Iran - The Geology.

Iran has major oil reserves yet to the discovered. According to one prominent Western geologist who once worked for the BP-led Consortium in this country, the undiscovered oil potential can be as large, if not larger, than that of Iraq. He says despite the enormous quantities of oil and gas discovered to date and the length of time spent in exploration, Iran is so large and so prolific that substantial new discoveries can be expected with confidence. The geology of the main producing areas is dominated by two important elements: (1) The sedimentary sequence of the Arabian Basin, with a thickness of over 15,000 metres, has a regional dip off the Arabian Arch north-eastwards beneath Persian Gulf waters and the southern plains of the Tigris and Euphrates Rivers. (2) On the north-eastern side of the Gulf, the sedimentary section becomes involved in the strongly north-west/south-easterly trending folds of the Zagros Belt and, ultimately, in the complex structural deformation of the main Zagros Thrust Zone. The Zagros Thrust Zone is located along the suture where the north-easterly directed Arabian Plate is subducted below the Iranian (Asian) Plate. Within this simplistic picture, two further trends can be detected: 1. A large regional high, the Qatar Arch, extends northwards from the Arabian Peninsula and appears to have a major control on Gulf geology. To the west of the arch, on the Arabian side of the Gulf and stretching to some extent into Iranian waters, the main orientation of structures and their associated oilfields is north-south, e.g. Dukhan (Qatar, Bahrain), Burgan (Kuwait), Ghawar (Saudi Arabia), etc. 2. To the east of the arch, in the Southern Gulf Basin and southern Iran, salt-related halokinetic structures are common. Many of these have pierced to the surface. But others have failed to penetrate the overburden and have produced large, gentle swells many of which are now the sites of major oil and gas fields. The influence of the Qatar Arch and its northerly extension on Iranian geology has been profound since it effectively marks the south-eastern limit of the major oilfields of the Zagros Fold Belt. Coincidentally, nearly all the major Permian gas fields of Iran are located on the northern nose of the arch. And the sedimentary section of the Iranian producing areas comprises over 15,000 metres of post-Precambrian sediments with all the major periods represented, except perhaps for the Devonian. Until the early 1990s, little was known of the pre-Permian section but it was already supposed that clastic sediments were better represented than they were in the younger section. Developments in Qatar's North Field in recent years revived Iran's interest in the Pars structures, both South Pars and North Pars. South Pars, the more recent of the two, was discovered as an extension of the North Field. Exploration wells drilled there in the summer of 1992 struck light crude oil as well as a high ratio of associated gas and condensates. The light crude oil, with very low sulphur content, was found in a Mesozoic formation after a rich Cretaceous formation was discovered. Major reserves of non-associated gas and condensate were found in the Permo-Triassic. Heavy crude oil was discovered in the upper region of Silurian sediments. There were indications of big pre-Permian potentials. The discoveries occurred during the drilling of the second well (called SP-2), as the first one (SP-1) was abandoned after reaching a depth of 3,400 metres. A third well (SP-3) was drilled to greater depths in 1992, with NIOC sources then informing APS that a pre-Permian layer was under study (see Gas Market Trends). The Permo-Triassic sequence consists of thick limestones, dolomites and anhydrites, which are lateral equivalents of the Khuff Group of the Arabian Peninsula. In both North Pars and South Pars, gas/condensate reserves in the Permo-Triassic sequence are believed to be potentially as large as those of the North Field. There is also a large Khuff gas reservoir beneath the Salman oilfield, an extension into Iranian waters of Abu Dhabi's Abu Al Bukhoosh field. The gas reserves beneath Salman are estimated by NIOC at 6 BCF and Iranian experts say there are also about 105 million barrels of condensate. The post-Triassic section of the Gulf and of the Zagros Fold Belt is a predominantly marine sequence of limestones, dolomites, shales and evaporites. Locally sandy clastic sequences occur but these are of relatively minor importance on the Iranian side of the Gulf. The main concentrations of evaporites are in the Late Jurassic, being the Gotnia/Hith section of the Miocene Gachsaran. Between these two, the section is dominated by limestones, some of which are shelfal and others are deep- water deposits in origin. Some of the deep-water limestones