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In structurally simple fields, the main control on production behavior is the distribution of lithofacies. In structurally complex fields, faults and ~~fractures~~ provide major elements influencing production performance. This article discusses the data used to establish the presence of faults and how faults are mapped for reservoir models. The reservoir structure can be analyzed at two different scales: the seismic scale and the well scale. The interpretation of faults and structure at the seismic scale is made by the seismic interpreter whereas the production geologist analyzes the structures from core and log data. Having established a fault framework for a field, it is important to know whether or not fluid flow communication occurs across the faults. Techniques are available to predict the likelihood of this. Sometimes sealing faults break down and open up to flow after a field has been producing for a few years. This reflects the change in the stress state of the reservoir as a result of pressure depletion.

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In structurally simple fields, the main control on production behavior is the distribution of [[lithofacies]]. In structurally complex fields, faults and [[fracture]]s provide major elements influencing production performance. This article discusses the data used to establish the presence of faults and how faults are mapped for reservoir models. The reservoir structure can be analyzed at two different scales: the seismic scale and the well scale. The interpretation of faults and structure at the seismic scale is made by the seismic interpreter whereas the production geologist analyzes the structures from core and log data. Having established a fault framework for a field, it is important to know whether or not fluid flow communication occurs across the faults. Techniques are available to predict the likelihood of this. Sometimes sealing faults break down and open up to flow after a field has been producing for a few years. This reflects the change in the stress state of the reservoir as a result of pressure depletion.

[[file:M91Ch6FG47.JPG|thumb|300px|{{figure number|1}}Seismic line and equivalent interpretation through the Penguin C South field, UK North Sea (from Dominguez<ref name=Dominguez_2007>Dominguez, R., 2007, Structural evolution of the Penguins cluster, UK northern North Sea, in S. J. Jolley, D. Barr, J. J. Walsh, and R. J. Knipe, eds., Structurally complex reservoirs: Geological Society (London) Special Publication 292, p. 25–48.</ref>). Reprinted with permission from the Geological Society.]]

==Seismic interpretation of faults==

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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]]).

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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]]).

==Structural core logging==

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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° depiction of the structure comparable to the display shown by borehole image logs. Commercial rigs are also available, which take 360° photographs of the core for the same purpose.

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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: [http://store.aapg.org/detail.aspx?id=612 AAPG Methods in Exploration Series 10], p. 122–124.</ref>

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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: [http://store.aapg.org/detail.aspx?id=612 AAPG Methods in Exploration Series 10], p. 122–124.</ref>

Structural core logging provides a variety of useful information for the reservoir model. For example,

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==Mapping faults==

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[[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.]]

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[[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 [[contour]]ed 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.]]

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]]).

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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]]).

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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]]).

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.

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A study on the Big Hole Fault in Utah based on core data showed a significant permeability reduction within the damage zone.<ref name=Shiptonetal_2002>Shipton, Z. K., J. P. Evans, K. R. Robeson, C. B. Forster, and S. Snelgrove, 2002, [http://archives.datapages.com/data/bulletns/2002/05may/0863/0863.htm Structural heterogeneity and permeability in faulted eolian sandstone: Implications for subsurface modeling of fault]s: AAPG Bulletin, v. 86, no. 5, p. 863–883.</ref> Probe permeameter measurements of permeability range from more than 2000 md in the undeformed host sandstone to less than 0.1 md in fault-damaged rocks near the fault. Whole-core tests showed that the permeability of individual deformation bands vary between 0.9 and 1.3 md. The transverse permeability modeled over 5–10-m (16–32-ft)-length scales across the fault zone was estimated as 30–40 md. This is approximately 1–4% of the permeability for the undeformed host rock.

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The general consensus in the industry is that damage zones around faults probably baffle flow across them rather than acting as barriers to fluid movement.<ref name=Sternlofetal_2004 /> <ref name=Fossenandbale_2007 /> The exception may be in deep reservoirs with high reservoir temperatures (more than 120°C). Here, accelerated quartz cementation at high temperature can decrease the pore throat diameters in the deformation bands to the extent that they become 100% water wet through capillary action. They thus become effective barriers to oil flow.<ref name=Hesthammeretal_2002>Hesthammer, J., P. A. Bjorkum, and L. Watts, 2002, [http://archives.datapages.com/data/bulletns/2002/10oct/1733/1733.htm The effect of temperature on sealing capacity of faults in sandstone reservoirs: Examples from the Gullfaks and Gullfaks Sor fields, North Sea]: AAPG Bulletin, v. 86, no. 10, p. 1733–1751.</ref>

