View source for Niger Delta Basin (geology)

[[File:Niger Delta Basin Location.png|thumb|The Niger Delta Basin is located in the Gulf of Guinea on the west coast of Africa.]]

[[File:Niger Delta Geologic map.png|thumb|Geologic map of the Niger Delta Basin and the Benue trough, and the oil fields in the region.]]

The '''Niger Delta Basin''', also referred to as the '''Niger Delta province''',  is an extensional rift basin located in the [[Niger Delta]] and the [[Gulf of Guinea]] on the passive continental margin near the western coast of Nigeria<ref name="United States Geologic Survey">{{cite web|last1=Tuttle|first1=Michele|last2=Charpentier|first2=Ronald|last3=Brownfield|first3=Michael|title=The Niger Delta Petroleum System:  Niger Delta Province, Nigeria, Cameroon, and Equatorial Guinea, Africa|url=http://pubs.usgs.gov/of/1999/ofr-99-0050/OF99-50H/ChapterA.html|website=United States Geologic Survey|publisher=United States Geologic Survey|access-date=6 March 2015}}</ref> with suspected or proven access to Cameroon, Equatorial Guinea and São Tomé and Príncipe. This basin is very complex, and it carries high economic value as it contains a very productive [[petroleum reservoir|petroleum system]]. The Niger delta basin is one of the largest subaerial basins in Africa. It has a subaerial area of about 75,000&nbsp;km<sup>2</sup>, a total area of 300,000&nbsp;km<sup>2</sup>, and a sediment fill of 500,000&nbsp;km<sup>3</sup>.<ref name="United States Geologic Survey" /> The sediment fill has a depth between 9–12&nbsp;km.<ref name="Sequence stratigraphy of the Pliocene-Pleistocene strata and shelf-margin deltas of the eastern Niger Delta, Nigeria">{{Cite thesis |last=fatoke |first=oluwaseyi adedamola |title=sequence stratigraphy of the pliocene-pleistocene strata and shelf-margin deltas of the eastern niger delta, nigeria |type=ph.d.|year=2010 |publisher=university of houston}}</ref> It is composed of several different [[geologic formations]] that indicate how this basin could have formed, as well as the regional and large scale [[tectonics]] of the area. The Niger Delta Basin is an [[extensional tectonics|extensional basin]] surrounded by many other basins in the area that all formed from similar processes. The Niger Delta Basin lies in the south westernmost part of a larger tectonic structure, the [[Benue Trough]]. The other side of the basin is bounded by the [[Cameroon Volcanic Line]] and the transform passive continental margin.<ref name="Sequence stratigraphy of the Pliocene-Pleistocene strata and shelf-margin deltas of the eastern Niger Delta, Nigeria" />

==Basin formation==
The Niger Delta Basin was formed by a failed rift [[triple junction|junction]] during the separation of the [[South American Plate]] and the [[African Plate]], as the South Atlantic began to open. Rifting in this basin started in the late [[Jurassic]] and ended in the mid [[Cretaceous]]. As rifting continued, several [[Fault (geology)|faults]] formed many of them [[thrust faults]]. Also at this time syn-rift sands and then shales were deposited in the late Cretaceous. This indicates that the shoreline regressed during this time. Concurrently, the basin had been undergoing extension resulting in high angle normal faults and fault block rotation. At the beginning of the Paleocene there was a significant shoreline transgression.<ref name="Sequence stratigraphy of the Pliocene-Pleistocene strata and shelf-margin deltas of the eastern Niger Delta, Nigeria" /> During the Paleocene, the [[Akata Formation]] was deposited, followed by the [[Agbada Formation]] during the Eocene. This loading caused the underlying shale [[Akata Formation]] to be squeezed into shale diapirs. Then in the Oligocene the [[Benin formation]] was deposited, which is still being deposited today. The overall basin is divided into a few different zones due to its tectonic structure.<ref name="United States Geologic Survey" /> There is an extensional zone, which lies on the continental shelf, caused by the thickened crust. Moving basinward is a transition zone, and a contraction zone, which lies in the deep-sea part of the basin.

