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==Tectonic setting==
==Tectonic setting==
Iraq is located in the northern part of the Arabian tectonic plate. The basement rocks are presumed to be accretionary rocks assembled during the Late Precambrian Pan-African orogeny, although no wells reach the basement rock in Iraq. Bouguer gravity values are low in the northeast, presumably due to the extreme thickness of sedimentary rocks ([[:file:M106Ch12Fig02.jpg|Figure 2]]), and become progressively higher to the west onto the Khleisia High (Rutbah subzone of Jassim and Goff<ref name=Jassimandgoff_2006 />) along the Iraq–Syria border. North- and northwest-trending basement grains are interpreted from gravity and magnetic data in southern, western, and northwestern Iraq as shown in [[:file:M106Ch12Fig02.jpg|Figure 2]].
[[file:M106Ch12Fig03.jpg|thumb|300px|{{figure number|3}}Map of tectonic provinces of Iraq, showing major structural features. Fields and significant wells with oil and/or gas shows are shown; numbers refer to Tables 1–5. Red lines are approximate locations of stratigraphic charts in [[:file:M106Ch12Fig04.jpg|Figure 4]] and [[:file:M106Ch12Fig05.jpg|Figure 5]].]]
Northeastern Iraq, contiguous with Iran and Turkey, is part of the Zagros foldbelt, consisting of two zones ([[:file:M106Ch12Fig03.jpg|Figure 3]]). The High Zagros Nappe zone is a topographically high region characterized by tightly folded rocks and by ophiolite which was obducted onto the Arabian plate in the Late Cretaceous ([[:file:M106Ch12Fig04.jpg|Figure 4]]). Cretaceous and older sedimentary rocks and a suite of igneous and metamorphic rocks crop out in this region ([[:file:M106Ch12Fig05.jpg|Figure 5]]). There are no oil and gas discoveries in the High Zagros Nappe zone, although rocks containing bitumen and organic-rich rocks are exposed.
[[file:M106Ch12Fig04.jpg|thumb|300px|{{figure number|4}}Regional geologic cross section, which extends NE across central Iraq, from the Arabian platform of southwestern Iraq, at the border with Saudi Arabia, across the Mesopotamian Foredeep and Zagros foldbelt in eastern Iraq and adjacent Iran, and ending at the Sirjan-Sanandaj zone in Iran.]]
The main part of the Zagros foldbelt is topographically lower and is characterized by large anticlines trending to the northwest. In the southwestern part of the foldbelt, synclinal areas are covered by Neogene sediments. In the northwestern part of the foldbelt, Paleogene strata are exposed in synclines. The Zagros foldbelt forms a large structural embayment, called the Kirkuk embayment, in the overall Zagros foldbelt. Many oil and gas fields occur within this zone, as shown in [[:file:M106Ch12Fig03.jpg|Figure 3]], including the super-giant Kirkuk field.
[[file:M106Ch12Fig05.jpg|thumb|300px|{{figure number|5}}Regional geologic cross section which extends NE across western Iraq, from the Arabian platform at the border with Jordan, across the Mesopotamian Foredeep and Zagros foldbelt in northwestern Iraq, and ending in adjacent Turkey.]]
The central portion of Iraq is the Mesopotamian foredeep, an area of relatively deep-water deposition in the Mesozoic and Cenozoic that formed in front of, and which is less deformed than, the Zagros foldbelt ([[:file:M106Ch12Fig06.jpg|Figure 6]]). Structural features trend to the northwest in the Mesopotamian foredeep, parallel to the Zagros foldbelt trends, and are formed by both detached and basement-involved faults ([[:file:M106Ch12Fig04.jpg|Figure 4]], [[:file:M106Ch12Fig06.jpg|Figure 6]]). Bouguer-gravity values are low in this foredeep ([[:file:M106Ch12Fig02.jpg|Figure 2]]), reflecting the thick sedimentary section. There are many oil and gas fields in the Mesopotamian foredeep ([[:file:M106Ch12Fig03.jpg|Figure 3]]), including the large East Baghdad field.
[[file:M106Ch12Fig06.jpg|thumb|300px|{{figure number|6}}Interpretation from megaseismic line 7 (reproduced from Mohammed,<ref name=Mohammed_2007>Mohammed, S. A. G., 2006, Megaseismic section across the northeastern slope of the Arabian plate, Iraq: GeoArabia, v. 11, no. 4, p. 77–102.</ref> by permission from GeoArabia), which extends from the Arabian platform of southwestern Iraq across the Mesopotamian Foredeep to the Zagros foldbelt in eastern Iraq. 50 km (31.1 mi).]]
The southern part of the Mesopotamian foredeep is contiguous with the northern extension of the Gotnia Basin, an area of subsidence and deep-shelfal salt deposition during the Jurassic that extends into adjacent Kuwait. The Gotnia Basin (called the Zubair zone by Jassim and Goff<ref name=Jassimandgoff_2006 />) is characterized by broad, low-angle anticlines that trend and plunge to the north. These anticlines form many oil and gas fields, including the super-giant Rumaila, West Qurna, Zubair, and Majnoon fields ([[:file:M106Ch12Fig03.jpg|Figure 3]]). Values of both Bouguer gravity and total magnetics are low ([[:file:M106Ch12Fig02.jpg|Figure 2]]), indicating a thick sedimentary sequence with a non-magnetic basement.
