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Fault zones in siliciclastic sequences consist of various proportions of sand and shale. There is a relationship between seal behavior and the inferred sand-shale ratio of the fault zone. Sand-rich portions of the fault zone leak; shale-rich portions of the fault zone seal. Given the complex structure and composition of fault zones and the many factors that could affect a seal, it has been surprising to find such a simple relationship, basin after basin.

[[Fault]] zones in siliciclastic sequences consist of various proportions of [[sand]] and [[shale]]. There is a relationship between seal behavior and the inferred sand-shale ratio of the fault zone. Sand-rich portions of the fault zone leak; shale-rich portions of the fault zone seal. Given the complex structure and composition of fault zones and the many factors that could affect a seal, it has been surprising to find such a simple relationship, basin after basin.

==Example: smear gouge==

==Example: smear gouge==

[[file:evaluating-top-and-fault-seal_fig10-20.jpg|300px|thumb|{{figure number|1}}Smear gouge along a normal fault found in a sequence of Permian-Carboniferous sand and shales near Lynemouth, U.K..]]

[[file:evaluating-top-and-fault-seal_fig10-20.jpg|300px|thumb|{{figure number|1}}Smear gouge along a normal fault found in a sequence of Permian-Carboniferous sand and shales near Lynemouth, U.K..]]

[[:file:evaluating-top-and-fault-seal_fig10-20.jpg|Figure 1]] is an example of smear gouge along a normal fault found in a sequence of Permian-Carboniferous sand and shales near Lynemouth, U.K. Ductile, light gray shales have been incorporated in the fault zone as a continuous layer sourced from a bed in the footwall (upper left). Similarly, a bed of coal (black) in the footwall and sandstone in the hanging wall have been incorporated as discontinuous fragments in the fault zone. The coal, however, is intensely fractured, reflecting [[Brittleness|brittle]] rather than [[Ductility|ductile]] [[deformation]].

[[:file:evaluating-top-and-fault-seal_fig10-20.jpg|Figure 1]] is an example of smear gouge along a [[normal fault]] found in a sequence of [[Permian]]-[[Carboniferous]] sand and shales near Lynemouth, U.K. Ductile, light gray shales have been incorporated in the fault zone as a continuous layer sourced from a bed in the [[footwall]] (upper left). Similarly, a bed of coal (black) in the footwall and sandstone in the [[hanging wall]] have been incorporated as discontinuous fragments in the fault zone. The coal, however, is intensely fractured, reflecting [[Brittleness|brittle]] rather than [[Ductility|ductile]] [[deformation]].

Shale-rich gouge zones can create impermeable barriers. The following, a normal fault in Jurassic and Triassic sediments near Lilstock, U.K., shows a gouge zone approximately [[length::5 m]] thick. The gouge consists predominantly of ductile shales and marls as well as small amounts of brittle limestones. Faults with similar shale-rich gouge in producing fields create seals.

Shale-rich gouge zones can create impermeable barriers. The following, a normal fault in [[Jurassic]] and [[Triassic]] sediments near Lilstock, U.K., shows a gouge zone approximately [[length::5 m]] thick. The gouge consists predominantly of ductile shales and marls as well as small amounts of brittle limestones. Faults with similar shale-rich gouge in producing fields create seals.

==Example: spectrum of gouge compositions==

==Example: spectrum of gouge compositions==