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How fault zones affect seal (view source)
Revision as of 13:54, 31 January 2014
, 13:54, 31 January 2014Initial import
{{publication
| image = exploring-for-oil-and-gas-traps.png
| width = 120px
| series = Treatise in Petroleum Geology
| title = Exploring for Oil and Gas Traps
| part = Predicting the occurrence of oil and gas traps
| chapter = Evaluating top and fault seal
| frompg = 10-1
| topg = 10-94
| author = Grant M. Skerlec
| link = http://archives.datapages.com/data/specpubs/beaumont/ch10/ch10.htm
| pdf =
| store = http://store.aapg.org/detail.aspx?id=545
| isbn = 0-89181-602-X
}}
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==
The following figure 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 brittle rather than ductile deformation.
Both brittle sands and coals as well as ductile shales are incorporated in the fault zone. The mechanism of gouge formation is much more complex than a simple “smearing” of ductile shales into the fault zone. Although faults exhibit a complete spectrum of behavior from brittle to ductile, interbedded sand-shale sequences with strong ductility contrast commonly exhibit this transitional behavior.
[[file:evaluating-top-and-fault-seal_fig10-19.png|thumb|{{figure number|10-19}}See text for explanation.]]
Shale-rich gouge zones can create impermeable barriers. The following figure, 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.
[[file:evaluating-top-and-fault-seal_fig10-20.png|thumb|{{figure number|10-20}}See text for explanation.]]
Fault gouge can range from sand rich to shale rich. The following figure shows three fault zones exhibiting a spectrum of sand-shale ratios. The table below describes the position of the three faults, their sequences and characteristics, and their location.
{| class = "wikitable"
|-
! Position
! Sequence
! Characteristics
! Location
|-
| Left
| Sand–siltstone
| Gouge with high sand–shale ratio
| Tertiary, Isle of Wight, U.K.
|-
| Center
| Limestone–shale
| Gouge with intermediate “sand”–shale ratio
| Jurassic, Kilve, U.K.
|-
| Right
| Limestone–shale
| Gouge with low “sand”–shale ratio
| Jurassic, Kilve, U.K.
|}
[[file:evaluating-top-and-fault-seal_fig10-21.png|thumb|{{figure number|10-21}}See text for explanation.]]
==See also==
* [[Quantitative fault seal analysis]]
* [[Quantitative fault seal analysis]]
* [[Limitations of quantitative fault seal analysis]]
* [[Example of routine fault seal analysis: gulf coast]]
==External links==
{{search}}
* [http://archives.datapages.com/data/specpubs/beaumont/ch10/ch10.htm Original content in Datapages]
* [http://store.aapg.org/detail.aspx?id=545 Find the book in the AAPG Store]
[[Category:Predicting the occurrence of oil and gas traps]]
[[Category:Evaluating top and fault seal]]
| image = exploring-for-oil-and-gas-traps.png
| width = 120px
| series = Treatise in Petroleum Geology
| title = Exploring for Oil and Gas Traps
| part = Predicting the occurrence of oil and gas traps
| chapter = Evaluating top and fault seal
| frompg = 10-1
| topg = 10-94
| author = Grant M. Skerlec
| link = http://archives.datapages.com/data/specpubs/beaumont/ch10/ch10.htm
| pdf =
| store = http://store.aapg.org/detail.aspx?id=545
| isbn = 0-89181-602-X
}}
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==
The following figure 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 brittle rather than ductile deformation.
Both brittle sands and coals as well as ductile shales are incorporated in the fault zone. The mechanism of gouge formation is much more complex than a simple “smearing” of ductile shales into the fault zone. Although faults exhibit a complete spectrum of behavior from brittle to ductile, interbedded sand-shale sequences with strong ductility contrast commonly exhibit this transitional behavior.
[[file:evaluating-top-and-fault-seal_fig10-19.png|thumb|{{figure number|10-19}}See text for explanation.]]
Shale-rich gouge zones can create impermeable barriers. The following figure, 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.
[[file:evaluating-top-and-fault-seal_fig10-20.png|thumb|{{figure number|10-20}}See text for explanation.]]
Fault gouge can range from sand rich to shale rich. The following figure shows three fault zones exhibiting a spectrum of sand-shale ratios. The table below describes the position of the three faults, their sequences and characteristics, and their location.
{| class = "wikitable"
|-
! Position
! Sequence
! Characteristics
! Location
|-
| Left
| Sand–siltstone
| Gouge with high sand–shale ratio
| Tertiary, Isle of Wight, U.K.
|-
| Center
| Limestone–shale
| Gouge with intermediate “sand”–shale ratio
| Jurassic, Kilve, U.K.
|-
| Right
| Limestone–shale
| Gouge with low “sand”–shale ratio
| Jurassic, Kilve, U.K.
|}
[[file:evaluating-top-and-fault-seal_fig10-21.png|thumb|{{figure number|10-21}}See text for explanation.]]
==See also==
* [[Quantitative fault seal analysis]]
* [[Quantitative fault seal analysis]]
* [[Limitations of quantitative fault seal analysis]]
* [[Example of routine fault seal analysis: gulf coast]]
==External links==
{{search}}
* [http://archives.datapages.com/data/specpubs/beaumont/ch10/ch10.htm Original content in Datapages]
* [http://store.aapg.org/detail.aspx?id=545 Find the book in the AAPG Store]
[[Category:Predicting the occurrence of oil and gas traps]]
[[Category:Evaluating top and fault seal]]