==Lithology==
[[File:Niger Delta Sealevel Stand.PNG|thumb|Sealevel highstand and lowstand mapped on a crossection of the basin]]

The sediment fill in the Niger Delta basin is characterized by three major depobelts. These three cycles show that the basin experienced an overall [[marine regression|regression]] throughout time as the sediments go from deep sea mud sized grains to [[fluvial]] denser sand sized grains. The [[lithologies]] of the area experience changes due to several factors. The sediment [[provenance]] from the onshore highlands which feed into the [[river delta|delta]] control the mineralogy of the grains. Additionally, the impact of sea level on sediment deposition is well known; relative sea level will control the basinward extent of lithologies (see [[sequence stratigraphy]]). [[Volcanic]] activity in the area may also result in thin deposits of ash ([[bentonite]]). The early Cretaceous sediments are thought to be from a [[tide]]-dominated system that were deposited on a concave shoreline, and throughout time the shoreline became [[wikt:convex|convex]] and it is currently a [[wave]]-dominated system.

;Basement
The oceanic basement rock is the oldest rock in the basin and is [[basaltic]] in composition. Also closer to the coast [[Precambrian]] continental basement [[crops]] out onshore.

;Cretaceous
There is a section of rock in this basin from the middle to late Cretaceous which is poorly understood due to its significant burial depth. It is believed to be composed of sediments from a tide-dominated coastline,<ref name="United States Geologic Survey" /> and there are believed to be several layers of shales, although their distribution is unconstrained.

;Akata Formation 
The Akata Formation is [[Paleocene]] in age. It is composed of thick shales, turbidite sands, and small amounts of silt and clay. The clay content resulted in it being a [[ductile]] shale formation which was squeezed into shale diapirs in the basin. The Akata Formation formed during [[lowstands]] in relative sea level and [[Anoxic waters|anoxic]] conditions. This formation is estimated to be up to 7,000 meters thick.<ref name="United States Geologic Survey" />

;Agbada Formation 
The Agbada Formation dates back to [[Eocene]] in age. It is a marine facies defined by both freshwater and deep sea characteristics. This is the major [[oil]] and [[natural gas]]-bearing facies in the basin. The hydrocarbons in this layer formed when this layer of rock became subaerial and was covered in a [[marsh]]-type environment rich in [[Organic matter|organic]] content. It is estimated to be 3,700 meters thick.<ref name="United States Geologic Survey" />

;Benin Formation 
The Benin Formation is [[Oligocene]] and younger in age. It is composed of continental flood plain sands and alluvial deposits. It is estimated to be up to 2,000 meters thick.<ref name="United States Geologic Survey" />

==Tectonic structures==
The tectonic structures in the Niger Delta Basin are typical of an extensional rift system, but the added shale [[diapirism]] due to loading makes this basin unique. The main method of deformation is gravitational collapse of the basin, although the older faulting and deformation in the basin are related to the continental breakup and rifting of the African plate and South American plates.<ref name="United States Geologic Survey" /> The overall basin is divided into a few different zones due to its tectonic structure. There is an extensional zone, which lies on the continental shelf, that is caused by the thickened crust. There is a transition zone, and then there is a contraction zone, which lies in the deep sea part of the basin.

;Basin inversion
Basin inversion is caused by uplift and/or compression in this basin. The compression is caused by the toe detachment of the shale diapirs. Basin inversion forms anticline structures, which serve as a great oil trap. Clay smears in the sediments seal the formations so oil does not escape out.<ref name="United States Geologic Survey" />

;Basinward dipping reflectors
Basinward dipping reflectors are a common feature of extensional type rift basins. As fault blocks extend they rotate to dip towards the center of the basin. At the top of these fault blocks sub basins can form.

