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).