The Zagros foldbelt and Mesopotamian foredeep end at the Khleisia High (Rutbah subzone of Jassim and Goff<ref name=Jassimandgoff_2006 />), a region of shallow basement and thin sedimentary cover, as interpreted from the high values of the Bouguer gravity map ([[:file:M106Ch12Fig02.jpg|Figure 2]]) and from the deep Khleisia 1 well which drilled to the Ordovician ([[:file:M106Ch12Fig05.jpg|Figure 5]]). Several east-trending extensional grabens that formed in the Late Cretaceous cut across and form the northern and southern boundaries of the Khleisia High ([[:file:M106Ch12Fig03.jpg|Figure 3]]). These include the Anah and Ashtar Grabens to the south ([[:file:M106Ch12Fig05.jpg|Figure 5]]), which appear as relatively low in the total magnetics map ([[:file:M106Ch12Fig02.jpg|Figure 2]]), and the inverted Sinjar Graben to the north, which is a low on Bouguer gravity. There are only a few small fields in northwestern Iraq ([[:file:M106Ch12Fig03.jpg|Figure 3]]).
Western and southern Iraq is part of the Arabian platform, a tectonically stable area which is only mildly deformed ([[:file:M106Ch12Fig04.jpg|Figure 4]]). The Ga’ara uplift in western Iraq is a large basement-involved high area where erosion has exposed Paleozoic strata. There are some small oil and gas discoveries on the Arabian platform in Iraq, mostly along the eastern edge, which is a structural monocline that flexes downward into the Mesopotamian foredeep ([[:file:M106Ch12Fig03.jpg|Figure 3]]). The most significant discovery is the Akkas field, located south of the Anah Graben, which contains gas in Paleozoic rocks.
==Oil and gas fields==
Table 1 lists the discoveries in Iraq (information in part from Jassim and Goff<ref name=Jassimandgoff_2006 />; Verma et al.<ref name=Vermaetal_2004>Verma, M. K., Ahlbrandt, T. S., and Al-Gailani, M., 2004, Petroleum reserves and undiscovered resources in the total petroleum systems of Iraq: Reserve growth and production implications: GeoArabia, v. 9, no. 3, p. 51–74.</ref>), along with the hydrocarbon type and age of the main reservoir. The numbers refer to the locations of the fields as shown in [[:file:M106Ch12Fig03.jpg|Figure 3]]. Table 2 lists the fields by hydrocarbon type, structural province, structural style and trap type, source name, age and strata type, and volumes of in-place, estimated ultimate recoverable (EUR), produced, and remaining ultimate recoverable (RUR) oil and gas. The volumes data for oil and gas come mainly from Verma et al.<ref name=Vermaetal_2004 /> and Jassim and Goff,<ref name=Jassimandgoff_2006 /> except as noted in other references in this article.
Table 3 lists the reservoir horizons and seals in fields that have produced oil and gas. The same information is shown for undeveloped fields in Table 4 and for small discoveries and wells with shows in Table 6.
There are 107 oil discoveries and nine gas discoveries in Iraq. The majority of these are in the Zagros foldbelt (53), the Mesopotamian foredeep (22), and the Gotnia Basin (18), and they account for over 98% of the oil and gas discovered.
The oil discoveries contain recoverable reserves of about 137 billion barrels of oil (gbo) and 93 trillion cubic feet of gas (tcfg). The largest oil fields are Rumaila (30 gbo), Kirkuk (25 gbo), and East Baghdad (16 gbo). Oil (and minor condensate) is about 89% of the per-barrel oil equivalent in Iraq.
The nine gas discoveries contain recoverable reserves of about 13 tcfg and 280 million barrels of liquid (mbl). The largest gas fields are Mansuriya (3.3 tcfg) and Akkas (2.5 tcfg).
To date, about 25 gbo and 11 tcfg have been produced from about 23 fields. The most has been from the Kirkuk (about 14 gbo) and Rumaila (about 8 gbo) fields.
The major reservoirs in Iraq are Cretaceous, and these are the main reservoirs in 69 discoveries. The recoverable reserves in Cretaceous reservoirs in all fields are estimated to be between 59 and 101 gbo and 15 to 55 tcfg. Similar, there are 32 discoveries with mainly Cenozoic reservoirs, with 24–46 gbo and 14–41 tcfg recoverable in all Cenozoic reservoirs. Only 10 discoveries are mainly in Triassic reservoirs, with recoverable reserves of 400–500 mbo and up to 6 tcfg, and three discoveries mainly from Jurassic reservoirs, with 580–1500 mbo and no gas. Estimates of up to 6 tcfg of gas occur in the Paleozoic at Akkas field, with shows in Khleisia 1.
Several conclusions can be drawn from these data:
* Iraq is an oil province, with a lot of oil generated in superb source rocks.
* The Cretaceous and Cenozoic are the most important reservoir intervals, and there are fewer discoveries in Jurassic and older rocks.
* Some accumulations are very large, with large trap and reservoir volumes, commonly in stacked reservoir-seal pairs.
==See also==
==See also==