IRAQ - The Geology.
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. 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 and south-east 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.
Details of the deep geology in Iraq are lacking. It seems reasonable to suppose that the N-S 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 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 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 Fms.
It is said 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 many Asmari producing 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 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 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.
Individual 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 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 wants to get both the giant Nahr Umr oilfield and Majnoon, a super-giant. LUKoil of Russia insists it has a PSA for West Qurna, another super-giant, but the Saddam government cancelled its deal in late 2002.
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 a 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 rising 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 112.5 bn barrels.
Geologists at the oil ministry said foreign firms to be involved in E&P will have to establish a minimum discovery rate of 2 bn barrels/year of additional oil reserves in the post-embargo period. They said this will be necessary for the government to keep a balance between planned expansion of Iraq's oil production capacity to more than 6.35m b/d and the maintenance of a reserve level commensurate with such a production profile for the longer run.
For this, they said, exploration should concentrate on defined leads within zones of high potential.
There was also to be exploration in deeper areas. The number of deep wells drilled so far is very limited, compared to Iraq's neighbours: Iran, Kuwait, Saudi Arabia, Syria and Turkey.
Other areas of high prospectivity yet to be explored include the Palaeozoic and older formations. Major discoveries have been made in parts of Iraq's western desert. That region has become attractive for exploration both for the Palaeozoic play and for stratigraphic traps.
The Permian Khuff reservoir is not properly studied in Iraq. A big Khuff gas field found in Iran is close to Iraq's border.
Natural gas has not received much attention in Iraq. Nearly 3.36 TCM (117.6 TCF) of gas reserves have been discovered so far, of which 10% are in gas caps. Most of the gas reserves found are in Tertiary reservoirs - in seven gas fields located in the north-east of the country.
Gas found in western Iraq lies in Ordovician sandstones.
Exploration in the past was largely in the hands of IPC and, after the 1972 nationalisation of IPC, in the hands of the oil ministry's upstream companies. A big number of prospects remain to be explored. Some of the discovered areas are to be tested.
Interesting to know is to what extent potential reservoirs, especially deep plays, have been investigated within existing fields.