are the products of restricted intrashelf basins presumed to have sourced most of the oil and associated gas. So, in general terms, the sedimentary section throughout the Persian Gulf area is remarkably uniform over the entire basin. The major differences in the types and distribution of hydrocarbons appear to be related to local features; hence the habitat of hydrocarbons will be broadly sketched by major sub-divisions. In onshore Iran, the Khuzistan region north-west of Bushehr comprises the main oil province. This is dominated by long, elongate, NW-SE trending anticlines. They are associated with the thrusting caused by the closure of the Gulf geosyncline. Decollement, associated with the evaporitic layers, means that frequently the structure seen at the surface is not always coincidental with the structure at the likely hydrocarbon producing levels. The major reservoirs in the Khuzistan region are the Asmari (Miocene) limestone and limestones of the Late Cretaceous Bangestan group. The Asmari group is more interesting. A characteristic of the Asmari is that, although it generally has poor matrix porosity, it is fractured within the folds. It has high fracture porosity and incredibly high permeability. (The wellhead of the "M.i.S. F7 Well", now standing outside BP's London headquarters, records that this one well produced 53m barrels of oil over a 15-year period). The ultimate seal to the Asmari is the Gachsaran evaporite. It is a measure of the efficiency of this seal that it has remained intact in the same structures where the underlying limestone has become a highly fractured, high pressure reservoir. At least one Permian gas field has been discovered at the northern end of the trend. The presence of enormous quantities of oil and associated gas preclude any doubts about the presence of source rocks, believed to be Late Cretaceous in age. Oil has either been trapped in the Bangestan group or migrated vertically into the Asmari. Despite the long history of exploration and development and the size of the structures in the northern Zagros Fold Belt, it is likely that there must be considerable future potential for successful exploration in these areas. With the use of new technology, which did not exist when the Western consortium was operating in Iran, re-exploration there could lead to major oil discoveries. In an area of complex folding, where there is evidence of detachment tectonics, the location of subsurface traps is dependent on accurate seismic surveying. The region has not been the subject of intense scrutiny, or of modern seismic techniques, which an area of this prospectivity warrants. The presence of Permian gas fields at both ends of the region suggests there may be potential for further deep gas discoveries within the main oil area itself, a theory under investigation by companies interested in Iran's prospects. However, it is also possible that depth of burial may have had a deleterious effect on the quality of any such gas occurring. Offshore, Northern Gulf: The fields which have been located to date in the Iranian waters of the northern Gulf show more affinity with the fields of the northern Arabian Peninsula than with the Zagros Fold Belt. Oil in these fields is reservoired in Miocene (Asmari) carbonates, as in the adjacent onshore areas. But for fields further offshore, the reservoirs more closely resemble those of the northern Arabian Peninsula, i.e. Miocene clastics (Ghar) and Early Cretaceous sandstones (Nahr Umr and Burgan). Even further south, oil is reservoired in Lower Cretaceous carbonates similar to those of the Saudi fields. The offshore Iranian fields are believed to have developed over deep-seated salt swells similar to those on the Arab side of the Gulf. They also have a similar north-south trend. To the north-east, the fields closest inshore, however, are overprinted by the Zagros Fold trend. Exploration in the northern Gulf waters, seriously disrupted by the Iran-Iraq war since it was the area most vulnerable to attack, is now one of the priorities for NIOC. It seems reasonable to suppose the area, once becoming the focus of renewed exploration, should yield further small to medium sized oil discoveries. In early 1990, deep drilling successes in Kuwait triggered interest in the northern Gulf. There is also potential for Permian gas, especially on the western flank of the Qatar Arch. The northern and southern regions are separated by the Qatar Arch. This is a northerly trending geanticlinal swell which extends from Qatar, beneath the Gulf waters, and into onshore Iran. Qatar's offshore portion of the arch is the site of one of the world's largest gas fields - the North Field, where reserves in place are estimated at more than 380 TCF of dry gas (after liquids removal). The field is rich in gas liquids.