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The general consensus in the industry is that damage zones around faults probably baffle flow across them rather than acting as barriers to fluid movement.<ref name=Sternlofetal_2004 /> <ref name=Fossenandbale_2007 /> The exception may be in deep reservoirs with high reservoir temperatures (more than 120°C). Here, accelerated [[quartz]] cementation at high temperature can decrease the pore throat diameters in the deformation bands to the extent that they become 100% water wet through capillary action. They thus become effective barriers to oil flow.<ref name=Hesthammeretal_2002>Hesthammer, J., P. A. Bjorkum, and L. Watts, 2002, [http://archives.datapages.com/data/bulletns/2002/10oct/1733/1733.htm The effect of temperature on sealing capacity of faults in sandstone reservoirs: Examples from the Gullfaks and Gullfaks Sor fields, North Sea]: AAPG Bulletin, v. 86, no. 10, p. 1733–1751.</ref>

Because of the abundance of low-permeability baffles and poorly connected volumes, production wells drilled in fault damage zones can significantly underperform. For example, wells drilled in fault-damaged zones in the North La Barge Shallow Unit of Wyoming are the poorest producers in the field.<ref name=Miskimins_2003>Miskimins, J. L., 2003, Analysis of hydrocarbons production in a critically-stressed reservoir: Presented at the Society of Petroleum Engineers Annual Technical Conference and Exhibition, October 5–8, Denver, Colorado, SPE Paper 84457, 8 p.</ref> It is generally not a good idea to plan a new well trajectory too close to a large fault because of this.

==Faults and fluid flow==

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Faults can have a significant impact on the fluid flow patterns within a reservoir. They can juxtapose one reservoir interval with another creating the potential for cross flow between the units. It is pragmatic to assume that all sand to sand juxtapositions allow fluid transfer across faults unless proven otherwise.<ref name=Jamesetal_2004>James, W. R., L. H. Fairchild, G. P. Nakayama, S. J. Hippler, and P. J. Vrolijk, 2004, [http://archives.datapages.com/data/bulletns/2004/07jul/0885/0885.HTM Fault-seal analysis using a stochastic multi-fault approach]: AAPG Bulletin, v. 88, no. 7, p. 885–904.</ref> Alternatively, juxtaposition of reservoir with nonreservoir rocks can cause the trapping of hydrocarbons against the fault. Deformation and cementation within the fault zone itself can create a zone of zero or very low permeability, which can cause the fault plane to act as a barrier to fluid flow. In some instances, fractures in the fault zone itself can act as conduits for fluid flow.

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Faults can have a significant impact on the fluid flow patterns within a reservoir. They can juxtapose one reservoir interval with another creating the potential for cross flow between the units. It is pragmatic to assume that all sand to sand juxtapositions allow fluid transfer across faults unless proven otherwise.<ref name=Jamesetal_2004>James, W. R., L. H. Fairchild, G. P. Nakayama, S. J. Hippler, and P. J. Vrolijk, 2004, [http://archives.datapages.com/data/bulletns/2004/07jul/0885/0885.HTM Fault-seal analysis using a stochastic multi-fault approach]: AAPG Bulletin, v. 88, no. 7, p. 885–904.</ref> Alternatively, juxtaposition of reservoir with nonreservoir rocks can cause the trapping of hydrocarbons against the fault. [[Deformation]] and cementation within the fault zone itself can create a zone of zero or very low permeability, which can cause the fault plane to act as a barrier to fluid flow. In some instances, fractures in the fault zone itself can act as conduits for fluid flow.

[[file:M91Ch13FG89.JPG|thumb|300px|{{figure number|11}}Allan diagrams show the reservoir stratigraphy of both the hanging wall and footwall blocks of a fault superimposed along the fault plane. At a glance, it can be seen where reservoir and nonreservoir lithologies are juxtaposed with potential for juxtaposition sealing.]]

