Changes

==Seismic interpretation of faults==

==Seismic interpretation of faults==

The seismic data set is interpreted primarily using vertical time sections. These are displays that show a series of vertical seismic traces displayed side by side ([[:file:M91Ch6FG47.JPG|Figure 1]]). The peaks or the troughs are filled in with black shading or color. Continuous reflections stand out as an overlapping array of peaks or troughs. These create patterns on a seismic section that give a representation of the geological structure in the subsurface. The seismic interpreter will look for discontinuities in the seismic reflections likely to represent faulting. Various techniques can help in picking faults. The interpretation can be cross checked against attribute maps showing changes in seismic dip (magnitude of the time gradient), azimuth (direction of maximum dip), or abrupt changes in amplitude (Dalley et al., 1989; Hesthammer and Fossen, 1997). Another method is to use semblance data to detect edges in the data (see [[Lithofacies maps]]).

The seismic data set is interpreted primarily using vertical time sections. These are displays that show a series of vertical seismic traces displayed side by side ([[:file:M91Ch6FG47.JPG|Figure 1]]). The peaks or the troughs are filled in with black shading or color. Continuous reflections stand out as an overlapping array of peaks or troughs. These create patterns on a seismic section that give a representation of the geological structure in the subsurface. The seismic interpreter will look for discontinuities in the seismic reflections likely to represent faulting. Various techniques can help in picking faults. The interpretation can be cross checked against attribute maps showing changes in seismic dip (magnitude of the time gradient), azimuth (direction of maximum dip), or abrupt changes in amplitude.<ref name=Dalleyetal_1989>Dalley, R. M., E. E. A. Gevers, G. M. Stampli, D. J. Davies, C. N. Gastaldi, P. R. Ruijetnberg, and G. J. D. Vermeer, 1989, Dip and azimuth displays for 3-D seismic interpretation: First Break, v. 7, p. 86–95.</ref> <ref name=Hesthammerandfossen_1997>Hesthammer, J., and H. Fossen, 1997, Seismic attribute analysis in structural interpretation of the Gullfaks field, northern North Sea: Petroleum Geoscience, v. 3, no. 1, p. 13–26.</ref> Another method is to use semblance data to detect edges in the data (see [[Lithofacies maps]]).

[[file:M91Ch13FG80.JPG|thumb|300px|{{figure number|2}}Dipmeter or image data can be used to pick likely fault planes in wells. Changes in dip amplitude or azimuth can indicate that a fault is present. Drag patterns may also be seen on the dip data above and below the fault intersection in a well (from Schlumberger, 1981). Courtesy of Schlumberger.]]

[[file:M91Ch13FG80.JPG|thumb|300px|{{figure number|2}}Dipmeter or image data can be used to pick likely fault planes in wells. Changes in dip amplitude or azimuth can indicate that a fault is present. Drag patterns may also be seen on the dip data above and below the fault intersection in a well (from Schlumberger<ref name=Schlumberger_1981>Schlumberger, 1981, Dipmeter interpretation, volume 1—Fundamentals: New York, Schlumberger, 61 p.</ref>). Courtesy of Schlumberger.]]

==Structural core logging==

==Structural core logging==

[[file:M91Ch13FG81.JPG|thumb|300px|{{figure number|3}}The stratigraphy in a well penetrating a normal fault will be incomplete due to fault cutout.]]

[[file:M91Ch13FG81.JPG|thumb|300px|{{figure number|3}}The stratigraphy in a well penetrating a normal fault will be incomplete due to fault cutout.]]

Core goniometry is a method for graphically depicting the structure in the core. The whole core is wrapped around with acetate film, and the structures and main bedding planes in the core are traced directly with felt tip marker pens. The unrolled film shows a 360&deg; depiction of the structure comparable to the display shown by borehole image logs. Commercial rigs are also available, which take 360deg photographs of the core for the same purpose.

Core goniometry is a method for graphically depicting the structure in the core. The whole core is wrapped around with acetate film, and the structures and main bedding planes in the core are traced directly with felt tip marker pens. The unrolled film shows a 360&deg; depiction of the structure comparable to the display shown by borehole image logs. Commercial rigs are also available, which take 360&deg; photographs of the core for the same purpose.

