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Nearly all of the oil and gas occurs in fields located within the Mesopotamian foredeep, Gotnia Basin, and Zagros foldbelt. Minor discoveries and shows have been found on the Arabian platform along the western flank of the Mesopotamian foredeep. There is one gas discovery (Akkas field) on the Arabian platform in western Iraq.

Nearly all of the oil and gas occurs in fields located within the Mesopotamian foredeep, Gotnia Basin, and Zagros foldbelt. Minor discoveries and shows have been found on the Arabian platform along the western flank of the Mesopotamian foredeep. There is one gas discovery (Akkas field) on the Arabian platform in western Iraq.

Ninety-eight percent of the oil and gas occurs in reservoirs of Cenozoic and Cretaceous age. The largest reserves occur in: 1) carbonate rocks of the Kirkuk Group (Lower Miocene–Oligocene), in fields within the Zagros foldbelt of northeastern Iraq, the largest being Kirkuk field; 2) carbonate rocks of the Mishrif Formation (Turonian–Cenomanian), in fields within the Mesopotamian foredeep and Zagros foldbelt in southern and central Iraq, including Rumaila, West Qurna, Majnoon, Halfayah, Zubair, and Buzurgan fields; and 3) siliciclastic rocks of the Zubair Formation (Albian–Barremian), in fields within the Mesopotamian foredeep and Zagros foldbelt in southern and central Iraq, including East Baghdad, Rumaila, West Qurna, and Zubair fields. Large reserves also occur in carbonate rocks of the Upper Cretaceous above the Mishrif Formation and in the Lower Cretaceous below the Zubair Formation. Smaller reserves occur in other Neogene and Paleogene carbonates and siliciclastics, in Jurassic and Triassic carbonates, and in Ordovician siliciclastics.

Ninety-eight percent of the oil and gas occurs in reservoirs of Cenozoic and Cretaceous age. The largest reserves occur in: 1) [[carbonate rock]]s of the Kirkuk Group (Lower Miocene–Oligocene), in fields within the Zagros foldbelt of northeastern Iraq, the largest being Kirkuk field; 2) carbonate rocks of the Mishrif Formation (Turonian–Cenomanian), in fields within the Mesopotamian foredeep and Zagros [[foldbelt]] in southern and central Iraq, including Rumaila, West Qurna, Majnoon, Halfayah, Zubair, and Buzurgan fields; and 3) siliciclastic rocks of the Zubair Formation (Albian–Barremian), in fields within the Mesopotamian foredeep and Zagros foldbelt in southern and central Iraq, including East Baghdad, Rumaila, West Qurna, and Zubair fields. Large reserves also occur in carbonate rocks of the Upper Cretaceous above the Mishrif Formation and in the Lower Cretaceous below the Zubair Formation. Smaller reserves occur in other Neogene and Paleogene carbonates and [[siliciclastic]]s, in Jurassic and Triassic carbonates, and in Ordovician siliciclastics.

Most of the oil and gas that have been discovered were generated from organic-rich, oil-prone carbonates of the Jurassic Sargelu and Naokelekan Formations. These source rocks are widely distributed and mature for oil and gas generation across the Mesopotamian foredeep and Zagros foldbelt. Lesser amounts of oil and gas are derived from: 1) Upper and Lower Cretaceous oil-prone source rocks within the Zagros foldbelt; 2) Triassic oil-prone source rocks in northwestern Iraq; and 3) Silurian gas-prone source rocks in western Iraq. The oil generated from the Jurassic source rocks migrated vertically to fill stacked reservoir intervals in many fields. Lateral migration of oil occurred along the western margin of the Mesopotamian foredeep, as proven by small fields and large seeps that are located where source rocks are absent or immature for oil or gas generation.

