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ABU DHABI - Geology - The Permian - Gas-Rich Khuff Formation.

The Upper Permian-Lower Triassic Khuff formation, where huge 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 in Saudi Arabia 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 fossi-liferous; (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. The, 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 formation 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.

The Khuff's porous zones are preserved to some extent. ADNOC's publication (ADNOC News, March 1999) said preservation of porosity was "possibly attributed to initial deposition environment, subsequent diagenesis and associated ever-pressuring". However, it added, "it appears that early hydrocarbon emplacement is a major contributory factor to significant porosity preservation".

Production testing has proved that the Khuff reservoir zones are of variable significance 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.
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Publication:APS Review Gas Market Trends
Date:Jan 1, 2001
Words:509
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