Having established the structures in the core, it is important to know how they were originally oriented within the reservoir. Dipmeter data, scribed core, and paleomagnetic data have all been used to work out the spatial orientation of the core (Davison and Haszeldine, 1984; Bleakly, 1992).

Having established the structures in the core, it is important to know how they were originally oriented within the reservoir. Dipmeter data, scribed core, and paleomagnetic data have all been used to work out the spatial orientation of the core.<ref name=Davisonandhaszeldine_1984>Davison, I., and R. S. Haszeldine, 1984, Orienting conventional cores for geological purposes: A review of methods, Journal of Petroleum Geology, v. 7, no. 4, p. 461–466.</ref> <ref name=Bleakly_1992>Bleakly, D. C., 1992, [[Core orientation]], ''in'' D. Morton-Thompson and A. M. Woods, eds., Development geology reference manual: AAPG Methods in Exploration Series 10, p. 122–124.</ref>

[[file:M91Ch13FG82.JPG|thumb|300px|{{figure number|4}}Repeated sections can be seen in a vertical well drilled through a reverse fault or with a highly deviated well penetrating a normal fault.]]

[[file:M91Ch13FG82.JPG|thumb|300px|{{figure number|4}}Repeated sections can be seen in a vertical well drilled through a reverse fault or with a highly deviated well penetrating a normal fault.]]

==Fault detection methods==

==Fault detection methods==

Dipmeter or borehole image data can be used to establish if and where any faults cut a well (Bengtson, 1981, 1982; Goetz, 1992; Adams and Dart, 1998). A sharp change in dip amplitude or azimuth on a dipmeter log can indicate that a fault is present. Drag patterns may also be seen on the dip data above and below the fault intersection in the well ([[:file:M91Ch13FG80.JPG|Figure 2]]).

Dipmeter or borehole image data can be used to establish if and where any faults cut a well.<ref name=Bengtson_1981>Bengtson, C. A., 1981, [http://archives.datapages.com/data/bulletns/1980-81/data/pg/0065/0002/0300/0312.htm Statistical curvature analysis techniques for structural interpretation of dipmeter data]: AAPG Bulletin, v. 65, no. 2, p. 312–332.</ref> <ref name=Bengtson_1982>Bengtson, C. A., 1982, [http://archives.datapages.com/data/specpubs/basinar2/data/a130/a130/0001/0000/0031.htm Structural and stratigraphic uses of dip profiles in petroleum exploration], in M. T. Halbouty, ed., The deliberate search for the subtle trap: AAPG Memoir 32, 351 p.</ref> <ref name=Goetz_1992>Goetz, J. F., 1992, [[Dipmeters]], ''in'' D. Morton-Thompson and A. M. Woods, eds., Development geology reference manual: AAPG Methods in Exploration Series 10, p. 158–162.</ref> <ref name=Adamsanddart_1998>Adams, J. T., and C. Dart, 1998, The appearance of potential sealing faults on borehole images, in G. Jones, Q. J. Fisher, and R. J. Knipe, eds., Faulting, fault sealing and fluid flow in hydrocarbon reservoirs: Geological Society (London) Special Publication 147, p. 71–86.</ref> A sharp change in dip amplitude or azimuth on a dipmeter log can indicate that a fault is present. Drag patterns may also be seen on the dip data above and below the fault intersection in the well ([[:file:M91Ch13FG80.JPG|Figure 2]]).

An anomalously thin reservoir section, perhaps in conjunction with the absence of a reasonably persistent marker horizon, may be caused by a normal fault cutting out part of the stratigraphic section in a well ([[:file:M91Ch13FG81.JPG|Figure 3]]). The thickness of missing section can be estimated by comparison to nearby wells with unfaulted sections.

An anomalously thin reservoir section, perhaps in conjunction with the absence of a reasonably persistent marker horizon, may be caused by a normal fault cutting out part of the stratigraphic section in a well ([[:file:M91Ch13FG81.JPG|Figure 3]]). The thickness of missing section can be estimated by comparison to nearby wells with unfaulted sections.

