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[[Fault]]s play an important role in creating hydrocarbon traps. For a better appreciation of the risks associated with fault-controlled prospects and of the production from faulted fields, it is important to understand the processes that contribute to fault seals. Given certain information about a fault cutting a reservoir sequence, it is desirable to predict the likely [[Fault seal behavior|sealing behavior]] of each part of the fault system.

[[Fault]]s play an important role in creating hydrocarbon traps. For a better appreciation of the risks associated with fault-controlled prospects and of the production from faulted fields, it is important to understand the processes that contribute to [[fault seal]]s. Given certain information about a fault cutting a reservoir sequence, it is desirable to predict the likely [[Fault seal behavior|sealing behavior]] of each part of the fault system.

==Shale smear factor==

==Shale smear factor==

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Lindsay et al.<ref name=Lindsay>Lindsay, N. G., F. C. Murphy, J. J. Walsh, and J. Watterson, 1993, Outcrop studies of shale smear on fault surfaces: International Association of Sedimentologists Special Publication 15,  p. 113-123.</ref> described outcrop studies of shale smears in a Carboniferous fluvio-deltaic sequence. In contrast to the sequence described by Weber et al.,<ref name=Weber /> these rocks were lithified at the time of faulting (burial depth about 2 km). Lindsay et al.<ref name=Lindsay /> recognized three types of shale smear: shear, abrasion, and injection.

Lindsay et al.<ref name=Lindsay>Lindsay, N. G., F. C. Murphy, J. J. Walsh, and J. Watterson, 1993, Outcrop studies of shale smear on fault surfaces: International Association of Sedimentologists Special Publication 15,  p. 113-123.</ref> described [[outcrop]] studies of [[shale]] smears in a [[Carboniferous]] fluvio-deltaic sequence. In contrast to the sequence described by Weber et al.,<ref name=Weber /> these rocks were lithified at the time of faulting (burial depth about 2 km). Lindsay et al.<ref name=Lindsay /> recognized three types of shale smear: shear, abrasion, and injection.

# Shear smears are analogous to those described by Weber et al.<ref name=Weber>Weber, K. J., G. Mandl, W. F. Pilaar, F. Lehner, and R. G. Precious, 1978, The role of faults in hydrocarbon migration and trapping in Nigerian growth fault structures: Offshore Technology Conference 10, paper OTC 3356, p. 2643-2653.</ref>([[:File:Shale Smear Factor Fig1.png|Figure 1]]). The thicknesses of the smears generally decrease with distance from the source bed, reaching a minimum in the region midway between the hanging-wall and footwall bed terminations.

# Shear smears are analogous to those described by Weber et al.<ref name=Weber>Weber, K. J., G. Mandl, W. F. Pilaar, F. Lehner, and R. G. Precious, 1978, The role of faults in hydrocarbon migration and trapping in Nigerian growth fault structures: Offshore Technology Conference 10, paper OTC 3356, p. 2643-2653.</ref>([[:File:Shale Smear Factor Fig1.png|Figure 1]]). The thicknesses of the smears generally decrease with distance from the source bed, reaching a minimum in the region midway between the hanging-wall and footwall bed terminations.

<math>\text{SSF} = \frac{\text{fault throw}}{\text{shale layer thickness}}</math>

<math>\text{SSF} = \frac{\text{fault throw}}{\text{shale layer thickness}}</math>

The shale smear factor remains constant between the offset terminations because it does not depend on smear distance (although lateral variations in fault throw would have a corresponding effect on the calculated SSF). SSF thus models the profile of abrasion-type smears. From a study of 80 faults (excluding composite smears), Lindsay et al.<ref name=Lindsay /> concluded that shale smears may become incomplete for an SSF greater than 7. Smaller values of SSF are more likely to correspond to continuous smears and therefore to a sealing layer on the fault surface. The values of SSF are not additive for compound smears because thin shales give higher SSF and dominate the sum. In such cases, a simple application of SSF values would take the minimum value (most sealing) from the relevant shale beds at that point on the fault.

The shale smear factor remains constant between the [[offset]] terminations because it does not depend on smear distance (although [[lateral]] variations in fault throw would have a corresponding effect on the calculated SSF). SSF thus models the profile of abrasion-type smears. From a study of 80 faults (excluding composite smears), Lindsay et al.<ref name=Lindsay /> concluded that shale smears may become incomplete for an SSF greater than 7. Smaller values of SSF are more likely to correspond to continuous smears and therefore to a sealing layer on the fault surface. The values of SSF are not additive for compound smears because thin shales give higher SSF and dominate the sum. In such cases, a simple application of SSF values would take the minimum value (most sealing) from the relevant shale beds at that point on the fault.

