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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 69 (1985)

Issue: 6. (June)

First Page: 899

Last Page: 912

Title: Depositional Environment, Reservoir Units Evolution, and Hydrocarbon Habitat of Shuaiba Formation, Lower Cretaceous, Abu Dhabi, United Arab Emirates

Author(s): A. S. Alsharhan (2)


The Shuaiba Formation (Aptian) is a thick, porous shelf carbonate, present in the subsurface over much of the Arabian Gulf. It is the uppermost formation of the Thamama Group (Lower Cretaceous), and is overlain by the Nahr Umr Formation (Albian) and overlies the Kharaib Formation (Barremian). The formation is divided into nine reservoir units, A to I (from the base upward). Reservoir units A, B, and C represent calcareous algal platform sediments showing some open-marine influence. They are dominated by chalky lime mudstones to wackestones with abundant Codiacian green algae (Bacinella irregularis and Lithocodium aggregatum) and stylolite seams. Reservoir units D and F are deeper open-marine shelf facies dominated by dense, argillaceous, lime mudstones to wackestones whic are burrowed and bioturbated. Reservoir units G and I represent lagoonal facies dominated by dense, argillaceous, wackestones to packstones, which are sparsely fossiliferous and commonly stylolitic. Reservoir unit H represents a shallow-water, coarser grained shelf limestone containing abundant rudists, algae, orbitolinids, and other foraminifers. Reservoir unit Ho represents a shoal facies characterized by wackestone to packstone sediment with abundant Orbitolina and scattered rudist debris and other foraminifers. In northeast and southeast Dhabi, the basinal Shuaiba equivalent (Bab Member) was deposited under slightly deeper, quieter marine conditions, and is dominated in parts of the section by dense, argillaceous lime mudstones and shaly wackestone, and calcareous shale i tervals.

The major reefal Shuaiba buildup in Abu Dhabi trends northwest-southeast. It is in this trend that the major oil fields (Shah, Bu Hasa, and Zurarah-Shaybah) were discovered and from which some wells produce in western Abu Dhabi (e.g., Ruwais-2). Hydrocarbons also occur outside of the rudist trend in the Shuaiba Formation in the Asab, Jarn Yaphour, and Sahil onshore oil fields, and the Zakum and Mandous offshore oil fields.

Local reefal anomalies are also encountered in parts of the deep marine basin of eastern Abu Dhabi. These are formed by an unusual pre-reefal growth of orbitolinids, rudists, and algae. An isolated buildup (patch reef) consisting of coralgal boundstones, encountered in Jarn Yaphour and Zabbara fields, may represent organic growth over either a rising salt structure or an uplifted zone formed by deep-seated faults.

Geochemical studies reveal that the main source for the Thamama oils are the Upper Jurassic Dukhan Formation (onshore Abu Dhabi) or Diyab Formation (offshore Abu Dhabi), however, some Shuaiba source rocks may have existed in intrashelf basinal areas in the upper Thamama zone. The Nahr Umr Shale acts as a seal for the Shuaiba reservoir in this area.

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