A fault-repeated section is sometimes seen in a well ([[:file:M91Ch13FG82.JPG|Figure 4]]). Near-vertical or gently dipping wells cutting reverse faults will show a repeated pattern. A repeat section can also occur where a highly deviated well cuts through a normal fault at a shallower angle than the dip of the fault plane ([[:file:M91Ch13FG82.JPG|Figure 4]]) (Mulvany, 1992).

A fault-repeated section is sometimes seen in a well ([[:file:M91Ch13FG82.JPG|Figure 4]]). Near-vertical or gently dipping wells cutting reverse faults will show a repeated pattern. A repeat section can also occur where a highly deviated well cuts through a normal fault at a shallower angle than the dip of the fault plane ([[:file:M91Ch13FG82.JPG|Figure 4]]).<ref name=Mulvany_1992>Mulvany, P. S., 1992, [http://archives.datapages.com/data/bulletns/1992-93/data/pg/0076/0006/0000/0895.htm A model for classifying and interpreting logs of boreholes that intersect faults in stratified rocks]: AAPG Bulletin, v. 76, no. 6, p. 895–903.</ref>

[[file:M91Ch13FG84.JPG|thumb|300px|{{figure number|6}}This faulted top reservoir map from the Staffa field in the UK North Sea is represented by a contoured surface and fault polygons. The fault polygons show the hanging wall and footwall fault cuts for the interpreted surface. The downthrown (hanging wall) side of the fault is indicated by a blocked out symbol (from Gluyas and Underhill, 2003). Reprinted with permission from the Geological Society.]]

[[file:M91Ch13FG84.JPG|thumb|300px|{{figure number|6}}This faulted top reservoir map from the Staffa field in the UK North Sea is represented by a contoured surface and fault polygons. The fault polygons show the hanging wall and footwall fault cuts for the interpreted surface. The downthrown (hanging wall) side of the fault is indicated by a blocked out symbol (from Gluyas and Underhill<ref name=Gluyasandunderhill_2003>Gluyas, J. G., and J. R. Underhill, 2003, The Staffa field, Block 3/8b, UK North Sea, in J. G. Gluyas and H. M. Hichens, eds., United Kingdom oil and gas fields, commemorative millennium volume: Geological Society (London) Memoir 20, p. 327–333.</ref> Reprinted with permission from the Geological Society.]]

==Well tests and faults==

==Well tests and faults==

One method for locating faults is to check the results of reservoir engineering pressure transient analyses of well tests. The basis for these tests is that a production well, while it is flowing, will draw down the pressure for a considerable distance out into the surrounding reservoir. If the well is shut in and production is stopped, the pressure will build up as a result of the radial inflow of fluid toward the pressure sink in the immediate vicinity of the borehole. If a sealing fault or a feature likely to disrupt horizontal fluid inflow is present within the drainage radius of the well, then this can often be detected. The fault will disrupt the rate of pressure build-up once the catchment area for inflow of fluid increases outward with time and comes in contact with the fault plane ([[:file:M91Ch13FG83.JPG|Figure 5]]). Analytical methods are available to make a rough estimate of how far away the fault is from the wellbore.

One method for locating faults is to check the results of reservoir engineering pressure transient analyses of well tests. The basis for these tests is that a production well, while it is flowing, will draw down the pressure for a considerable distance out into the surrounding reservoir. If the well is shut in and production is stopped, the pressure will build up as a result of the radial inflow of fluid toward the pressure sink in the immediate vicinity of the borehole. If a sealing fault or a feature likely to disrupt horizontal fluid inflow is present within the drainage radius of the well, then this can often be detected. The fault will disrupt the rate of pressure build-up once the catchment area for inflow of fluid increases outward with time and comes in contact with the fault plane ([[:file:M91Ch13FG83.JPG|Figure 5]]). Analytical methods are available to make a rough estimate of how far away the fault is from the wellbore.

