ABU DHABI - Geology - 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. 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 now contained in Abu Dhabi's Jurassic and Cretaceous reservoirs. The Shilaif is a high quality potential oil source rock; but in Abu Dhabi, 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 permeability can exceed 100 md.

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 Umm Shaif.

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.