IRAQ - The Geology.

Iraq embraces the north-western portion of the prolific Arabian Basin and the Zagros Fold Belt. From south-west to north-east, the thick post-Precambrian sediments of the Arabian Basin dip gently off the Arabian Shield, beneath the plains of the Tigris and Euphrates rivers, and later become involved in the strongly folded Zagros Fold Belt.

Close to the Iranian border in the north-east, the land rises to the complex Zagros Thrust Zone. Structural trends are similar to those seen in the Arabian Peninsula and in Iran, i.e., in the Zagros foothills the trend is north-west/south-east. The folds frequently have surface expression.

In the southern marshes of the country lies Majnoon, a super-giant oilfield found in 1975 by a unit of Petrobras with more than 30 bn barrels of oil in place. At least 11 bn barrels are recoverable at low cost. Partly developed, Majnoon can be brought on stream within 26 months of concentrated work for an initial capacity of 450,000 b/d which will rise to 600,000 b/d later and could eventually reach up to 3m b/d. Later it was found that Majnoon was part of the Great Rumaila Triangle.

Majnoon was first estimated to have 7 bn barrels with the oil's quality varying from heavy to medium according to several pay zones. Heavier oil in deeper zones were in 1990 put at 23-25 bn barrels. Located in the Hawizah marshes, mostly under man-made islands close to Iran's border north of Basra, Majnoon extends 60 km inland. Its development was held up in the 1980s by a flooding of the area to prevent an Iranian invasion. Most of it was occupied by Iran for some time. On the Iranian side, NIOC has discovered an extension of Majnoon with a huge oil reserve. Majnoon's first pay zone is the Hartha (Upper Cretaceous), close to the surface, which in 1976 tested 4,000 b/d of gas-rich oil. Reservoir pressure was so high that the rig was destroyed. Other pay zones include Mishrif carbonates, Nahr Umr sands (Middle Cretaceous), and Shuaiba carbonates and Zubair sands (Lower Cretaceous).

In late 2002 Iraqi geologists announced an exploration programme to cover the Cainozoic and Mesozoic, to be followed by work on older zones down to the Palaeozoic and Permian Khuff - judging by big discoveries in those horizons made in Iran, Saudi Arabia, etc. This was to determine the full potential of the Greater Rumaila Triangle.

By September 1980, when the war with Iran broke out, Braspetro had reached the engineering phase for production facilities at Majnoon. Before work was suspended, it had 14 drilling rigs in operation at Majnoon and at the nearby Nahr Umr, a giant. It had drilled more than 20 exploration wells in Majnoon. One well had penetrated 14 oil-bearing zones. Before March 2003, the state-owned South Oil Co. (SOC) was producing 50,000 b/d from a part of Majnoon which it had developed.

Along Iraq's southern border with Iran and the nearby offshore, new prospects yet to be explored include several extremely important zones. On the Iranian side of the border there, super-giant oilfields have been found in recent years. Most lie in the oil-rich Abadan Plain Basin, a huge onshore-offshore structure near Abadan. The super-giants there include Azadegan (26 bn barrels in place), Dasht-e Abadan (26 bn bls) and Ferdowsi (about 30.6 bn bls). The latest find there is Yadavaran with a potential to produce up to 400,000 b/d (see gmt14IranGeoApr2-07 and gmt15IranFieldsApr9-07).

In the far north-west of the country, close to the Turkish and Syrian borders, the Zagros trend swings to a westerly direction. This is reflected in the orientation of the fields in north-western Iraq and north-eastern Syria. Particularly in areas bordering Syria, prospects under study since 1989 have pointed to the possibility of high quality oil in source rocks extending from the Mesopotamian foredeep. There is one potentially large oilfield in a border area adjacent to Deir El Zor (Syria). The part explored on the Syrian side appears to be a small extension of a large reservoir, probably a giant, on the Iraqi side containing high quality oil.

To the south of Iraq, the structural orientation is more north-south. It parallels trends seen in Kuwait and Saudi Arabia. Folds have little surface expression and have largely been located by seismology. Rumeila, a super-giant oilfield about 80 km long, extends some 3.6 km into Kuwait and was one cause of Iraq's August 1990 invasion of the emirate.