Care has to be taken that a feature such as a sand pinch-out or channel margin is not mistaken for a fault. It is a useful exercise for the reservoir engineer to have a working session with the seismic interpreter in order to compare test data for all the wells in the field with the interpreted fault pattern. An example of this is given by Marquez et al. (2001) for the LL-04 reservoir in the Tia Juana field, Venezuela. An integrated reservoir characterization study was made to identify reserve growth opportunities. Part of this study involved cross checking the seismic interpretation of faults with evidence of compartmentalization from engineering data. In places where inferred reservoir compartments and faults did not coincide, the seismic interpretation was rechecked to see if a fault had been missed. If no fault could be located, the geologists then investigated the possibility that stratigraphic pinch-outs could be the cause of compartmentalization.

Care has to be taken that a feature such as a sand pinch-out or channel margin is not mistaken for a fault. It is a useful exercise for the reservoir engineer to have a working session with the seismic interpreter in order to compare test data for all the wells in the field with the interpreted fault pattern. An example of this is given by Marquez et al.<ref name=<Marquezetal_2001>Marquez, L. J., M. Gonzalez, S. Gamble, E. Gomez, M. A. Vivas, H. M. Bressler, L. S. Jones, S. M. Ali, and G. S. Forrest, 2001, Improved reservoir characterization of a mature field through an integrated multi-disciplinary approach, LL-04 reservoir, Tia Juana field, Venezuela: Presented at the Society of Petroleum Engineers Annual Technical Conference and Exhibition, September 30–October 3, New Orleans, Louisiana, SPE Paper 71355, 10 p.</ref> for the LL-04 reservoir in the Tia Juana field, Venezuela. An integrated reservoir characterization study was made to identify reserve growth opportunities. Part of this study involved cross checking the seismic interpretation of faults with evidence of compartmentalization from engineering data. In places where inferred reservoir compartments and faults did not coincide, the seismic interpretation was rechecked to see if a fault had been missed. If no fault could be located, the geologists then investigated the possibility that stratigraphic pinch-outs could be the cause of compartmentalization.

==Mapping faults==

==Mapping faults==

Structure maps show the contoured depth surface and a representation of any faults cutting the surface. The faults are drawn as fault polygons marking the hanging wall and footwall fault cuts for the interpreted surface. The hanging wall is the rock volume above the fault plane, and the footwall is the rock volume that lies beneath it ([[:file:M91Ch13FG81.JPG|Figure 3]], [[:file:M91Ch13FG82.JPG|Figure 4]], [[:file:M91Ch13FG84.JPG|Figure 6]]).

Structure maps show the contoured depth surface and a representation of any faults cutting the surface. The faults are drawn as fault polygons marking the hanging wall and footwall fault cuts for the interpreted surface. The hanging wall is the rock volume above the fault plane, and the footwall is the rock volume that lies beneath it ([[:file:M91Ch13FG81.JPG|Figure 3]], [[:file:M91Ch13FG82.JPG|Figure 4]], [[:file:M91Ch13FG84.JPG|Figure 6]]).

Faults on structure maps should be checked for consistency. The fault polygons represent the length of the fault that can be picked from seismic data. Where the fault throw is less than the seismic resolution, the fault will not be mapped by the interpreter. The limits of the seismically mapped faults will therefore not represent the actual fault tips in the subsurface, the points at either end of the real fault where the fault displacement is zero. Estimates can be made of the extent of the actual fault tips for a seismically mapped fault. The ideal normal fault trace will have an elliptical shape with the maximum displacement in the center of the fault, decreasing gradually to zero at the fault tips (Barnett et al., 1987). If a linear length-to-displacement ratio is assumed, it is possible to use this geometry to extend the seismic fault traces to a feasible location of the fault tips in the subsurface (Pickering et al., 1997).

