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Abu Dhabi - Geology.

The Abu Dhabi emirate has an onshore area of 77,700 sq km, compared with about 84,000 sq km for the whole of the UAE, plus some 30,000 sq km of offshore waters on the Arab side of the Gulf. The emirate is 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 wide-spread 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 up-thrust mountains of Oman's Musandam Peninsula. To the north, on the far side of the 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 ages. 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, up-lifting younger rocks to form the traps for hydrocarbons.

Licenced areas for onshore exploration over the past 25 years exceed 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, a 25,000-foot pile 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 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 inter-bedded 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 offshore Umm Shaif and Abu al-Bukhoosh oilfields. In the extreme south-east of onshore Abu Dhabi, continental clastic beds of the Minjur formation (Fm) were deposited in the Late Triassic. This unit is not represented in the offshore. And the Triassic is dis-conformably over-lain 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 Ara'ej and Uwainat Fms 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 Fms.

The Rus, like the older Hith anhydrite, loses its evaporitic character to the east. All three Fms pass laterally into the basinal limestones of the Pabdeh Fm in the east.

The Qatar Arch and the western part of Abu Dhabi were up-lifted in the Early Oligocene and part of the Eocene was eroded.

During the subsequent transgression, the Asmari limestone Fm was deposited in the eastern portion of Abu Dhabi, extending westwards to the edge of the Pabdeh basin. Succeeding Gachsaran and Mishan Fms thickened from west to east and comprise carbonates, anhydrites, marls and shales.

During the Late Miocene and Pliocene, the Alpine Orogeny produced the Zagros and Oman mountains, creating the structural framework seen today.
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Publication:APS Review Gas Market Trends
Geographic Code:7UNIT
Date:Jan 12, 2015
Words:777
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