr and
Manuel Willemse. Because of this history, my friends deserve some credit
for this work but I must accept all blame.
About this section. This webpage is a guide to fracture identification that should function like a field guide for identifying different types of animals. The page describes the different types of industrially significant natural rock fractures, their distinguishing characteristics in core, outcrop and image logs, and when complete will provide extensive links to photographs, schematic and example log images, and other information on this and other websites. A flow-chart is provided to help you identify fractures in core, image logs and outcrop. Fracture classification: The Good, the Bad and the Ugly. The statement "that feature is either a fracture or a fault" is exactly equivalent to saying "that animal is either a dog or a poodle" because fracture is a general term for any type of brittle failure and a fault is a specific type of fracture. So what? Is it really important to correctly identify fractures or is classification just an academic exercise? There are important practical reasons to use correct fracture nomenclature:
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Types of Natural Rock Fractures |
Let's define a natural rock fracture as follows:
Fracture. A general term for any non-sedimentary mechanical discontinuity thought to represent a surface or zone of mechanical failure. Chemical processes such as solution and stress corrosion may have played an important role in the failure process. The term is used to describe a natural feature either when available evidence is inadequate for exact classification or when distinction between fracture types is unimportant.With this definition as a foundation, we can describe the different types of natural rock fractures relevant to the oil industry.
Fracture mode.
The foundation of fracture taxonomy is the fracture
mode terminology of standard engineering fracture
mechanics. Figure 1 shows geologic fracture names in terms
of fracture mode terminology. Three fundamental modes of
fracture are possible: mode I (mode-one), mode
II (mode-two) and mode III (mode-three). All
three modes can occur separately or in any combination. Fractures in
which two or more modes were operative are termed mixed-mode
fractures. For example, a fracture might be termed a mixed
mode I-mode II fracture. |
Figure 1. Fracture mode and geologic fracture names.
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Movement sense.
The left and right sides of Figure 1 show opposite senses of each
mode. Under a given set of conditions, the physical mechanisms of
fracture are identical for opposite senses of mode II and mode III
failure. However, the two senses of mode I failure occur by
different physical mechanisms. Standard geological fracture terminology is largely based on the engineering terminology, although standard engineering terminology can be used for natural or induced rock fractures. Note that fracture is a general term so that joints and faults are different types of fractures. |
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Fracture Types and Subtypes |
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Joint.
A natural rock fracture formed predominantly by mode I movement
(Engelder, 1987; Pollard and Aydin, 1988). Plumose
surface morphology is diagnostic of jointing (Kulander,
Barton and Dean, 1979). Unmineralized joints are normally quite permeable.
Contained joints are joints
that are contained within individual beds of a brittle lithology (Gross, 1993;
Gross et al, 1995). The density of contained joints can be quantified precisely
using special methods. Two special types of joints are useful because they provide
slip-sense, and sometimes slip-direction, criteria for the fault movement.
Pinnate joints are a type
of joint that forms adjacent to faults during fault movement and/or propagation
(Hancock, 1985). Tail joints or
wing cracks form at the tip
of a propagating fault (Horii and Nemat-Nasser, 1985).
Fault.
A type of natural rock fracture formed predominantly by mode II and/or mode
III movements. Natural rock fractures that initially formed as joints and
were then reactivated as sliding-mode fractures are also termed faults. (Some
workers use the term faulted joint.)
Faults have a wide range of morphologies and fill types. Faults range from
highly permeable to highly impermeable depending on the manner of formation
and type of fill. Fault slip-sense
and slip-direction often can
be determined from surface
features. Fault-type names reflect their
slip-sense and slip-direction. Fault nomenclature is
reviewed here and
in standard structural geology texts (e.g. Twiss and Moore 1992).
Fault zone.
A fault represented by a zone of intensely deformed rock >1 cm thick. The
thickness limitation insures routine application of the term only to zones
thick enough to clearly distinguish in image logs. In any specific case,
the distinction between a fault-zone and a fault is dependent on the user's
interest in the fault rocks and their fluid-flow properties. If these are
not of interest then thick fault-zones may simply be termed faults.
