IRAN - The Geology.

Iran is a vast country with oil and gas rich basins yet to be properly explored. One Western geologist who once worked for the BP-led Consortium in this country says the undiscovered potential can be as large, if not larger, than Iran's current oil and gas reserves. He says despite the enormous quantities of oil and gas discovered to date and the length of time spent in exploration, Iran is 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, there are two further trends:

A large regional high, the Qatar Arch, extends northwards from the Arabian Peninsula and has 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.

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.

The sedimentary section of Iran's 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 supposed that clastic sediments were better represented than they were in the younger section. Developments in Qatar's North Field 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 Qatar's 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 oil, with low sulphur content, was found in a Mesozoic formation after a rich Cretaceous reservoir was discovered. Major reserves of non-associated gas and condensate were found in the Permo-Triassic. Heavy crude oil was found in the upper region of Silurian sediments. There were indications of big pre-Permian potentials.

The discoveries occurred during the drilling of a second well (SP-2), as the first one (SP-1) was abandoned after reaching a depth of 3,400 metres. SP-3 was drilled to greater depths in 1992, with NIOC sources then informing APS that a pre-Permian layer was under study.

The Permo-Triassic sequence consists of thick limestones, dolomites and anhydrites, which are lateral equivalents of the Khuff Group of the Arabian Peninsula. In North Pars and South Pars, gas/condensate reserves in the Permo-Triassic sequence are believed to be as large as those of the North Field. There is a big Khuff gas reservoir (in Iran also called Upper Dalan) 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 105m barrels of condensate.

The post-Triassic section of the Gulf and the Zagros Fold Belt is a predominantly marine sequence of limestones, dolomites, shales and evaporites. Sandy clastic sequences occur, but on the Iranian side of the Gulf these are of relatively minor importance.

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", which has been standing outside BP's London HQ, records 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. Permian gas has been found at the northern end of the trend.

The presence of enormous quantities of oil and associated gas precludes 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, there must be considerable potential for successful exploration there. With new technology, which did not exist when the Western consortium operated 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. Despite major oil and gas discoveries there made by NIOC in recent years, the region is yet to be intensely scrutinised with the use of modern seismic techniques.

The presence of Permian gas fields at both ends of the region suggests there may be potential for deep gas finds within the main oil area, a theory under investigation by companies interested in Iran's prospects. But it is 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 Persian 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 Iranian areas. But for fields further offshore, the reservoirs more closely resemble those of the northern Arabian Peninsula, such as the Miocene clastics (Ghar) and Early Cretaceous sandstones (Nahr Umr & Burgan). Even further south, oil is reservoired in Lower Cretaceous carbonates similar to those of the Saudi fields.

The offshore Iranian fields have developed over deep-seated salt swells similar to those on the Arab side. They 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. The area should yield further 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 extending from Qatar, beneath the Persian Gulf waters, and into Iran. Qatar's offshore part of the arch is the site of the world's largest gas field - North Field, where reserves are now estimated at more than 900 TCF of dry gas (after liquids removal). The field is rich in gas liquids. The field's extension to the Iranian side of the waters, South Pars, is estimated to contain 436 TCF (12.3 TCM) of gas (compared to an earlier estimate of 350 TCF or 9.9 TCM).

In this part of the Gulf, in Iran opposite Qatar, substantial hydrocarbons other than those of Pars gas have been found as a pronounced NW-SE Zagros trend. They are genetically related to the Tertiary and Cretaceous oil fields of the north-western Zagros. One of these is a gas field, Kangan (discovered in 1973 by Elf of France). It has a liquid yield similar to that of the North Field. It is similar to North Pars (discovered by Shell in 1966, about four years before Shell found the North Field in Qatar). The presence of Mesozoic and Permo-Triassic formations discovered in 1992 in South Pars, together with a pre-Permian possibility, aroused the interest of foreign companies. The oil potential increases to the north as the arch gets progressively deeper.

South-east of the Qatar Arch, the southern Zagros Fold Belt takes on a more easterly trend and culminates at the northern extension of the Oman Mountain Front. In this area there are numerous Hormuz salt piercement structures, i.e. Palaeozoic or older salt has penetrated through the overburden to reach the surface over a wide area. Comparatively little exploration drilling has been done in this region. Success achieved before the Khomeini revolution had resulted in a small number of gas and oil discoveries in the Straits of Hormuz.

Gas in the southern Zagros Fold Belt occurs at a variety of levels from Khuff to the Miocene Guri member. It seems the area lacks sufficient source rocks for oil and is mostly gas prone. The amounts of non-hydrocarbon constituents in the gas is higher than elsewhere in the region. It was assumed in the 1970s that any deep gas between the Qatar Arch and Hormuz may have an even higher non-combustible content.