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 subsequently 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 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. There, particularly in areas bordering with 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 and south-east of Iraq, the structural orientation is more north-south. It parallels trends seen in Kuwait and Saudi Arabia. Folds here have little surface expression and have largely been located by seismology. Rumeila, a super-giant oilfield about 80 kilometres long, extends some 3.6 km into Kuwait and was one cause of Iraq's August 1990 invasion (see Kuwait survey to be serialised in 2003).

Details of the deep geology in Iraq are lacking. It seems reasonable to suppose 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 NW-SE 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 involved 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 addition, however, 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 reservoir. It was not until 1927 that a similar discovery was made in Iraq in an equivalent reservoir at Kirkuk (see profiles of Iraq's fields in next week's Oil, Downstream and Gas Market Trends). It was then established that the main Tertiary hydrocarbons of the basin in Iraq, Iran and Saudi Arabia came from the Oligo-Miocene carbonate formation 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 the Kirkuk fields 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 formations.

It is said that 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. It was established that there were excellent overlapping source beds for the northern fields, where deeper drilling in domes (Kirkuk and NW Iraq, Ain Zalah-Butmah, as well as on many Asmari producing structures in Iran), gave evidence of the 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 of fields around Basra are the sandstone members of the Zubair and Nahr Umr formations. But the Cretaceous Mishrif formation 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 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 separate levels. This factor contributes to the generally large reserves held in individual structures - hence the "Great Rumaila Triangle". The triangle includes Majnoon and a number of other fields and is, perhaps, the largest hydrocarbon 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 by the Gulf crisis, after Iraq invaded Kuwait in August 1990.

Individual fields in Iraq, therefore, frequently contain significantly different qualities of oil. And it is not unknown for fields to contain both high quality medium to light oil and also 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 in the country. Since 1989, the oil ministry has given much attention to fields with condensates that 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 seriously negotiating PSAs or service contracts. Companies from various parts of the world have been discussing such deals in the past 12 years. They include TotalFinaElf, which hopes to get both the giant Nahr Umr oilfield and Majnoon, a super-giant. LUKoil of Russia has signed a PSA for West Qurna, another super-giant.

Background Of Reserve Estimates: Historically, Iraq's proven oil and gas reserves data have been grossly conservative, with many single well discoveries passed over by IPC (BP, Shell, Total, Exxon, Mobil & Partex). Gas discoveries were neglected and associated gas was flared. Oil ministry officials claimed in the 1990s that IPC had deliberately kept reserves figures low in order for successive governments in Baghdad not to press for higher oil production levels and not to attract competitive E&P offers from other foreign companies.

Oil reserves were proven at 25 bn barrels in 1960, and 27 bn in 1961 when Law No. 80 reduced the concession area of IPC and its affiliates (Mosul Petroleum Co. & Basra Petroleum Co.) to the limits of their producing oilfields. Published data reduced reserves figures from 26.5 bn barrels in 1962 to 23.5 bn in 1968, when the Baath Party came to power. But Baathist governments in the 1970s and in most of the 1980s also were conservative in their reserves estimates. Published figures of proven oil reserves stood at 35.9 bn barrels in 1972, when IPC was nationalised, and dropped to the 29-32 bn range through the rest of the 1970s. In 1980 they were reported at 31 bn barrels, falling to 30 bn in 1981-82, and then rising gradually from 41 bn in 1983 to about 48ybn barrels in 1987.

In 1988 the government announced a new estimate of 100 bn barrels, based on both re-evaluation of data and new discoveries made by the oil ministry's upstream companies. By then the entire petroleum sector had been restructured, with INOC abolished and its various divisions coming under the direct control of the oil ministry. In 1995, the oil ministry said Iraq's proven oil reserves stood at more than 112 billion barrels.