Similarly, zones thinner than 1 cm may be termed fault zones if
fine-scale fracture properties are of interest and the features are
distinguishable with the available data. Fault zone refers to
the intensely deformed volume of breccia, gouge
and/or smear across which most of the slip occurred
and does not encompass damage (such as pinnate
jointing) in the halo around the fault zone. In
other words, material in a fault zone represents disrupted material that
is no longer continuous with the parent rock.
Deformation band.
A natural rock fracture defined by a zone of grain crushing and
compaction developed by mixed anti-mode I + mode II and/or III
movement. Deformation bands are cm-scale braided
accumulations of crushed zones roughly
0.5-1 mm thick that contain characteristic ramp-and-eye
structures. Deformation bands often develop
as conjugate sets. They
are important to the petroleum industry because they only form in highly
porous (>15%) sandstones and chalks (which make good reservoirs)
and because the material within a deformation band is
about 3 orders of magnitude less permeable
than the host rock. Deformation bands
cause severe compartmentalization of oil
fields in the North Sea,
Indonesia, US and elsewhere. (Antonellini and Aydin, 1995a, 1995b)
Compaction band.
Compaction bands are unusual features similar to deformation bands.
However, they develop by pure anti-mode I movement and may lack the
characteristic braided appearance. They can be planar to wavy and
may even develop as networks with a
mudcrack-like geometry indicating constrictional strain
(very rare). Volume loss may be accommodated by extensive grain crushing, in
which case compaction bands can be considered a special case of
deformation bands. However, some compaction bands develop by grain
sliding and rearrangement of grains with little or no crushing so
that they do not meet a strict definition of
fracture. Compaction bands developed
primarily by grain sliding, are rare, and
develop only in highly porous (>20%) sandstones having
grain sizes of at least 0.3-0.8 mm. (Mollema and Antonellini, 1996)
Stylolite (pressure solution seam).
A stylolite (pronounced style-o-light) is a zone of
insoluble residue produced by
stress-enhanced dissolution.
Stylolites typically have a cone-in-cone
structure that produces a characteristic
zig-zag appearance in
cross section. Most geologists do not consider stylolites to be
fractures. I used to think the same thing, but I was wrong. Stylolites
are fractures! They are stress-corrosion anticracks! Before
you blow a fuse, please read
this
and sleep on it.
Stylolites should serve as flow barriers because the insoluble
residue is very fine-grained and clay-rich. However, stylolites
are very weak and are easily reactivated as joints by later
tectonic events. They are often reported to be permeable in
hydrocarbon reservoirs. A slickolite is
a type of stylolite in which the teeth are inclined <90° to the
plane of the stylolite (Hancock, 1985). Slickolites form at an angle
to s1, often by dissolution along a preexisting fracture. Slickolites are
surfaces of shear displacement as well as shortening.
Induced fracture.
Any rock fracture produced by human activities, such as drilling,
accidental or intentional hydrofracturing, core handling, etc.
(Kulander, Dean and Ward,1990; Distinguishing
natural from induced fractures in image logs).
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1.1.2 - Fracture orientations relative to the principal stress orientations |
Earth stress.
Stress is defined as the force per unit area acting on a plane. Any
stress state at a point in a solid body can be described completely
by the orientations and magnitudes of three stresses
called principal stresses. The principal
stresses are oriented perpendicular to each other and to the three
planes of no resolved shear stress at the point. The drawing shows a
block of rock having constant stress throughout. The symbol
(sigma) designates compressive or tensile stress. Subscripts identify
specific stresses. The principal stresses are defined:
1 >
2 >
3.
Compressive stress and shortening strain are considered positive in
rock mechanics and structural geology because in the earth all three
principal stresses are always compressive (except around underground
voids such as caves or very near to the earth's surface). Joints (extensional fractures)
seem paradoxical because they are one of the most common types of natural
rock fracture even though they require an effectively tensile
driving stress. Pore-fluid pressure drives most joints by producing
tensile effective stress through poroelastic loading of flaws that
are orders of magnitude larger than typical pores. See
Mechanics of jointing for
a detailed discussion.
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Figure 2. Significance of fracture orientations.