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==Fault seal==

Estimates can be made using Allan diagrams as to the probability that a fault will seal within a reservoir. In the first instance, fault seal can result from the juxtaposition of reservoir with nonreservoir rock. However, experience from many petroleum provinces has shown that faults can seal even where reservoir quality sand bodies are juxtaposed across a fault. The most common mechanism for sealing results from the incorporation of fine grained or dense material into the fault plane. Five different processes may cause this:<ref name=Mitra_1988>Mitra, S., 1988, [http://archives.datapages.com/data/bulletns/1988-89/data/pg/0072/0005/0500/0536.htm Effects of deformation mechanisms on reservoir potential in central Appalachian overthrust belt]: AAPG Bulletin, v. 72, no. 5, p. 536–554.</ref> <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>

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* Clay smear: Faults in clay-rich sediments are believed to form clay smears by the shearing of mudstone beds into the fault zone.<ref name=Weberetal_1978>Weber, K. J., L. J. Urai, W. F. Pilaar, F. Lehner, and R. G. Precious, 1978, The role of faults in hydrocarbon migration and trapping in Nigerian growth fault structures: 10th Annual Offshore Technology Conference Proceedings, v. 4, p. 2643–2653.</ref> <ref name=Lehnerandpilaar_1997>Lehner, F. K., and F. K. Pilaar, 1997, The emplacement of clay smears in synsedimentary normal faults: Inferences and field observations near Frechen Germany, in P. Moller-Pederson and A. G. Koestler, eds., Hydrocarbon seals: Importance for exploration and production: Norwegian Petroleum Society Special Publication 7, p. 15–38.</ref>

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* Clay smear: Faults in clay-rich sediments are believed to form clay smears by the shearing of [[mudstone]] beds into the fault zone.<ref name=Weberetal_1978>Weber, K. J., L. J. Urai, W. F. Pilaar, F. Lehner, and R. G. Precious, 1978, The role of faults in hydrocarbon migration and trapping in Nigerian growth fault structures: 10th Annual Offshore Technology Conference Proceedings, v. 4, p. 2643–2653.</ref> <ref name=Lehnerandpilaar_1997>Lehner, F. K., and F. K. Pilaar, 1997, The emplacement of clay smears in synsedimentary normal faults: Inferences and field observations near Frechen Germany, in P. Moller-Pederson and A. G. Koestler, eds., Hydrocarbon seals: Importance for exploration and production: Norwegian Petroleum Society Special Publication 7, p. 15–38.</ref>

* Cataclasis (shale gouge): Fault movement affecting clean sandstones will cause grain crushing and the breakage of rock in the fault plane, which will form a fault gouge.<ref name=Lindsayetal_1993>Lindsay, N. G., F. C. Murphy, J. J. Walsh, and J. Watterson, 1993, Outcrop studies of shale smear on fault surfaces, in S. S. Flint and I. D. Bryant, eds., The geological modelling of hydrocarbon reservoirs and outcrop analogs: International Association of Sedimentologists Special Publication 15, p. 113–123.</ref>

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* Diagenesis or cementation: Fine grained fault rock and associated open fractures in fault zones can be prone to cementation. Fluids migrating up the fault zone can cause the mineralization of the host rock. It is a common observation to find carbonate-cemented intervals in wells drilled close to faults, whereas wells drilled farther away from the faults do not contain carbonate cements (e.g., Reynolds et al.<ref name=Reynoldsetal_1998>Reynolds, A. D., et al., 1998, [http://archives.datapages.com/data/bulletns/1998/01jan/0025/0025.htm Implications of outcrop geology for reservoirs in the Neogene productive series: Apsheron Peninsula, Azerbaijan]: AAPG Bulletin, v. 82, no. 1, p. 25–29.</ref>). This is an indication that the fault zones have acted as the locus for the fluids causing carbonate cementation.

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* [[Diagenesis]] or cementation: Fine grained fault rock and associated open fractures in fault zones can be prone to cementation. Fluids migrating up the fault zone can cause the mineralization of the host rock. It is a common observation to find carbonate-cemented intervals in wells drilled close to faults, whereas wells drilled farther away from the faults do not contain carbonate cements (e.g., Reynolds et al.<ref name=Reynoldsetal_1998>Reynolds, A. D., et al., 1998, [http://archives.datapages.com/data/bulletns/1998/01jan/0025/0025.htm Implications of outcrop geology for reservoirs in the Neogene productive series: Apsheron Peninsula, Azerbaijan]: AAPG Bulletin, v. 82, no. 1, p. 25–29.</ref>). This is an indication that the fault zones have acted as the locus for the fluids causing carbonate cementation.

* Pore volume collapse: Ductile deformation during fault movement can cause poorly sorted sediments to mix and homogenize with a resultant decrease in porosity.