;Shale diapirs
The shale diapirs are from the Akata Formation. This structure is formed due to the improper dehydration of the formation and the over pressuring by the overlying and denser Agbada Formation.<ref name="United States Geologic Survey" />

;High-angle normal faulting
High-angle [[normal fault]]ing is a feature of the extensional portion of the rifting in this basin. It is considered a growth fault and the feature lies closer towards the basins edge and transitions to the toe detachment faulting as you continue down the basin.<ref name=AAPG>{{cite journal|last1=Short|first1=K.C.|last2=Staeuble|first2=A.J.|title=Outline of geology of Niger delta|journal=AAPG Bulletin|volume=51|issue=5|pages=761–799|doi=10.1306/5d25c0cf-16c1-11d7-8645000102c1865d }}</ref>

==Oil and Gas==
[[File:Nigerdeltapetroleumsys.gif|thumb|Minimum and maximum petroleum system]]

The Niger Delta is the twelfth largest province in the world by known oil and gas resources.<ref name="USGs1999">{{cite web |last1=Tuttle |first1=Michele L. W. |last2=Brownfield |first2=Michael E. |last3=Charpentier |first3=Ronald R. |title=Niger Delta (Akata-Agbada) Petroleum System OF99-50H, Chapter A |url=https://pubs.usgs.gov/of/1999/ofr-99-0050/OF99-50H/ChapterA.html#:~:text=Known%20oil%20and%20gas%20resources,recoverable%20gas%20have%20been%20discovered. |website=pubs.usgs.gov |access-date=12 August 2021}}</ref> The Niger Delta Basin produces around 2 million barrels of oil per day. The entire system is predicted to contain 34.5 billion barrels of oil and 94 trillion feet<sup>3</sup> of natural gas. This area is still very heavily explored by oil companies today. It is one of the largest oil producers in the world.<ref name="United States Geologic Survey" />

===Oil Pollution===
{{Excerpt|Environmental issues in the Niger Delta}}
In July 1979, an incident  on the coastal zone of the Forcado tank 6 terminal spilled 570 thousand barrels of oil into the Forcados estuary, affecting the sea species and surrounding swamp forest.<ref name="USGs1999"/> In January 1980, the Funiwa No.5 Well in Funiwa Field spilled 421 thousand barrels of oil into the ocean, damaging 836 acres of mangrove forest.<ref name="USGs1999"/> In 1983, 5 000 barrels were spilled causing high mortality in crabs, fish and shrimps.<ref name="USGs1999"/> Until 1999, an average of 240 thousand barrels of crude oil was spilled in the zone every year.<ref name="USGs1999"/>

== See also ==
{{Portal|Nigeria|Energy}}
* [[Bonga Field]]
* [[Cameroon line]]

==References==
{{Reflist}}

== External links ==
*[http://geology.cr.usgs.gov/energy/WorldEnergy/OF99-50H/ The Niger Delta Petroleum System:  Niger Delta Province, Nigeria, Cameroon, and Equatorial Guinea, Africa], [[United States Geological Survey]], 1999
*[https://web.archive.org/web/20030810165603/http://www.nigeriasaotomejda.com/Pages/geological%20-summary.html Regional Geology and Petroleum Prospectivity], Nigeria - São Tomé and Príncipe Joint Development Zone (JDZ)

{{Major African geological formations|state=collapsed}}

{{coord missing|Nigeria}}

[[Category:Basins of Africa]]
[[Category:Gulf of Guinea]]
[[Category:Niger River Delta]]
[[Category:Geology of Cameroon]]
[[Category:Geology of Equatorial Guinea]]
[[Category:Geology of Nigeria]]
[[Category:Geology of São Tomé and Príncipe]]
[[Category:Geologic provinces]]
[[Category:Oil fields of Africa]]
[[Category:Oil fields of Nigeria]]
[[Category:Geography of Cameroon]]
[[Category:Geography of Equatorial Guinea]]
[[Category:Geography of Nigeria]]
[[Category:Geography of São Tomé and Príncipe]]
[[Category:Sedimentary basins of Africa]]
[[Category:Mesozoic rifts and grabens]]