ABU DHABI - The Geology.

The Abu Dhabi emirate has an onshore area of 77,700 square kilometres, compared with about 84,000 sq km for the whole of the UAE, plus some 30,000 sq km of offshore area on the Arabian side of the Persian Gulf.

The emirate is located on a broadly subsiding shelf wherein sedimentation is very thick. Excellent reservoir rocks developed over wide areas with remarkable lateral continuity. The shales, anhydrites and tight limestones were equally widespread to provide extremely efficient sealing mechanisms.

Major oil and gas reserves have been discovered in the emirate since the 1950s, essentially in the Cretaceous, Jurassic and Permian reservoirs.

Abu Dhabi lies in a broad synclinal area between the basement shield of the Arabian Peninsula and the upthrust mountains of Oman's Musandam Peninsula. To the north, on the far side of the Persian Gulf, the sedimentary basin is controlled by the Zagros mountain front. Major warps on a north-south axis can be distinguished under the Ghawar trend in Saudi Arabia and beneath the Qatar Arch. Within the territorial limits of Abu Dhabi there are few exposures of rocks older than the Plio-Pleistocene and Recent cover. Deep wells drilled have penetrated the pre-Khuff clastics of Permian and pre-Permian age. Lower Paleozoic salt, possibly Cambrian in age, is believed to underlie much of Abu Dhabi, especially in the offshore. Salt structures similar to those exposed to the north in Iran are believed to have been responsible for much of the structuration within the basin, uplifting younger rocks to form the traps for hydrocarbons.

Licenced areas for onshore exploration over the past 19 years have reached about 65,000 sq km. Offshore acreages with rich sedimentation have reached about 30,000 sq km. Water depths off Abu Dhabi seldom exceed 40 metres.

The thickness of the sedimentary pile is more than 30,000 feet. Of this 25,000 feet is post-salt. A deep well at Zakum reached 17,370 feet and was still above the salt pillow which forms this large, low-amplitude feature whose surface area is 660 sq km. Elsewhere in offshore Abu Dhabi and along the coast, the salt has punched up through the more recent sediments to breach the surface.

In contrast to the non-piercement structures, the offshore piercement features form narrow, slim chimneys resulting in islands with surface areas of 3 to 6 sq km. The salt itself has dissolved away leaving a chaotic melange of exotic blocks brought up with the salt.

The sedimentary succession from Permian to Recent consists almost entirely of limestones and dolomite rocks interbedded with shales and evaporites. The pre-Permian does contain some coarser sandstones. The frequent repetition of shallow-water limestones and sabkha-like deposits suggests that the type of conditions existing at present in Abu Dhabi have occurred at regular intervals in the past. The Permian Khuff group is present in most of Abu Dhabi and its territorial waters, mainly beneath the Umm Shaif and Abu Al Bukhoosh offshore oilfields.

In the extreme south-east of onshore Abu Dhabi, continental clastic beds of the Minjur formation were deposited in the Late Triassic. This unit is not represented in the offshore. And the Triassic is disconformably overlain by the Lower and Middle Jurassic.

The Lower Jurassic began with a major marine transgression during which the Hamlah and Izhara were deposited. The Hamlah consists of dolomites, argillaceous limestones and shales. The overlying Izhara is lithogically similar but contains some darker, more basinal limestones. The Middle Jurassic was characterised by shallow, warm seas in which the clean, grainy limestone of the Araej and Uwainat formations were laid down. In general, conditions remained remarkably uniform across the greater part of Abu Dhabi during Permian to Middle Jurassic times and rock units can be correlated with ease over wide areas.

During the Lower Tertiary, Abu Dhabi lay on the margins of a basin centred on the northern emirates. In the west shelfal conditions saw the deposition of dolomites, limestones and anhydrites of the Umm Er Radhuma, Rus and Dammam formations. The Rus, like the older Hith anhydrite, loses its evaporitic character to the east. All three formations pass laterally into the basinal limestones of the Pabdeh formation in the east.

The Qatar Arch and the western part of Abu Dhabi were uplifted in the Early Oligocene and part of the Eocene was eroded. During the following transgression, the Asmari limestone formation was deposited in the eastern portion of Abu Dhabi, extending westwards to the edge of the Pabdeh basin. Succeeding Gachsaran and Mishan formations thickened from west to east and comprise carbonates, anhydrites, marls and shales. During Late Miocene and Pliocene, the Alpine Orogeny produced the Zagros and Oman mountains creating the structural framework seen today.