Faults on structure maps should be checked for consistency. The fault polygons represent the length of the fault that can be picked from seismic data. Where the fault throw is less than the seismic resolution, the fault will not be mapped by the interpreter. The limits of the seismically mapped faults will therefore not represent the actual fault tips in the subsurface, the points at either end of the real fault where the fault displacement is zero. Estimates can be made of the extent of the actual fault tips for a seismically mapped fault. The ideal normal fault trace will have an elliptical shape with the maximum displacement in the center of the fault, decreasing gradually to zero at the fault tips.<ref name=Barnettetal_1987>Barnett, J. A. M., J. Mortimer, J. H. Rippon, J. J. Walsh, and J. Watterson, 1987, [http://archives.datapages.com/data/bulletns/1986-87/data/pg/0071/0008/0900/0925.htm Displacement geometry in the volume containing a single normal fault]: AAPG Bulletin, v. 71, no. 8, p. 925–937.</ref> If a linear length-to-displacement ratio is assumed, it is possible to use this geometry to extend the seismic fault traces to a feasible location of the fault tips in the subsurface.<ref name=Pickeringetal_1997>Pickering, G., D. C. P. Peacock, D. J. Sanderson, and J. M. Bull, 1997, [http://archives.datapages.com/data/bulletns/1997/01jan/0082/0082.htm Modeling tip zones to predict the throw and length characteristics of faults]: AAPG Bulletin, v. 81, no. 1, p. 82–89.</ref>

If the structure is computer mapped, the contours interpolated by the mapping algorithm around faults can sometimes be rather untidy. It is not unusual for a computer map to show spurious fault reversal along the length of the fault. Thus, it is important to check and edit the contour maps by hand where this has happened.

If the structure is computer mapped, the contours interpolated by the mapping algorithm around faults can sometimes be rather untidy. It is not unusual for a computer map to show spurious fault reversal along the length of the fault. Thus, it is important to check and edit the contour maps by hand where this has happened.

[[file:M91Ch13FG85.JPG|thumb|300px|{{figure number|7}}The structural framework of a reservoir can be shown to be valid if it can be taken apart and restored to its predeformed state without any gaps showing (from Zamora Valcarce et al., 2006).]]

[[file:M91Ch13FG85.JPG|thumb|300px|{{figure number|7}}The structural framework of a reservoir can be shown to be valid if it can be taken apart and restored to its predeformed state without any gaps showing (from Zamora Valcarce et al.<ref name=ZamoraValcarceetal_2006>Zamora Valcarce, G., T. Zapata, A. Ansa, and G. Selva, 2006, [http://archives.datapages.com/data/bulletns/2006/03mar/0307/0307.HTM Three-dimensional modeling and its application for development of the El Porton field, Argentina]: AAPG Bulletin, v. 90, no. 3, p. 307–319.</ref>).]]

==Fault validation==

==Fault validation==

Computer methods are available for validating the consistency of a reservoir fault framework. The most sophisticated of these will allow the geologist to examine the faulted model in 3-D and move the various fault blocks interactively back to the prefaulted undeformed state (Williams et al., 1997). If this can be achieved without any gaps appearing, then the fault model is valid in a geometric sense. However, if large gaps cannot be removed, then there are serious problems with the structural interpretation. Sometimes it can take several attempts at making a fault interpretation before a validated fault model is obtained.

Computer methods are available for validating the consistency of a reservoir fault framework. The most sophisticated of these will allow the geologist to examine the faulted model in 3-D and move the various fault blocks interactively back to the prefaulted undeformed state.<ref name=Williamsetal_1997>Williams, G. D., S. J. Kane, T. S. Buddin, and A. J. Richards, 1997, Restoration and balance of complex folded and faulted rock volumes: Tectonophysics, v. 273, no. 3–4, p. 203–218.</ref> If this can be achieved without any gaps appearing, then the fault model is valid in a geometric sense. However, if large gaps cannot be removed, then there are serious problems with the structural interpretation. Sometimes it can take several attempts at making a fault interpretation before a validated fault model is obtained.

Zamora Valcarce et al. (2006) used fault restoration to validate the El Porton field structure in Argentina prior to building a 3-D model of the field. The model was to be built to help plan the trajectories of new development wells. The idea behind validating the structural model was to give extra confidence that a planned well could be expected to intersect with the intended reservoir target given the structural complexities of the reservoir ([[:file:M91Ch13FG85.JPG|Figure 7]]).

Zamora Valcarce et al.<ref name=Zamoravalcarceetal_2006 /> used fault restoration to validate the El Porton field structure in Argentina prior to building a 3-D model of the field. The model was to be built to help plan the trajectories of new development wells. The idea behind validating the structural model was to give extra confidence that a planned well could be expected to intersect with the intended reservoir target given the structural complexities of the reservoir ([[:file:M91Ch13FG85.JPG|Figure 7]]).

