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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.
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.]]
[[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.]]
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.
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.
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>
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.
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.
==Fault seal==
==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>
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>
* 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>
* 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>
* 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>
* [[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.
* [[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.
==Growth faults==
==Growth faults==
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>
[[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==
==Faults as flow conduits==