The Cretaceous: The succeeding Early Cretaceous Thamama rocks are dominated by shallow water carbonates of remarkable, widespread homogeneity. These rocks are of significant commercial importance and comprise a series of porous, clean, pellet and fossiliferous limestones with interbedded tight (often stylolitic lime) mudstones and packstones. They comprise, in ascending order the Habshan, Lekhwair, Kharaib and Shuaiba formations. These are better known by their informal oilfield nomenclature of Thamama Zones I to VI offshore, or Zones A to F onshore. (According to oilfield practice the zones are numbered or lettered from top downwards in order of penetration). Shuaiba differs in that it contains referral build-ups of rudists surrounded by dense basinal limestones of the Bab member.

Following Thamama Group deposition, there has been a break in sedimentation in Abu Dhabi, with the hiatus most pronounced in the south-eastern area where there was active erosion. The Thamama is succeeded by the Middle Cretaceous Wasia Group consisting, in ascending order, of the Nahr Umr, the Mauddud and the Salabikh (or Shilaif) and its partial lateral equivalent, the Mishrif. The Nahr Umr is the first major non-carbonate formation in the succession and consists of shales and minor glauconitic siltstones which pass laterally to limestones in the Oman mountain front to the east.

Further north in the Gulf, Nahr Umr is a sandstone of major importance as a reservoir, particularly in Iraq. Clean shelfal limestones of the Mauddud overlie the Nahr Umr. They are thinly developed in Abu Dhabi. Following the Mauddud, deep water subsidence once again affected the region with a north-south trending basin forming beneath Abu Dhabi. In this basin thick bituminous "oligestinal" limestones of the Shilaif formation were deposited, initially across the entire area. Later basinal conditions were restricted to the axis. Clean, shelfal and rudistic Mishrif limestones were deposited on the flanks of the basin which gradually became more constricted. In the centre of Abu Dhabi, the culmination of the basinal phase resulted in the deposition of the Tuwayil and Ruwadha members.

Middle Cretaceous deposition closed with some uplift and renewed subsidence resulting in the deposition of the Late Cretaceous Aruma Group. This group's basal member, the Laffan, is a widespread homogeneous brown-to-red open marine shale capping the Mishrif and Shilaif. In north-eastern Abu Dhabi and adjacent parts of Dubai there is evidence that the Mishrif was subject to sub-areal erosion prior to the deposition of the Laffan, and the latter shows some fresh-water characteristics. The Laffan is succeeded in turn by the Halul or Ilam limestones, the Fiqa shale and the Simsima carbonate. With rare exceptions, these are not commercially important in Abu Dhabi. Both the Fiqa and Simsima lose their characteristic identity in eastern Abu Dhabi and pass into the Gurpi formation, a unit deposited in a basin forming in the northern UAE.

The Jurassic: Around the close of the Middle Jurassic, waters deepened; and the Late Jurassic sequence begins with the deposition of deep water limestones and limy mudstones of the Diya (or Dukhan) formation which has been identified as the major source of the oils in Abu Dhabi. This unit passes laterally eastwards into more shelfal limestones.

Near the close of the Upper Jurassic, shallow water cyclic sedimentation was re-established with the deposition of the carbonates and evaporites of the Arab and succeeding Hith formations. These can be easily distinguished in western parts of Abu Dhabi. But they lose their separate identities to the east. In particular the Hith, which is a well defined anhydrite in the west, passes laterally into dolomites to the east. The end of the Jurassic was marked by a period of non-deposition and possibly local erosion.

The Upper Permian-Lower Triassic Khuff formation, where gas accumulations have been discovered in the north-western offshore areas of Abu Dhabi, remains an interesting subject of study. The formation was first studied in 1958 by Western geologists (M. Steineke, R.A Bramkamp and N. J. Sander) at the type locality of Ain Khuff near the Riyadh-Jeddah road in Saudi Arabia. The Permian covers the upper part of the Palaeozoic era (after the Pennsylvanian), thought to have covered a period between 280 and 225 million years ago. It also covers a corresponding system of rocks. It is named after the Russian province of Perm west of the Urals mountains, where rocks of this age were first studied.

At Ain Khuff the Khuff formation is 562 feet thick, with alternating lithology from top to bottom being as follows: (1) a 92.5-foot thick aphanitic layer of calcarenitic limestone, commonly marly and fossilifer-ous; (2) a 233-foot thick aphanitic limestone; and (3) a 110.5-foot thick layer of dolomite and shale, with angular granitic sand and fine conglomerate at the bottom. There (at Ain Khuff), the reported age of the formation is Late Permian and the underlying rocks are of the Pre-Cambrian basement. Wajid sandstone and Saq sandstone, with discomformable contact, and the overlying rocks are of the Lower Triassic Sudair formation.

