Changes

The clay smear potential is calculated for a particular point on the fault plane as a function of the distance of that point from a shale bed acting as the source for the clay smear and the shale bed thickness<ref name=Bouvieretal_1989>Bouvier, J. D., C. H. Kaars-Sijpesteijn, D. F. Kluesner, C. C. Onyejekwe, and R. C. Van der Pal, 1989, [http://archives.datapages.com/data/bulletns/1988-89/data/pg/0073/0011/1350/1397.htm Three-dimensional seismic interpretation and fault sealing investigations, Nun River field, Nigeria]: AAPG Bulletin, v. 73, p. 1397–1414.</ref> <ref name=Fulljamesetal_1996>Fulljames, J. R., L. J. J. Zijerveld, and R. C. M. W. Fransen, 1997, Fault seal processes: Systematic analysis of fault seals over geological and production time scales, in P. Moller-Petersen and A. G. Koestler, eds., Hydrocarbon seals, importance for exploration and production: Norwegian Petroleum Society Special Publication 7, p. 51–79.</ref> ([[:file:M91Ch13FG90.JPG|Figure 12]]).

The clay smear potential is calculated for a particular point on the fault plane as a function of the distance of that point from a shale bed acting as the source for the clay smear and the shale bed thickness<ref name=Bouvieretal_1989>Bouvier, J. D., C. H. Kaars-Sijpesteijn, D. F. Kluesner, C. C. Onyejekwe, and R. C. Van der Pal, 1989, [http://archives.datapages.com/data/bulletns/1988-89/data/pg/0073/0011/1350/1397.htm Three-dimensional seismic interpretation and fault sealing investigations, Nun River field, Nigeria]: AAPG Bulletin, v. 73, p. 1397–1414.</ref> <ref name=Fulljamesetal_1996>Fulljames, J. R., L. J. J. Zijerveld, and R. C. M. W. Fransen, 1997, Fault seal processes: Systematic analysis of fault seals over geological and production time scales, in P. Moller-Petersen and A. G. Koestler, eds., Hydrocarbon seals, importance for exploration and production: Norwegian Petroleum Society Special Publication 7, p. 51–79.</ref> ([[:file:M91Ch13FG90.JPG|Figure 12]]).

The shale smear factor (SSF) is dependent on the shale bed thickness and the fault throw but not on the smear distance (Lindsay et al., 1993) ([[:file:M91Ch13FG90.JPG|Figure 12]]). Smaller values of the SSF correspond to a more continuous development of smear on the fault plane. A large fault is likely to seal where the SSF is equal to or less than 4.<ref name=Farseth_2006 />

The shale smear factor (SSF) is dependent on the shale bed thickness and the fault throw but not on the smear distance (Lindsay et al., 1993) ([[:file:M91Ch13FG90.JPG|Figure 12]]). Smaller values of the SSF correspond to a more continuous development of smear on the fault plane. A large fault is likely to seal where the SSF is equal to or less than 4.<ref name=Farseth_2006>Farseth, R. B., 2006, Shale smear along large faults: Continuity of smear and fault seal capacity: Journal of the Geological Society (London), v. 163, p. 741–751.</ref>

The shale gouge ratio works on the assumption that the sealing capacity is related directly to the percentage of shale beds or clay material within the slipped interval.<ref name=Yieldingetal_1997 /> The shale gouge ratio is the proportion of the sealing lithology in the rock interval that has slipped past a given point on the fault ([[:file:M91Ch13FG90.JPG|Figure 12]]). To calculate the shale gouge ratio, the proportion of shale and clay in a window equivalent to the throw is measured.

The shale gouge ratio works on the assumption that the sealing capacity is related directly to the percentage of shale beds or clay material within the slipped interval.<ref name=Yieldingetal_1997 /> The shale gouge ratio is the proportion of the sealing lithology in the rock interval that has slipped past a given point on the fault ([[:file:M91Ch13FG90.JPG|Figure 12]]). To calculate the shale gouge ratio, the proportion of shale and clay in a window equivalent to the throw is measured.

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=Lindseyetal_1993 /> <ref name=Foxfordetal_1998 /> 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=Childesetal_1997 /> <ref name=Jamesetal_1997 /> 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 /> 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 /> 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.