* Grain contact dissolution: Fault zones can act as planes for intergranular grain contact dissolution and subsequent recementation of the dissolved material. This can be an important mechanism for fault sealing in carbonate rocks.<ref name=Peacocketal_1998>Peacock, D. C. P., Q. J. Fisher, E. J. M. Willemse, and A. Aydin, 1998, The relationship between faults and pressure solution seams in carbonate rocks and the implications for fluid flow, in G. Jones, Q. J. Fisher, and R. J. Knipe, eds., Faulting and fluid flow in hydrocarbon reservoirs: Geological Society (London) Special Publication 147, p. 105–115.</ref>

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==Fault sealing characteristics==

Fine grained fault rock will have a higher capillary entry pressure compared to the undeformed host rock. Brown<ref name=Brown_2003>Brown, A., 2003, [http://archives.datapages.com/data/bulletns/2003/03mar/0381/0381.HTM Capillary effects on fault-fill sealing]: AAPG Bulletin, v. 87, no. 3, p. 381–395.</ref> described how the seal behavior of water-wet fault fill defines three potential zones within a fault.

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* A fault can seal because the petroleum phase has insufficient buoyancy pressure to invade and displace water from the fine grained material within the fault rock; this has been termed membrane sealing.<ref name=Watts_1987 />

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* A fault can seal because the petroleum phase has insufficient [[buoyancy pressure]] to invade and displace water from the fine grained material within the fault rock; this has been termed membrane sealing.<ref name=Watts_1987 />

* Higher within the petroleum column, the buoyancy pressure can increase to the point at which the oil or gas can invade the fault rock and thus leak through it. However, the fault rock will have a very low permeability, and the rate of leakage can be trivial, even over geological time.<ref name=Heum_1996>Heum, O. R., 1996, A fluid dynamic classification of hydrocarbon entrapment: Petroleum Geoscience, v. 2, no. 2, p. 145–158.</ref> The fault can then be considered to be effectively lsquosealingrsquo by hydraulic resistance.<ref name=Watts_1987>Watts, N. L., 1987, Theoretical aspects of cap-rock and fault seals for single and two phase columns: Marine and Petroleum Geology, v. 4, no. 4, p. 274–307.</ref>

* Where an exceptionally thick petroleum column exists, even low-permeability fault rocks can leak at significant rates. This is the zone of fault fill seal failure.

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The prediction of fault seal is based on the assumption that if there is enough shale in the section undergoing faulting, then sealing is likely. There is often a continuous shale gouge or shale smear along fault planes where there is sufficient mudstone material available to be incorporated.<ref name=Lindsayetal_1993 /> <ref name=Foxfordetal_1998>Foxford, K. A., J. J. Walsh, J. Watterson, I. R. Garden, S. C. Guscott, and S. D. Burley, 1998, Structure and content of the Moab fault zone, Utah, U.S.A., and its implications for fault seal prediction, 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. 87–103.</ref> Nevertheless, a number of field studies show that fault zones can have a significant degree of complexity and variation in deformation style along their lengths.<ref name=Childsetal_1997>Childs, C., J. J. Walsh, and J. Watterson, 1997, Complexity in fault zones and its implications for fault seal prediction, in P. Moller-Pederson and A. G. Koestler, eds., Hydrocarbon seals: Importance for exploration and production: Norwegian Petroleum Society Special Publication 7, p. 61–72.</ref> <ref name=Jamesetal_1997>James, D. M. D., C. Childs, J. Watterson, and J. J. Walsh, 1997, Discussion on a model for the structure and development of fault zones: Reply: Journal of the Geological Society (London), v. 154, no. 2, p. 366–368.</ref> For example, Foxford et al.<ref name=Foxfordetal_1998 /> examined good exposures of the Moab fault in Utah. They found that the structure and content of the fault zone was so variable that it was impossible to predict the nature of the fault zone over even a 10-m (33-ft) distance. Doughty<ref name=Doughty_2003>Doughty, P. T., 2003, [http://archives.datapages.com/data/bulletns/2003/03mar/0427/0427.HTM Clay smear seals and fault sealing potential of an exhumed growth fault, Rio Grande rift, New Mexico]: AAPG Bulletin, v. 87, no. 3, p. 427–444.</ref> found that the clay smear along the Calabacillas fault in New Mexico showed numerous gaps particularly where minor faults within the fault zone complex cut out the shale smear associated with the major slip plane. The implication of these field studies is that fault seal can be predicted but is subject to chance factors affecting the reliability of the prediction. Because of this, any fault seal prediction should be calibrated against actual evidence that fault compartmentalization is present. Yielding et al.<ref name=Yieldingetal_1999>Yielding, G., J. A. Overland, and G. Byberg, 1999, [http://archives.datapages.com/data/bulletns/1999/06jun/0925/0925.htm Characterization of fault zones for reservoir modeling: An example from the Gullfaks field, northern North Sea]: AAPG Bulletin, v. 83, no. 6, p. 925–951.</ref> made a fault seal analysis for the Gullfaks field in the Norwegian North Sea. Areas of higher shale gouge ratios (>20%) were more likely to seal on the basis of pressure history and chemical tracer movement between wells.