[[file:M91Ch13FG86.JPG|thumb|300px|{{figure number|8}}Relay ramps are found in the zone between two overlapping faults. They potentially provide pathways for fluid flow across a fault zone (from Peacock and Sanderson, 1994).]]

[[file:M91Ch13FG86.JPG|thumb|300px|{{figure number|8}}Relay ramps are found in the zone between two overlapping faults. They potentially provide pathways for fluid flow across a fault zone (from Peacock and Sanderson<ref name=Peacockandsanderson_1994>Peacock, D. C. P., and D. J. Sanderson, 1994, Geometry and displacement of relay ramps on normal fault systems: AAPG Bulletin, v. 78, no. 2, p. 147–165.</ref>).]]

Fault restoration can also give insights into the structural history of an oil field. By determining the timing for episodes of faulting, uplift, and erosion, insights can be gained that allow the structural controls on reservoir development to be understood.

Fault restoration can also give insights into the structural history of an oil field. By determining the timing for episodes of faulting, uplift, and erosion, insights can be gained that allow the structural controls on reservoir development to be understood.

==Fault geometries, linked faults, and relay ramps==

==Fault geometries, linked faults, and relay ramps==

What appears to be a simple large fault on seismic data may be more complex than it looks. The imaged fault may in reality comprise several closely spaced, overlapping faults, but because the seismic data cannot resolve the detail of the fault zone, it is shown as a single fault trace. These fault zones comprise linked fault segments with relay ramps in the overlapping areas between them ([[:file:M91Ch13FG86.JPG|Figure 8]]) (Peacock and Sanderson, 1994; Needham et al., 1996).

What appears to be a simple large fault on seismic data may be more complex than it looks. The imaged fault may in reality comprise several closely spaced, overlapping faults, but because the seismic data cannot resolve the detail of the fault zone, it is shown as a single fault trace. These fault zones comprise linked fault segments with relay ramps in the overlapping areas between them ([[:file:M91Ch13FG86.JPG|Figure 8]]).<ref name=Peacockandsanderson_1994 /> <ref name=Needhametal_1996>Needham, D. T., G. Yielding, and B. Freeman, 1996, Analysis of fault geometry and displacement patterns, in P. G. Buchanan and D. A. Nieuwland, eds., 1996, Modern developments in structural interpretation, validation and modelling: Geological Society Special Publication 99, p. 189–199.</ref>

It can be important to map relay ramps, as they can potentially provide pathways for fluid flow across a fault zone (Hesthammer and Fossen, 1997; Rotevatn et al., 2007). Identification of relay ramps can be difficult in practice as the gap between overlapping faults are small (e.g., tens of meters) and difficult to resolve. However, there are ways in which relay ramps can be recognized, despite the limits of seismic resolution:

It can be important to map relay ramps, as they can potentially provide pathways for fluid flow across a fault zone.<ref name=Hesthammerandfossen_1997 /> <ref name=Rotevatnetal_2007>Rotevatn, A., H. Fossen, J. Hesthammer, T. E. Aas, and J. A. Howell, 2007, Are relay ramps conduits for fluid flow? Structural analysis of a relay ramp in Arches National Park, Utah, in L. Lonergan, R. Jolly, K. Rawnsley, and D. J. Sanderson, eds., Fractured reservoirs: Geological Society (London) Special Publication 270, p. 55–71.</ref> Identification of relay ramps can be difficult in practice as the gap between overlapping faults are small (e.g., tens of meters) and difficult to resolve. However, there are ways in which relay ramps can be recognized, despite the limits of seismic resolution:

* Areas where fault traces show kinks on maps are commonly an expression of unresolved relay ramps (Fossen at al., 2005).

