ABU DHABI - Geology - Source Rocks.

The basinal limestones of the Diyab formation are the source of most, if not all, 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 portions of the offshore area. 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 four zones A, B, C and D, with the latter the most significant. Porosities range up to 30% and permeability 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. Significant oil is found only at Umm Al Dalkh. Although porosities are excellent, permeability is moderate to low; but the main reason for failure appears to be lack of access to mature source rocks. The laterally equivalent Shilaif, despite its source potential, is said to be mostly too immature. Doubts about the efficiency of the Laffan seal may also exist.

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. But the source rocks for the Khuff gas discoveries in Abu Dhabi have not yet been proven. A suitable hydrocarbon source may be present in the Silurian shales, which have been penetrated in only one Khuff well in offshore Abu Dhabi, in Sahmah-1 well 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 hence 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 seal for the Khuff are the shales and evaporites of the overlying Sudair. The basinal limestones of the Diyab provide top seal for the Araej, but intraformational seals also are present. The ultimate seal for the Arab is the Hith anhydrites. Its spread may be a major controlling factor in the distribution of hydrocarbons in Abu Dhabi. No "pre-Hith" oil occurs in areas east of the Hith edge, i.e., where the Hith is absent. In the areas 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, although there is evidence that 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.