Iraq petroleum geology

Petroleum systems of the Tethyan region
Series	AAPG Memoir
Chapter	Iraq
Author	George J. Grabowski Jr.
Link	Web page
Store	AAPG Store

There are more than 100 oil and gas fields in Iraq, containing more than 137 billion barrels of recoverable oil and more than 106 TCF of recoverable gas. Of this large resource, about 25 billion barrels of oil and 11 TCF of gas have been produced as of 2014.

Introduction

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.

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.

Figure 1  Topographic map of Iraq, showing major cities and rivers, along with major oil and gas fields. Points of high elevation are indicated.

Physiography

The southern and western parts of Iraq are part of the broader Arabian platform of adjacent Syria, Jordan, Saudi Arabia, and Kuwait (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 (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.

Elevations rise in northeastern Iraq. Most of the Kirkuk embayment of the Zagros foldbelt is covered by Neogene sediments, except for elongated northwest-trending anticlines that expose older strata. Surface elevations rise up to 850 m (2788 ft) on the Qara Chauq Dagh anticline (Figure 1). There are many towns and cities in this region, including Mosul and Kirkuk.

The High Zagros Nappe zone further to the northeast is characterized by large anticlinal mountains that expose Cenozoic to Paleozoic sediments as well as igneous and metamorphic rocks. Elevations are higher across this region, reaching 3586 m (11765 ft) on the Iraq-Iran border east of Rowandaz (Figure 1). Major gorges cut by rivers, such as the Zab River that runs into the Dijlah River near Jabal Qara Chauq Dagh, form exposures that are the type sections for many formations. One effect of the surface relief of Iraq is the flow of groundwater deeply within the sediments of the Mesopotamian foredeep, from the somewhat higher Arabian platform in western and southern Iraq, as well as within synclines in the Kirkuk embayment of the Zagros foldbelt. Freshwater extends far into the subsurface because of these flows.

Figure 2 Bouguer gravity (right) and total magnetic field (left) maps of Iraq, from Jassim and Goff, compiled by GEOSURV. The gravity map shows low values in the Zagros foldbelt and Mesopotamian foredeep due to the thick sedimentary section. Orientations of features trend to the north and northwest in southern, western, and northwestern Iraq, interpreted in part to be caused by the structural grain of Precambrian accreted terrains. 200 km (124.3 mi).

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 (Figure 2), and become progressively higher to the west onto the Khleisia High (Rutbah subzone of Jassim and Goff) 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 Figure 2.

Figure 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 Figure 4 and Figure 5.

Northeastern Iraq, contiguous with Iran and Turkey, is part of the Zagros foldbelt, consisting of two zones (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 (Figure 4). Cretaceous and older sedimentary rocks and a suite of igneous and metamorphic rocks crop out in this region (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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  Figure 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.

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  Figure 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 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 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 (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 (Figure 4, Figure 6). Bouguer-gravity values are low in this foredeep (Figure 2), reflecting the thick sedimentary section. There are many oil and gas fields in the Mesopotamian foredeep (Figure 3), including the large East Baghdad field.

Figure 6 Interpretation from megaseismic line 7 (reproduced from Mohammed, 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) 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 (Figure 3). Values of both Bouguer gravity and total magnetics are low (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), a region of shallow basement and thin sedimentary cover, as interpreted from the high values of the Bouguer gravity map (Figure 2) and from the deep Khleisia 1 well which drilled to the Ordovician (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 (Figure 3). These include the Anah and Ashtar Grabens to the south (Figure 5), which appear as relatively low in the total magnetics map (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 (Figure 3).

Western and southern Iraq is part of the Arabian platform, a tectonically stable area which is only mildly deformed (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 (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; Verma et al.), along with the hydrocarbon type and age of the main reservoir. The numbers refer to the locations of the fields as shown in 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. and Jassim and Goff, except as noted in other references in this article.

Table 1 List of oil and gas fields and wells with oil and/or gas shows, giving the dominant hydrocarbon type (oil or gas) and reservoir age. Field number refers to locations shown in Figure 3