* Areas where fault traces show kinks on maps are commonly an expression of unresolved relay ramps.<ref name=Fossenetal_2005>Fossen, H., T. S. Johansen, J. Hesthammer, and A. Rotevatn, 2005, [http://archives.datapages.com/data/bulletns/2005/12dec/1593/1593.HTM Fault interaction in porous sandstone and implications for reservoir management, examples from southern Utah]: AAPG Bulletin, v. 89, no. 12, p. 1593–1606.</ref>

* Relay ramps may correspond to displacement minima along long faults (Needham et al., 1996).

* Relay ramps may correspond to displacement minima along long faults.<ref name=Needhametal_1996 />

* Some of the longer faults may show anomalous length to displacement ratios. This can indicate that a relay ramp has been overlooked (Willemse et al., 1996).

* Some of the longer faults may show anomalous length to displacement ratios. This can indicate that a relay ramp has been overlooked.<ref name=Willemseetal_1996>Willemse, E. J. M., D. D. Pollard, and A. Aydin, 1996, Three-dimensional analyses of slip distributions on normal fault arrays with consequences for fault scaling: Journal of Structural Geology, v. 18, no. 2–3, p. 295–309.</ref>

==Fault damage zone==

==Fault damage zone==

A large number of fractures, microfaults, and deformation bands can be found in a zone (up to 100 m [328 ft] or more wide) on either side of major fault planes (Aydin and Johnson, 1978; Jamison and Stearns, 1982; Antonellini and Aydin, 1995). These damage zones can be observed in outcrops and in cores from wells near large faults ([[:file:M91Ch13FG87.JPG|Figure 9]]).

A large number of fractures, microfaults, and deformation bands can be found in a zone (up to 100 m [328 ft] or more wide) on either side of major fault planes.<ref name=Aydinandjohnson_1978>Aydin, A., and A. M. Johnson, 1978, Development of faults as zones of deformation bands and as slip surfaces in sandstones: Pure and Applied Geophysics, v. 116, p. 931–942.</ref> <ref name=Jamisonandstearns_1982>Jamison, W. R., and D. W. Stearns, 1982, [http://archives.datapages.com/data/bulletns/1982-83/data/pg/0066/0012/2550/2584.htm Tectonic deformation of Wingate Sandstone, Colorado National Monument]: AAPG Bulletin, v. 66, no. 12, p. 2584–2608.</ref> <ref name=Antonelliniandaydin_1995>Antonellini, M., and A. Aydin, 1995, [http://archives.datapages.com/data/bulletns/1994-96/data/pg/0079/0005/0600/0642.htm Effects of faulting on fluid flow in porous sandstones: Geometry and spatial distribution]: AAPG Bulletin, v. 79, no. 5, p. 642–670.</ref> These damage zones can be observed in outcrops and in cores from wells near large faults ([[:file:M91Ch13FG87.JPG|Figure 9]]).

[[file:M91Ch13FG88.JPG|thumb|300px|{{figure number|10}}Deformation bands in the Aztec Sandstone, Valley of Fire, Nevada. Increased compaction compared to the undeformed rock causes the deformation bands to be more resistant to weathering and to stand out as ridges. Individual bands are approximately planar, showing distinct tips even where they are closely spaced (bottom left photo). Porosity loss resulting from granular rearrangement and clay accumulation in the bands results in lowered permeability (bottom right photo). DB = deformation band (from Sternlof et al., 2004).]]

[[file:M91Ch13FG88.JPG|thumb|300px|{{figure number|10}}Deformation bands in the Aztec Sandstone, Valley of Fire, Nevada. Increased compaction compared to the undeformed rock causes the deformation bands to be more resistant to weathering and to stand out as ridges. Individual bands are approximately planar, showing distinct tips even where they are closely spaced (bottom left photo). Porosity loss resulting from granular rearrangement and clay accumulation in the bands results in lowered permeability (bottom right photo). DB = deformation band (from Sternlof et al.<ref name=Sternlofetal_2004>Sternlof, K. R., J. R. Chapin, D. D. Pollard, and L. J. Durlofsky, 2004, [http://archives.datapages.com/data/bulletns/2004/09sep/1315/1315.HTM Permeability effects of deformation band arrays in sandstone]: AAPG Bulletin, v. 88, no. 9, p. 1315–1329.</ref>).]]