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.

Gibson<ref name=Gibson_1994 /> 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.

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.

[[file:M91Ch13FG91.JPG|thumb|300px|{{figure number|13}}Schematic illustration showing the character of fault zones in siliciclastic strata based on outcrop and core observations from onshore and offshore Trinidad (from Gibson<ref name=Gibson_1994 />).]]

[[file:M91Ch13FG91.JPG|thumb|300px|{{figure number|13}}Schematic illustration showing the character of fault zones in siliciclastic strata based on outcrop and core observations from onshore and offshore Trinidad (from Gibson<ref name=Gibson_1994 />).]]

Offshore, hydrocarbon columns up to 200 m (656 ft) thick are found within compartments interpreted as being sealed by clay smears along faults. The general observation is that the blanket of clay smear along faults only appears to be continuous and effective where the shale content of the displaced section exceeds 25%. The shale smear factor was estimated for faults from two of the fields in the basin. SSF values of between 1 and 4 were found for faults with throws more than 150 m (492 ft) that sealed the longest hydrocarbon columns. It was concluded that faults in this area could be modeled as sealing along their length provided the SSF did not exceed a value of 4.

Offshore, hydrocarbon columns up to 200 m (656 ft) thick are found within compartments interpreted as being sealed by clay smears along faults. The general observation is that the blanket of clay smear along faults only appears to be continuous and effective where the shale content of the displaced section exceeds 25%. The shale smear factor was estimated for faults from two of the fields in the basin. SSF values of between 1 and 4 were found for faults with throws more than 150 m (492 ft) that sealed the longest hydrocarbon columns. It was concluded that faults in this area could be modeled as sealing along their length provided the SSF did not exceed a value of 4.

[[file:M91Ch13FG92.JPG|thumb|300px|{{figure number|14}}Comparison between (a) depth-converted seismic interpretation from the Gullfaks field, Norwegian North Sea, and (b) a plaster model deformed by plane strain extension. The plaster model shows that many small-scale faults are expected to exist in the Gullfaks structure but are below seismic resolution (from Fossen and Hesthammer<ref name=Fossenandhesthammer_1998 />). Reprinted with permission from the Geological Society of London.]]

[[file:M91Ch13FG92.JPG|thumb|300px|{{figure number|14}}Comparison between (a) depth-converted seismic interpretation from the Gullfaks field, Norwegian North Sea, and (b) a plaster model deformed by plane strain extension. The plaster model shows that many small-scale faults are expected to exist in the Gullfaks structure but are below seismic resolution (from Fossen and Hesthammer<ref name=Fossenandhesthammer_1998>Fossen, H., and J. Hesthammer, 1998, Structural geology of the Gullfaks field, northern North Sea, in M. P. Coward, H. Johnson, and T. S. Daltaban, eds., Structural geology in reservoir characterization: Geological Society Special Publication 127, p. 231–261.</ref>). Reprinted with permission from the Geological Society of London.]]

==Subseismic faults==

==Subseismic faults==

Only the faults that the geophysicist can pick from seismic data will be mapped, that is, those faults with vertical displacements down to the limit of seismic resolution. As mentioned in [[Data: sources]], this can be about 20–40 m for reservoirs at moderate depths. However, a significant number of subseismic faults will probably be present with vertical displacements less than this ([[:file:M91Ch13FG92.JPG|Figure 14]], [[:file:M91Ch13FG93.JPG|Figure 15]]). Thus, the true degree of the structural complexity of a reservoir will be underrepresented.

Only the faults that the geophysicist can pick from seismic data will be mapped, that is, those faults with vertical displacements down to the limit of seismic resolution. As mentioned in [[Data: sources]], this can be about 20–40 m for reservoirs at moderate depths. However, a significant number of subseismic faults will probably be present with vertical displacements less than this ([[:file:M91Ch13FG92.JPG|Figure 14]], [[:file:M91Ch13FG93.JPG|Figure 15]]). Thus, the true degree of the structural complexity of a reservoir will be underrepresented.