During the Middle Permian in Abu Dhabi, the climate gradually became warmer and more arid. As a result of marine transgression, a carbonate platform was established throughout the region, on which the thick sequence of shallow-water carbonates and subordinate evaporites of the Khuff were deposited. These ranged in thickness from 2,668 to 3,026 feet. The formation was completely penetrated in a total of 17 Khuff structures in Abu Dhabi, in which ten large gas discoveries have been made in the offshore areas. Consisting mainly of slightly anhydritic dolomite and limestone, the formation is divided into Upper and Lower Khuff, which are separated by a widespread bed of 40/60-foot thick anhydrite. The latter is called Middle Anhydrite. The Upper and Lower Khuff are subdivided into reservoir units, with Upper Khuff having four units and Lower Khuff having three units. All are based on significant log picks, sedimentological support, well and outcrop data.

Khuff reservoirs vary from well to well and from field to field. The Upper Khuff is more productive than the Lower Khuff. High gas flow rates are due partly to production from fractures in zones of poor porosity development in the Lower Khuff. The absence of hydrocarbons in the onshore Khuff structures could be due to the fact that most of the existing structures were developed after the main period of hydrocarbon migration.

Source Rocks: The basinal limestones of the Diyab formation are the source of most of the oil and gas presently contained in the Jurassic and Cretaceous reservoirs in Abu Dhabi. The Shilaif is a high quality potential oil source rock; but within Abu Dhabi, apparently it has not reached sufficient maturity to have generated significant amounts of oil. In areas where its depth of burial is sufficient to have reached maturity, such as eastern-most Abu Dhabi and western Dubai, it may have sourced the oils in the Mishrif at Umm Al Dalkh and the offshore Dubai fields.

In terms of volume, the Thamama is the most important of Abu Dhabi's reservoirs. The unit produces over a wider area of Abu Dhabi than any other. It forms the producing reservoirs of Bab, Bu Hasa, Asab, and Sahil in the onshore, and Zakum and Mubarraz in the offshore. Also in the offshore Thamama, oil occurs at Umm Shaif and Abu Al Bukhoosh, in addition to their older reservoirs. In the onshore, production is mainly from the younger Thamama reservoirs, while in the offshore, at Zakum, the older zones have had the longer production history. The low permeability Zones I and II were developed more recently despite their vast volumes of oil in place. Cap gas is associated with the oil in Bab, where there has been a major gas/condensate development.

The Araej, comprising upper and lower members with the intervening Uwainat group, is an important reservoir in the western and central offshore. Light oil is present in Umm Shaif. But elsewhere only non-associated wet gas occurs. The Arab formation is also of major importance in the offshore areas, especially at Umm Shaif, subdivided into Zones A, B, C and D, with the latter the most significant. Porosities range up to 30% and permeabi-lity can exceed 100ymd. Despite its seemingly attractive development as a reefal facies built-up over large areas of Abu Dhabi, the Mishrif has failed to live up to expectations.

Small accumulations of oil appear in the Halul and Asmari formations at Mandous, in the Halul at Al Khair - both offshore and onshore in the Simsima at Shah. In the case of Mandous, the oil is likely to have migrated from deeper source rocks along faults associated with the Mandous salt plug. The source at Al Khair and Shah may be more localised.

Pre-Khuff clastics and Khuff, especially Upper Khuff carbonates, form huge non-associated gas reservoirs in offshore Abu Dhabi, mainly beneath the Umm Shaif oilfield. A suitable hydrocarbon source may be present in the Silurian shales, which have been penetrated in offshore Abu Dhabi, in western Oman, and in the Qusaybah formation in Saudi Arabia.

Khuff gas is predominantly dry and the content of inert components, nitrogen and carbon dioxide increases towards the east, to such an extent that Zakum gas is judged non-commercial. The decrease in gas quality appears to be related to depth and temperature. The Khuff reservoirs are frequently of moderate porosity with low matrix permeability - the latter enhanced by natural fractures.

The Seals: The Khuff and pre-Khuff have sufficient intraformational tight units to provide their own internal seals. The ultimate seals for the Khuff are the shales and evaporites of the overlying Sudair. The basinal limestones of Diyab provide top seal for the Araej, but intraformational seals also are present. The ultimate seals for the Arab are the Hith anhydrites. Their spread may be a major controlling factor in petroleum distribution in Abu Dhabi. No "pre-Hith" oil occurs in areas east of the Hith edge. Where the Hith occurs, the amounts of Thamama oil are reduced. In these areas the occurrence of post-Hith oil can be explained by either local faulting which has breached the Hith seal, or the Thamama oil is close enough to the Hith margin to have leaked around it. The Nahr Umr has proved to be a very efficient seal for the vast amounts of Thamama oil; the tight, stylotised limestones between the producing zones also act as intraformational seals. Where the Nahr Umr may have been breached by faulting, oil has escaped up to younger horizons; but these cases are few and far between.