==Shale gouge ratio (SGR)==

==Shale gouge ratio (SGR)==

[[File:Shale-gouge-ratio-fig3.png|thumb|300px|{{figure number|3}}Gouge ratio algorithms for estimating likelihood of clay entrainment in the fault gouge zone. The gouge ratio reflects the proportion of the sealing lithology in the rock interval that has slipped past a given point on the fault. (a) Calculation for explicit shale/clay beds in an otherwise shale-free sequence; Dz is the thickness of each shale bed. (b) Calculation for a sequence of reservoir zones; Dz is the thickness of each reservoir zone and Vcl is the clay volume fraction in the zone.]]

[[File:Shale-gouge-ratio-fig3.png|thumb|300px|{{figure number|3}}Gouge ratio algorithms for estimating likelihood of clay entrainment in the fault gouge zone. The gouge ratio reflects the proportion of the sealing lithology in the rock interval that has slipped past a given point on the fault. (a) Calculation for explicit shale/clay beds in an otherwise shale-free sequence; Dz is the thickness of each shale bed. (b) Calculation for a sequence of reservoir zones; Dz is the thickness of each reservoir zone and Vcl is the clay volume fraction in the zone.]]

The shale gouge ratio is simply the percentage of shale or clay in the slipped interval. [[:File:Shale-gouge-ratio-fig3.png|Figure 3a]] illustrates how this would be calculated, at a given point on a fault surface, for explicit shale beds (Equation 4):

The [[shale gouge ratio]] is simply the percentage of shale or [[clay]] in the slipped interval. [[:File:Shale-gouge-ratio-fig3.png|Figure 3a]] illustrates how this would be calculated, at a given point on a fault surface, for explicit shale beds (Equation 4):

<math>\text{SGR} = \frac{\Sigma (\text{shale bed thickness})}{\text{fault throw}} \times 100%</math>

<math>\text{SGR} = \frac{\Sigma (\text{shale bed thickness})}{\text{fault throw}} \times 100%</math>

Equation 5 reduces to equation 4 as the zonation approaches individual beds (assuming shale/clay beds are 100% clay material). The SGR represents, in a general way, the proportion of shale or clay that might be entrained in the fault zone by a variety of mechanisms. The more shaly the wall rocks, the greater the proportion of shale in the fault zone, and therefore the higher the capillary entry pressure. Although this is undoubtedly an oversimplification of the detailed processes occurring in the fault zone, it represents a tractable upscaling of the lithological diversity at the fault surface; the required information is simply fault displacement and shale fraction through the sequence.

Equation 5 reduces to equation 4 as the zonation approaches individual beds (assuming shale/clay beds are 100% clay material). The SGR represents, in a general way, the proportion of shale or clay that might be entrained in the fault zone by a variety of mechanisms. The more shaly the wall rocks, the greater the proportion of shale in the fault zone, and therefore the higher the capillary entry pressure. Although this is undoubtedly an oversimplification of the detailed processes occurring in the fault zone, it represents a tractable upscaling of the lithological diversity at the fault surface; the required information is simply fault displacement and shale fraction through the sequence.

The gouge ratio algorithm can be extended to include other lithologies in addition to shale/clay. For example, if numerous coal beds are present they may contribute to the fine-grained fault gouge, although less efficiently than smeared clay. In this case the coal units can be included in the summation and down-weighted with respect to the shale.

 

The gouge ratio algorithm can be extended to include other lithologies in addition to shale/clay. For example, if numerous [[coal]] beds are present they may contribute to the fine-grained fault gouge, although less efficiently than smeared clay. In this case the coal units can be included in the summation and down-weighted with respect to the shale.

Unfortunately both shale gouge ratio and smear gouge ratio are commonly called SGR.  They are inversely related.

Unfortunately both shale gouge ratio and smear gouge ratio are commonly called SGR.  They are inversely related.

==Smear gouge ratio (SGR)==

==Smear gouge ratio (SGR)==

Smear-gouge ratio is the ratio of sand to shale that has moved past some critical portion of the fault plane.<ref>Skerlec, G. M., 1996, Risking fault seal in the Gulf Coast (abs.): AAPG Annual Convention Program and Abstracts, v. 5,  p. A131.</ref> Although not simply relatable to the shale gouge ratio, the smear gouge ratio varies in an inverse manner; i.e., high shale gouge ratio corresponds to low smear gouge ratio and vice versa.

[[Smear-gouge ratio]] is the ratio of sand to shale that has moved past some critical portion of the fault plane.<ref>Skerlec, G. M., 1996, Risking fault seal in the Gulf Coast (abs.): AAPG Annual Convention Program and Abstracts, v. 5,  p. A131.</ref> Although not simply relatable to the shale gouge ratio, the smear gouge ratio varies in an inverse manner; i.e., high shale gouge ratio corresponds to low smear gouge ratio and vice versa.

==Further Reading==

==Further Reading==