Abstract

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 to date.

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.

The tectonics and sedimentation during the Phanerozoic created the source-rich setting of Iraq. Sedimentation was widespread across Iraq during most of the Paleozoic, although intervals of strata are absent and interpreted either not to have been deposited or to have been eroded following deposition. A notable example is the absence of Early and Middle Devonian strata from Iraq. The distribution of Pre-Permian strata, including the gas-prone Silurian source rocks, and therefore the Paleozoic hydrocarbon system and plays, was controlled in large part by erosion at the unconformity that separates Late and Early Carboniferous.

Following siliciclastic infill of the topography at this unconformity, Iraq was covered by platform-wide deposition of carbonate sediments on an east-dipping ramp in the Permian, Triassic, and Early Jurassic. Continental fragments rifted from the northern and eastern margins of Arabia during these times, after which a large portion of eastern Iraq subsided. This intra-shelf basin, named the Gotnia Basin, was the location of organic-rich “starved” sedimentation in the Middle to Late Jurassic that formed the important Sargelu and Naokelekan source rocks. The Gotnia Basin was rimmed by carbonate shelf margins.

The Gotnia Basin filled with anhydrite and salt layers in the latest Jurassic and then continued to fill throughout the Cretaceous, primarily by prograding carbonate and siliciclastic sediments that entered the basin along the southern, western, and northern margins. These progradational deposits contain the major reservoir units of Iraq, whereas the basinal sediments include most of the sealing facies, predominantly marine shales, as well as some locally important source rocks. The basinal area continued to contract by sedimentary infill until the Turonian, when the Arabian plate was disturbed by a compressional event related to the initial closing of the Tethys Ocean. This event is marked by a hiatus in deposition and, at least locally, erosion at an unconformity. Faults were inverted and folding of anticlines in the Mesopotamian foredeep (and probably in the Zagros foldbelt) occurred at this event.

Marine waters flooded across this unconformity in the Late Turonian and Coniacian, with renewed carbonate deposition. Ophiolites were obducted onto Arabia in the middle part of the Late Cretaceous and are present in far northeastern Iraq. At the same time, thick calcareous and muddy sediments filled rapidly subsiding extensional grabens present in western Iraq. A subsiding trough formed in the position of the existing Mesopotamian foredeep and Zagros foldbelt of Iraq, and siliciclastic sediments started to fill the deep-water trough from the north and northeast due to the encroaching Tethys closure. Basinal carbonate and shale were deposited in the center of the trough in the Paleocene and Eocene, while the basin was partly filled by prograding carbonate shelf margins located on the southwestern and northeastern sides of the basin. This continued through the Oligocene and Early Miocene, until the remaining basins were filled with anhydrite of the Dhiban Formation and the overlying carbonates of the Jeribe Formation in the Early Miocene. Deposition throughout the Late Cretaceous and Cenozoic provided the burial for the Mesozoic source rocks to generate oil and gas.

Closure of the Tethys Ocean continued, with local uplift and erosion of areas of Iraq in the Early Miocene. Thick evaporitic deposits of the Fatha Formation lap onto and over the older Cenozoic strata. The marine seaway along the Mesopotamian foredeep was closed by the Middle Miocene, with ensuing non-marine siliciclastic deposition and folding associated with west-verging shortening deformation in the Zagros foldbelt.

Oil and gas plays are proven and their distribution is well known for the Cenozoic and most of the Cretaceous. The deeper plays of the Lower Cretaceous, Jurassic, and older reservoirs are less well explored in the Mesopotamian foredeep, Gotnia Basin, and Kirkuk embayment of the Zagros foldbelt. Exploration is occurring in the marginally drilled interior of the Kurdistan region of the Zagros foldbelt in northeastern Iraq. Paleozoic plays in western Iraq are also marginally explored.