Joints (GREEN).
1.
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1.1.3 - Regional Fracture Orientations |
| Figures 3, 4 and 5 represent the three most common regional earth stress regimes which are termed the Andersonian stress regimes after the geologist (E.M. Anderson) who first recognized and described them in 1905. In Andersonian regimes one principal stress is vertical so that the other two are horizontal. Be aware that although Andersonian regimes are the most common, inclined stressfields are not unusual. The figures schematically show the average regional orientations in which different types of natural fractures form relative to Andersonian stress regimes. |
Figure 3.
Figure 4.
Figure 5.
Natural fracture orientations are often unrelated to the orientations of the present-day stresses in a
rock mass for the following reasons (Engelder 1992):
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Fault slip and fault names |
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This section reviews basic fault nomenclature. If you're already
comfortable with fault nomenclature then skip this section and go
directly to the Fracture identification
flowchart or return to the
technical directory to check-out other fun stuff,
like fracturing during folding or
learn how to avoid making
hideous mistakes when you interpret breakouts.
Figure 6 shows the basic fault-slip parameters. The blue arrow indicates the right-hand rule strike, the green arrow dip. The hangingwall is the fault block above the fault plane; the footwall is the fault block below the fault plane. |
Fault heave and throw can be deceptive because:
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Figure 7: Vertical throw vs. stratigraphic throw
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Basic fault names based on slip direction.
Fault names indicate the rake of the movement
direction of the hangingwall. Rake is the angle
of the hangingwall slip-vector measured in the fault plane. There
are many different rake notations, but this website measures rake
from the dip vector. Positive rakes are clockwise (as seen looking
down on the fault plane) and rake can range either from +180° to
-180° or from 0°-360° depending on your preference. I prefer the
+180° to -180° scheme because rakes with absolute values <90°
indicate normal slip, >90° indicate reverse slip, positive rakes
indicate right-lateral movement, and negative rakes indicate
left-lateral movement.
Figure 8 shows how fault names relate to the
hangingwall-slip rake. The diagram looks perpendicularly down onto
the fault plane. The green arrow is the dip-vector; the blue arrow
is the right-hand rule strike. In normal usage faults with
slip-vectors lying within 10°-15° of the dip or
strike orientation (not
direction) are
termed normal/reverse or right-/left-lateral faults,
respectively. Faults with slip-vectors outside of these ranges are
given compound names. The vertical or horizontal slip component
of a vertical or horizontal fault is named arbitrarily.
Right-lateral vs. left-lateral.
Imagine that you are standing on one side of a steeply dipping
fault as it moves. If objects on the opposite block appear to
be moving to your right, then the fault has a right-lateral
component. If objects on the opposite block appear to be
moving to your left, then the fault has a left-lateral
component. The apparent movement is the same whether you are
standing on the hangingwall or footwall.
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Figure 8. The fault name describes the rake of the slip vector.
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Special fault names.
Geologists use many different names for special types of
faults. These names have complex and far-reaching implications
that depend to some extent on the user. When in doubt, use a
simple name based on the slip-sense and direction because you will
never be wrong and everyone will understand you. Gee, those sound
like pretty good reasons to avoid specialized jargon entirely....
Some important types of specialized fault names: Wrench vs. strike-slip fault. These terms are synonyms and indicate a fault with a slip-vector closely parallel to the fault strike. Some geologists reserve the term wrench for large, regional strike-slip faults, steeply-dipping regional strike-slip faults or as a synonym for tear fault. Thrust fault. This name once meant any reverse fault with a dip-angle of 30° or less. Now the term indicates faults with an originally low dip-angle that formed during regional compressional deformation. A single thrust fault may change its orientation as it crosscuts different lithologies. Folding can reorient thrusts so that they may have a variety of angles today. Detachment fault. A regional, low-angle, listric normal fault formed during crustal extension. Tear fault. Often used to indicate a steeply-dipping wrench fault that bounds or cuts the hangingwall of a thrust or normal fault, also used for mode III faults. |
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Continue to next page: Fracture identification flowchart |
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Copyright © 2000 - 2017 • Alfred Lacazette • All Rights Reserved