[[file:M91Ch13FG93.JPG|thumb|300px|{{figure number|15}}Fault maps of the East Pennine coalfield, United Kingdom. In map (a), only faults with throws of 20 m (64 ft) or more are shown. These are equivalent to faults that are detectable by seismic surveys at reservoir depths. In map (b), every mapped fault is shown, with fault throws of between 10 cm (4 in.) and 180 m (590 ft) (from Watterson et al.<ref name=Wattersonetal_1996 />). Reprinted with permission from the Journal of Structural Geology.]]

[[file:M91Ch13FG93.JPG|thumb|300px|{{figure number|15}}Fault maps of the East Pennine coalfield, United Kingdom. In map (a), only faults with throws of 20 m (64 ft) or more are shown. These are equivalent to faults that are detectable by seismic surveys at reservoir depths. In map (b), every mapped fault is shown, with fault throws of between 10 cm (4 in.) and 180 m (590 ft) (from Watterson et al.<ref name=Wattersonetal_1996>Watterson, J., J. J. Walsh, P. A. Gillespie, and S. Easton, 1996, Scaling systematics of fault sizes on a large-scale range fault map: Journal of Structural Geology, v. 18, no. 2/3, p. 199–214.</ref>). Reprinted with permission from the Journal of Structural Geology.]]

It is possible to input subseismic faults into a reservoir model using stochastic methods.<ref name=Muntheetal_1993 /> <ref name=Hollundetal_2002 /> In summary, this is a computerized procedure for randomly inserting shapes representing geological features into a 3-D model while still honoring predefined rules and statistics controlling the global distribution of the data.

It is possible to input subseismic faults into a reservoir model using stochastic methods.<ref name=Muntheetal_1993>Munthe, K. L., H. Omre, L. Holden, E. Damsleth, K. Heffer, T. S. Olsen, and J. Watterson, 1993, Subseismic faults in reservoir description and simulation, Presented at the Society of Petroleum Engineers Annual Technical Conference and Exhibition, October 3–6, Houston, Texas, [https://www.onepetro.org/conference-paper/SPE-26500-MS SPE Paper 26500], 8 p.</ref> <ref name=Hollundetal_2002>Hollund, K., P. Mostad, B. F. Nielsen, L. Holden, J. Gjerde, M. G. Contursi, A. J. McCann, C. Townsend, and E. Sverdrup, 2002, Havana—A fault modelling tool, in A. G. Koestler and R. Hunsdale, eds., Hydrocarbon seal quantification: Norwegian Petroleum Society Special Publication 11, p. 157–171.</ref> In summary, this is a computerized procedure for randomly inserting shapes representing geological features into a 3-D model while still honoring predefined rules and statistics controlling the global distribution of the data.

The first part of the method involves making an estimate of the number of subseismic faults by extrapolating from statistics on the length versus frequency of known seismic faults into the subseismic region. Fractal analysis has been used on the assumption that fault-size populations approximate to fractal distributions.<ref name=Gauthierandlake_1993 /> Statistics are also compiled on fault orientations, length to throw ratios, and fault densities per square kilometer. A further step is to determine those areas of the field where subseismic faults are more likely to be present than elsewhere. One method is to predict the paleostrain regime of the reservoir at the time of faulting.<ref name=Maertenetal_2006 /> On this basis, a model will be made, which will include both the seismic and subseismic faults. Fault seal analysis can be applied to the subseismic faults in the model to determine whether they are sealing or not.

The first part of the method involves making an estimate of the number of subseismic faults by extrapolating from statistics on the length versus frequency of known seismic faults into the subseismic region. Fractal analysis has been used on the assumption that fault-size populations approximate to fractal distributions.<ref name=Gauthierandlake_1993>Gauthier, B. D. M., and S. D. Lake, 1993, [http://archives.datapages.com/data/bulletns/1992-93/data/pg/0077/0005/0750/0761.htm Probabilistic modeling of faults below the limit of seismic resolution in Pelican field, North Sea, offshore United Kingdom]: AAPG Bulletin, v. 77, no. 5, p. 761–777.</ref> Statistics are also compiled on fault orientations, length to throw ratios, and fault densities per square kilometer. A further step is to determine those areas of the field where subseismic faults are more likely to be present than elsewhere. One method is to predict the paleostrain regime of the reservoir at the time of faulting.<ref name=Maertenetal_2006>Maerten, L., P. Gillespie, and J.-M. Daniel, 2006, [http://archives.datapages.com/data/bulletns/2006/09sep/BLTN05148/BLTN05148.HTM Three-dimensional geomechanical modeling for constraint of subseismic fault simulation]: AAPG Bulletin, v. 90, no. 9, p. 1337–1358.</ref> On this basis, a model will be made, which will include both the seismic and subseismic faults. Fault seal analysis can be applied to the subseismic faults in the model to determine whether they are sealing or not.