Field #	Field name	Hydrocarbon type	Age of main reservoirs	Field #	Field name	Hydrocarbon type	Age of main reservoirs	Field #	Field name	Hydrocarbon type	Age of main reservoirs	Field #	Field name	Hydrocarbon type	Age of main reservoirs
1	Tawake	Oil	Cretaceous	31	Chemchemal	Gas	Triassic	61	Tel Ghazal	Gas	Cenozoic	91	Nasiriyah	Oil	Cretaceous
2	Sufaiyah	Oil	Cretaceous	32	Kirkuk	Oil	Cenozoic	62	Balad	Oil	Cretaceous	92	Samawa	Oil	Cretaceous
3	Mushorah	Oil	Cretaceous	33	Ismail	Oil	Cenozoic	63	East Baghdad	Oil	Cretaceous	93	Diwan	Oil	Cretaceous
4	Boheira	Oil	Cretaceous	34	Bai Hassan	Oil	Cenozoic	64	Nahrawan	Oil	Cretaceous	94	Ghalaisan	Oil	Cretaceous
5	Abtakh	Oil	Cretaceous	35	Qara Chauq	Oil	Triassic	65	Fallujah	Oil	Cretaceous	95	Shawiyah	Oil	Cretaceous
6	Ain Zalah	Oil	Cretaceous	36	Khabbaz	Oil	Cenozoic	66	Nafatah	Oil	Cretaceous	96	Safawi	Oil	Cretaceous
7	Gullar	Oil	Cretaceous	37	Sadid	Oil	Cenozoic	67	Hit	Oil	Cenozoic	97	Ubaid	Oil	Cretaceous
8	Gusair	Oil	Cretaceous	38	Khanuqah	Oil	Cenozoic	68	Awasil	Oil	Cretaceous	98	Majnoon	Oil	Cretaceous
9	Jabal Kand	Oil	Cretaceous	39	Makhul	Oil	Jurassic	69	Abu Jir	Oil	Cretaceous	99	Nahr Umr/Bin Umar	Oil
10	Raffan	Oil	Jurassic	40	Kor Mor	Gas	Cenozoic	70	Badrah	Oil	Cretaceous	100	Sindbad	Oil	Cenozoic
11	Butmah	Oil	Triassic	41	Jambur	Oil	Cenozoic	71	Dhafriya	Oil	Cretaceous	101	Siba	Oil	Cretaceous
12	Alan	Oil	Triassic	42	Pulkhana	Oil	Cenozoic	72	Ahdab	Oil	Cretaceous	102	Umm Qasr	Oil	Cretaceous
13	Sarjoon	Oil	Cretaceous	43	Judaida	Oil	Cenozoic	73	Afaq	Oil	Cretaceous	103	Zubair	Oil	Cretaceous
14	Ain Ghazal	Oil	Cenozoic	44	Hamrin	Oil	Cenozoic	74	Musaiyib	Oil	Cretaceous	104	Jabal Sanam	Oil	Cretaceous
15	Tel Hajar	Oil	Cenozoic	45	Ajil	Oil	Cenozoic	75	Kifl	Oil	Cretaceous	105	Tuba	Oil	Cretaceous
16	Ibrahim	Oil	Triassic	46	Tikrit	Oil	Cretaceous	76	West Kifl	Oil	Cretaceous	106	West Qurna	Oil	Cretaceous
17	Atshan	Oil	Triassic	47	Samarra	Oil	Cretaceous	77	Merjan	Oil	Cretaceous	107	Rumaila	Oil	Cretaceous
18	Adaiyah	Oil	Triassic	48	Mileh Tharthar	Oil	Triassic	78	Abu Ghirab	Oil	Cenozoic	108	Ratawi	Oil	Cretaceous
19	Qalian	Oil	Triassic	49	Khleisia	Gas	Paleozoic	79	Jabal Fauqi	Oil	Cenozoic	109	Subba	Oil	Cretaceous
20	Qasab	Oil	Cenozoic	50	Anah	Oil	Triassic	80	Buzurgan	Oil	Cretaceous	110	West Luhais	Oil	Cretaceous
21	Jawan	Oil	Cenozoic	51	Akkas	Gas	Paleozoic	81	Noor	Oil	Cretaceous	111	Luhais	Oil	Cretaceous
22	Najmah	Oil	Cenozoic	52	Chia Surkh	Oil	Cenozoic	82	Huwaiza	Oil	Cretaceous	112	Rachi	Oil	Cretaceous
23	Qaiyarah	Oil	Cenozoic	53	Qamar	Oil	Cenozoic	83	Halfayah	Oil	Cretaceous	113	Jerishan	Oil	Cretaceous
24	Hibbarah	Oil	Cenozoic	54	Gilabat	Oil	Cenozoic	84	Amara	Oil	Cretaceous	114	Khidr Al Maa	Oil	Cretaceous
25	Makhmour	Oil	Cretaceous	55	Injana	Oil	Cretaceous	85	Rifae	Oil	Cretaceous	115	Abu Kheima	Oil	Cretaceous
26	Hawler (Benenan)	Oil	Jurassic	56	Khashm Al Ahmar	Gas	Cenozoic	86	Muhainya	Oil	Cretaceous	116	Shaikan	Oil	Jurassic
27	Demirdagh	Oil	Cretaceous	57	Jaria Pika	Gas	Cenozoic	87	Kumait	Oil	Cretaceous	117	Miran West	Oil	Cretaceous
28	Quwair	Oil	Cenozoic	58	Nau Doman	Oil	Cenozoic	88	Dujaila	Oil	Cretaceous	118	Kurdamir	Gas	Cenozoic
29	Binabawi	Gas	Triassic	59	Naft Khaneh	Oil	Cenozoic	89	Rafidain	Oil	Cretaceous	119	Shakal	Oil	Cenozoic
30	Taq Taq	Oil	Cretaceous	60	Mansuriya	Gas	Cenozoic	90	Gharraf	Oil	Cretaceous	120	Bijeel	Oil	Jurassic
												121	Bazian	Oil	Cretaceous

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.

Table 2 List of oil and gas fields and wells with oil and/or gas shows. Field number refers to locations shown in Figure 3. Hydrocarbon type: oil, gas, or oil and gas. Structural province refers to regions shown in Figure 3. Structural style of the trap: basement-involved fold, detached fold-thrust, or salt-supported dome. Trap type: anticline, faulted anticline, faulted dome, or fault trap. Source name and age given only where proven by biomarkers. Source strata type: basinal marine (includes calcareous and marly lithologies) or basinal shale. Volumes of oil and gas in-place, estimated ultimate recoverable (EUR), produced, and remaining ultimate recoverable (RUR) age given in millions of barrels of oil (MMBO) and billions of cubic feet of gas (BCFG). Condensate included in oil volumes. Information mainly from Verma et al. and Jassim and Goff. Additional references as shown. Comments reflect the impression of the author.

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

• Phanerozoic Tethys region
• Israel petroleum systems
• Libya hydrocarbon provinces
• Tethys region
• Jordan petroleum geology

External links

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estimated ultimately recoverable