Most of the oil and gas that have been discovered were generated from organic-rich, oil-prone carbonates of the Jurassic Sargelu and Naokelekan Formations. These source rocks are widely distributed and mature for oil and gas generation across the Mesopotamian foredeep and Zagros foldbelt. Lesser amounts of oil and gas are derived from: 1) Upper and Lower Cretaceous oil-prone source rocks within the Zagros foldbelt; 2) Triassic oil-prone source rocks in northwestern Iraq; and 3) Silurian gas-prone source rocks in western Iraq. The oil generated from the Jurassic source rocks migrated vertically to fill stacked reservoir intervals in many fields. Lateral migration of oil occurred along the western margin of the Mesopotamian foredeep, as proven by small fields and large seeps that are located where source rocks are absent or immature for oil or gas generation.

==Physiography==

==Physiography==

The southern and western parts of Iraq are part of the broader Arabian platform of adjacent Syria, Jordan, Saudi Arabia, and Kuwait ([[:file:M106Ch12Fig01.jpg|Figure 1]]). This is a sparsely populated area of desert and low hills, mostly 300–500 m (984–1640 ft) above sea level (msl). The highest elevation here occurs at Jabal ‘Unayzah, in the southwestern corner of the country, at 940 msl. Bedrock is exposed in these areas, largely Cenozoic but Mesozoic and Paleozoic in western Iraq.

The southern and western parts of Iraq are part of the broader Arabian platform of adjacent Syria, Jordan, Saudi Arabia, and Kuwait ([[:file:M106Ch12Fig01.jpg|Figure 1]]). This is a sparsely populated area of desert and low hills, mostly 300–500 m (984–1640 ft) above sea level (msl). The highest elevation here occurs at Jabal ‘Unayzah, in the southwestern corner of the country, at 940 msl. [[Bedrock]] is exposed in these areas, largely [[Cenozoic]] but Mesozoic and [[Paleozoic]] in western Iraq.

The land slopes down to the floodplains of the Al Furat (Euphrates) and Dijlah (Tigris) Rivers, which flow from Turkey and Syria through the length of Iraq to the Gulf of Arabia via the Shatt al Arab. The elevation at the Iraq–Syria border is 300–400 msl. Al Furat follows the Anah Graben until it reaches the subsiding Mesopotamian foredeep ([[:file:M106Ch12Fig01.jpg|Figure 1]]). The Dijlah flows around the uplifted Sinjar Graben to flow across the low portion of the Zagros foldbelt to the Mesopotamian foredeep. The floodplains are covered by alluvium, terrace gravels, and eolian sands. There are many towns and cities, including Baghdad, An Nasiriyah, and Basrah, in the floodplains of these rivers.

The land slopes down to the floodplains of the Al Furat (Euphrates) and Dijlah (Tigris) Rivers, which flow from Turkey and Syria through the length of Iraq to the Gulf of Arabia via the Shatt al Arab. The elevation at the Iraq–Syria border is 300–400 msl. Al Furat follows the Anah Graben until it reaches the subsiding Mesopotamian foredeep ([[:file:M106Ch12Fig01.jpg|Figure 1]]). The Dijlah flows around the uplifted Sinjar Graben to flow across the low portion of the Zagros foldbelt to the Mesopotamian foredeep. The floodplains are covered by alluvium, terrace gravels, and eolian sands. There are many towns and cities, including Baghdad, An Nasiriyah, and Basrah, in the floodplains of these rivers.

==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]].

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]].

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.

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.

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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.

The main part of the Zagros foldbelt is topographically lower and is characterized by large [[anticline]]s trending to the northwest. In the southwestern part of the foldbelt, synclinal areas are covered by Neogene [[sediment]]s. In the northwestern part of the foldbelt, [[Paleogene]] strata are exposed in [[syncline]]s. 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.

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.

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).]]

[[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).]]

==Oil and gas fields==

==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 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.

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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.

The major [[reservoir]]s 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.

{| class = "wikitable"

{| class = "wikitable"

Several conclusions can be drawn from these data:

Several conclusions can be drawn from these data:

* Iraq is an oil province, with a lot of oil generated in superb source rocks.

* Iraq is an oil province, with a lot of oil generated in superb [[source rock]]s.

* The Cretaceous and Cenozoic are the most important reservoir intervals, and there are fewer discoveries in Jurassic and older 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.

* Some accumulations are very large, with large trap and reservoir volumes, commonly in stacked reservoir-seal pairs.