Details of the deep geology in Iraq are lacking. It is assumed that the north-south oriented folds of southern Iraq owe their existence to deep-seated salt structuration similar to that postulated for many of the fields in the Arabian Peninsula. In contrast, the north-western/south-eastern folds to the north are associated with a complex structure developed in front of the Zagros Thrust Belt and were formed by the north-easterly vergence of the Arabian Plate against the Iranian (Asian) Plate.

The habitat of hydrocarbons is a paradigm for the whole Arabian-Iranian region to the south and south-east. Oil occurs in Miocene and older limestones in the Zagros Fold Belt and is frequently capped by younger evaporites. Porosity and permeability of the carbonate reservoirs are often enhanced in the folded structures by fracturing, a similar pattern to that seen in Iran. In the most northerly fields hydrocarbons have been found in older Mesozoic limestones, e.g. within the Triassic and the Jurassic.

The first commercial oil in the Middle East was found in Iran in 1908, at Masjid-i-Suleiman, in an Oligo-Miocene Asmari limestone formation (Fm). It was not until 1927 that a similar discovery was made in Iraq in an equivalent Fm at Kirkuk. It was then established that the main Tertiary hydrocarbons of the basin in Iraq, Iran and Saudi Arabia came from the Oligo-Miocene carbonate Fm of south-western Iran and its Iraq extensions, sealed under Lower Fars evaporites. In Kirkuk, production also comes from an older Eo-Paleocene carbonate reservoir.

Equally important is the view that the Oligo-Miocene oil of Kirkuk came into place by vertical migration from earlier pre-Zagros pools in the underlying Middle and Lower Cretaceous reservoirs, with fracture-type pools in some cases left "en route" in Upper Cretaceous Fms.

The heavy Asmari and Upper Cretaceous oils of the Mosul region had their origin in an Upper Jurassic source, and came into place by a combination of lateral and vertical migrations. There were excellent over-lapping source beds for the northern fields, where deeper drilling in domes (Kirkuk and NW Iraq, Ain Zalah-Butmah, and on Asmari fields in Iran) gave evidence of vertical migration emplacement of Tertiary oil. Evidence for the pre-Tertiary (Cretaceous) origins of Asmari accumulations has been established by experts using geochemical data, crude oil analysis, API gravity data, pressure data and a more precise understanding of the paleogeography.

In the south, the main reservoirs around Basra are the sandstone members of the Zubair and Nahr Umr Fms. The Cretaceous Mishrif Fm is also productive. These fields (like those of Kuwait and Saudi Arabia) are large, gently dipping folds - in contrast to the elongate, steeply dipping tight structures of the Zagros fold belt.

A characteristic of almost all the Iraqi fields, whether in the fold belt or in the Tigris-Euphrates plain, is that they are productive at several levels. This factor contributes to the generally large reserves held in individual structures - hence the Greater Rumaila Triangle". The triangle includes Majnoon and a number of other fields and is likely to become the largest petroleum province in the world.

In the north-west, the hydrocarbon potentials are also considerable. The area there, along the border with Syria, could prove to be rich in gas/condensates as well. There, evaporites present an excellent regional seal. Exploration efforts in that area were interrupted after Iraq invaded Kuwait in August 1990.

Fields in Iraq, therefore, frequently contain significantly different qualities of oil. Fields contain both high quality medium to light oils and large deposits of heavy oil at different levels. Some fields have large gas caps or high gas/oil ratios and hence substantial quantities of dissolved gas. The Pilsner Limestone in those areas was originally so named because of the frothy nature of the oil produced from it.

Gas and gas/condensate fields also occur elsewhere in the country. Since 1989, the oil ministry has given much attention to fields with condensates which could lighten Iraq's mix of export crude oils.

Details of the results of exploration since August 1990 have only been made available to companies negotiating PSAs or SCs. Companies from various parts of the world have been discussing such deals in the past 16 years. They include Total, which wants to get both Majnoon and Nahr Umr. LUKoil of Russia insists it has a PSA for West Qurna, another super-giant, but the Saddam government cancelled its deal in late 2002.