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Gibson<ref name=Gibson_1994>Gibson, R. G., 1994, [http://archives.datapages.com/data/bulletns/1994-96/data/pg/0078/0009/1350/1372.htm Fault-zone seals in siliciclastic strata of the Columbus basin, offshore Trinidad]: AAPG Bulletin, v. 78, no. 9, p. 1372–1385.</ref> provided a case history for fault seal analysis from the Columbus Basin, offshore Trinidad. Oil and gas fields occur in upper Miocene to Pleistocene deltaic sandstones of the Columbus Basin, located offshore to the southeast of the island of Trinidad. Numerous small faults dissect these reservoirs, and fault seal appears to be a critical feature controlling the size of these petroleum pools. Allan diagrams show that juxtaposition sealing is insufficient to explain the fault control on fluid contacts.

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Gibson<ref name=Gibson_1994>Gibson, R. G., 1994, [http://archives.datapages.com/data/bulletns/1994-96/data/pg/0078/0009/1350/1372.htm Fault-zone seals in siliciclastic strata of the Columbus basin, offshore Trinidad]: AAPG Bulletin, v. 78, no. 9, p. 1372–1385.</ref> provided a case history for fault seal analysis from the Columbus Basin, offshore Trinidad. Oil and gas fields occur in upper Miocene to Pleistocene deltaic sandstones of the Columbus Basin, located offshore to the southeast of the island of Trinidad. Numerous small faults dissect these reservoirs, and fault seal appears to be a critical feature controlling the size of these petroleum pools. Allan diagrams show that juxtaposition sealing is insufficient to explain the fault control on [[fluid contacts]].

The sediments that form the reservoirs offshore are also exposed onshore along the east coast of Trinidad. Outcrops onshore and cores offshore provide control on the nature of the fault rock. In these outcrops, shale smears are found where shale beds have been displaced along the fault. The shale smears range in thickness from millimeter- to centimeter-thick shale partings to complex zones up to several meters thick ([[:file:M91Ch13FG91.JPG|Figure 13]]).

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==Growth faults==

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Growth ~~faults~~ are faults that were active at the same time as the sediments were being deposited ([[:file:M91Ch13FG94.JPG|Figure 16]]). Many show a listric geometry with the fault soling out into shale horizons. They are common in areas with thick delta sequences. Growth faults can be recognized because sediments thicken into the hanging wall of a growth fault and the throw of the fault increases with depth. All the individual reservoir units may thicken up across a mapped growth fault. Alternatively, growth can be taken up by additional layers filling the accommodation space in the hanging wall.<ref name=Hodgettsetal_2001>Hodgetts, D., J. Imber, C. Childs, S. Flint, J. Howell, J. Kavanagh, P. Nell, and J. Walsh, 2001, [http://archives.datapages.com/data/bulletns/2001/03mar/0433/0433.htm Sequence-stratigraphic responses to shoreline-perpendicular growth faulting in shallow marine reservoirs of the Champion field, offshore Brunei Darussalam, South China Sea]: AAPG Bulletin, v. 85, no. 3, p. 433–457.</ref>

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[[Growth fault]]s are faults that were active at the same time as the sediments were being deposited ([[:file:M91Ch13FG94.JPG|Figure 16]]). Many show a listric geometry with the fault soling out into shale horizons. They are common in areas with thick delta sequences. Growth faults can be recognized because sediments thicken into the hanging wall of a growth fault and the throw of the fault increases with depth. All the individual reservoir units may thicken up across a mapped growth fault. Alternatively, growth can be taken up by additional layers filling the accommodation space in the hanging wall.<ref name=Hodgettsetal_2001>Hodgetts, D., J. Imber, C. Childs, S. Flint, J. Howell, J. Kavanagh, P. Nell, and J. Walsh, 2001, [http://archives.datapages.com/data/bulletns/2001/03mar/0433/0433.htm Sequence-stratigraphic responses to shoreline-perpendicular growth faulting in shallow marine reservoirs of the Champion field, offshore Brunei Darussalam, South China Sea]: AAPG Bulletin, v. 85, no. 3, p. 433–457.</ref>

==Faults as flow conduits==

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Faults: structural geology (view source)

Revision as of 16:51, 15 January 2024

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