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Natural Fracture Types

  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of material presented in the AAPG Geologic Atlas of Borehole Images (Lacazette, 2000), which in turn grew out of unpublished collaborative work with Terry Engelder, Wayne Narr and Manuel Willemse.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:

  • Different types of fractures form in different orientations relative to the earth stresses that prevailed at the time of fracturing. Correctly identifying fracture types is essential for predicting the orientations of fracture populations as a whole and therefore for planning optimum drilling directions and for building reservoir models.
  • Different types of fractures have different fluid-flow properties.
  • Certain types of fractures form only in specific rock types or in specific geologic environments.
  • Certain types of fractures have particular shape/size distributions and obey particular density (spacing) laws which can be used to build 3D reservoir models.
  • Geological terminology should remain consistent with fracture terminology in other technical disciplines. Poor usage of geological fracture terminology threatens to create yet more confusing and specialized jargon at a time when interdisciplinary communication is increasingly important. For example, fracture is a general term for a brittle failure of any kind and this usage is consistent between geology and other technical disciplines, such as engineering. The present trend to use the term joint (a natural mode I rock fracture) synonymously with fracture is both confusing and incorrect.
The Way, the Truth and The Light.   In the same way that a good biological taxonomy makes it easier to systematically identify animals, practical petroleum geology needs a simple, useful geological taxonomy for fractures. A well developed, reasonably standardized rock fracture terminology that is reasonably consistent with engineering fracture terminology has existed in the structural geology literature for many years. The summarized, organized and simplified version of traditional geologic fracture nomenclature presented here provides practical (industrially useful) fracture classifications. Please use it.

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.

Fracture mode nomenclature is purely descriptive, not genetic. For example, a mode I fracture can be formed by one or more mechanisms such as hydraulic fracturing, thermal contraction, and/or diagenetic shrinkage. Stating that a fracture is a mode I fracture only implies that the walls moved perpendicularly away from the fracture plane when the fracture formed.


Figure 1.

  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.

Fracture Types and Subtypes
  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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