General experience with inserting subseismic faults into simulation models is that they will influence the flow behavior.<ref name=Damslethetal_1998 /> <ref name=Englandandtownsend_1998 /> <ref name=Ottesenetal_2005 /> The critical feature seems to be whether the faults are sealing or not. Sealing faults can create an open framework of short baffles, which helps to improve sweep. The baffles increase the tortuosity of the flood front and delay water breakthrough. A large number of sealing subseismic faults in a reservoir will, on the other hand, create numerous dead ends, which will reduce the sweep efficiency of a waterflood. Nonsealing subseismic faults form cross-fault juxtapositions, which can improve vertical connectivity and enhance sweep.

General experience with inserting subseismic faults into simulation models is that they will influence the flow behavior.<ref name=Damslethetal_1998>Damsleth, E., V. Sangolt, and G. Aamodt, 1998, Sub-seismic faults can seriously affect fluid flow in the Njord field off western Norway—A stochastic fault modeling case study: Presented at the 1998 Society of Petroleum Engineers Annual Technical Conference and Exhibition, September 27–30, 1998, New Orleans, [https://www.onepetro.org/conference-paper/SPE-49024-MS SPE Paper 49024], 49 p.</ref> <ref name=Englandandtownsend_1998>England, W. A., and C. Townsend, 1998, The effects of faulting on production from a shallow marine reservoir: Presented at the 1998 Society of Petroleum Engineers Annual Technical Conference and Exhibition, September 27–30, 1998, New Orleans, [https://www.onepetro.org/conference-paper/SPE-49023-MS SPE Paper 49023], 16 p.</ref> <ref name=Ottesenetal_2005>Ottesen, S., C. Townsend, and K. M. Overland, 2005, [http://archives.datapages.com/data/specpubs/hedberg2/chapter09/CHAPTER09.HTM Investigating the effect of varying fault geometry and transmissibility on recovery: Using a new workflow for structural uncertainty modeling in a clastic reservoir], in P. Boult and J. Kaldi, eds., Evaluating fault and cap rock seals: AAPG Hedberg Series 2, p. 125–136.</ref> The critical feature seems to be whether the faults are sealing or not. Sealing faults can create an open framework of short baffles, which helps to improve sweep. The baffles increase the tortuosity of the flood front and delay water breakthrough. A large number of sealing subseismic faults in a reservoir will, on the other hand, create numerous dead ends, which will reduce the sweep efficiency of a waterflood. Nonsealing subseismic faults form cross-fault juxtapositions, which can improve vertical connectivity and enhance sweep.

[[file:M91Ch13FG94.JPG|thumb|300px|{{figure number|16}}Reservoir intervals thicken markedly across growth faults. They are common in areas with thick delta sequences and mobile substrates such as shale or salt. This example is from Upper Triassic deltaic sediments exposed in the coastal cliffs of Svalbard (from Edwards<ref name=Edwards_1976 />).]]

[[file:M91Ch13FG94.JPG|thumb|300px|{{figure number|16}}Reservoir intervals thicken markedly across growth faults. They are common in areas with thick delta sequences and mobile substrates such as shale or salt. This example is from Upper Triassic deltaic sediments exposed in the coastal cliffs of Svalbard (from Edwards<ref name=Edwards_1976>Edwards, M. B., 1976, [http://archives.datapages.com/data/bulletns/1974-76/data/pg/0060/0003/0300/0341.htm Growth faults in upper Triassic deltaic sediments, Svalbard]: AAPG Bulletin, v. 60, no. 3, p. 341–355.</ref>).]]

==Growth faults==

==Growth faults==