Clean, porous sandstones respond to localized strain by forming deformation bands ([[:file:M91Ch13FG88.JPG|Figure 10]]). These are tabular zones where the grains are reorganized by grain sliding, rotation, and commonly fracturing in response to deformation processes including dilation, shearing, and compaction (Fossen et al., 2007). Deformation bands are frequently sheared with shear offsets on a millimeter to centimeter scale. By comparison to open fractures, which tend to enhance permeability, deformation bands have a much reduced permeability compared to the undeformed host sandstone (Antonellini and Aydin, 1994). Given that a damage zone can contain hundreds of deformation bands, then it is clear that even sand-sand contact faults with damage zones can have significantly reduced permeability across them.

Clean, porous sandstones respond to localized strain by forming deformation bands ([[:file:M91Ch13FG88.JPG|Figure 10]]). These are tabular zones where the grains are reorganized by grain sliding, rotation, and commonly fracturing in response to deformation processes including dilation, shearing, and compaction.<ref name=Fossenetal_2007>Fossen, H., R. A. Schultz, Z. K. Shipton, and K. Mair, 2007, Deformation bands in sandstone: A review: Journal of the Geological Society (London), v. 164, p. 755–769.</ref> Deformation bands are frequently sheared with shear offsets on a millimeter to centimeter scale. By comparison to open fractures, which tend to enhance permeability, deformation bands have a much reduced permeability compared to the undeformed host sandstone.<ref name=Antonelliniandaydin_1994>Antonellini, M., and A. Aydin, 1994, [http://archives.datapages.com/data/bulletns/1994-96/data/pg/0078/0003/0350/0355.htm Effect of faulting on fluid flow in porous sandstones: Petrophysical properties]: AAPG Bulletin, v. 78, no. 3, p. 355–377.</ref> Given that a damage zone can contain hundreds of deformation bands, then it is clear that even sand-sand contact faults with damage zones can have significantly reduced permeability across them.

Damage zones in impure sandstones (those with 15–40% clay) contain phyllosilicate-framework fault rocks. These are anastomozing zones where the rock has been disaggregated and the clays have been mixed in with the framework grains to produce a more homogenous mixture of clays than is present in the undeformed host rock. Faults affecting clay-rich sandstones with more than 40% clay content form clay smears (Fisher and Knipe, 1998).

Damage zones in impure sandstones (those with 15–40% clay) contain phyllosilicate-framework fault rocks. These are anastomozing zones where the rock has been disaggregated and the clays have been mixed in with the framework grains to produce a more homogenous mixture of clays than is present in the undeformed host rock. Faults affecting clay-rich sandstones with more than 40% clay content form clay smears.<ref name=Fisherandknipe_1998>Fisher, Q. J., and R. J. Knipe, 1988, Fault sealing processes in siliciclastic sediments, ''in'' H. Jones, Q. J. Fisher, and R. J. Knipe, eds., Faulting, fault sealing and fluid flow in hydrocarbon reservoirs: Geological Society (London) Special Publication 147, p. 117–134.</ref>

The intensity of damage decreases away from the fault with the width of the damage zone roughly proportional to the throw of the fault (Knott, 1994; Knott et al., 1996). Field work on faulting in the Navajo Sandstone of Utah found that the summed width of the damage zones on either side of the fault core is approximately 2.5 times the total fault throw (Shipton and Cowie, 2001). Note that this observation is case specific for this locality. Large and rapid variations in damage zone thickness occur along many faults, and any estimate attempting to systematically relate damage zone thickness to fault throw is liable to a significant uncertainty as a result (Fossen and Bale, 2007).

The intensity of damage decreases away from the fault with the width of the damage zone roughly proportional to the throw of the fault (Knott, 1994; Knott et al., 1996). Field work on faulting in the Navajo Sandstone of Utah found that the summed width of the damage zones on either side of the fault core is approximately 2.5 times the total fault throw (Shipton and Cowie, 2001). Note that this observation is case specific for this locality. Large and rapid variations in damage zone thickness occur along many faults, and any estimate attempting to systematically relate damage zone thickness to fault throw is liable to a significant uncertainty as a result (Fossen and Bale, 2007).