ABSTRACT: Early Permian Alluvial Gharif Formation Reservoirs of Central Oman

LIVERA, STEPHEN E., Petroleum Development Oman, Muscat, Sultanate of Oman

The alluvial Gharif Formation was deposited in the stable intracratonic Rub al Khali Basin during a period of climatic warming between the Gondwana glaciogenic Al Khlata sediments and the regional Khuff platform carbonates. Middle and upper Gharif coastal plain to alluvial red-bed sediments overlie a highstand marine carbonate and contain more than 400 million cubic meters (2.5 billion bbl) of light oil (>25 degrees API). These deposits have a uniform thickness of 150-180 m in Central Oman, cover an area of 50,000 sq. kilometers, and are reservoirs in 26 oil fields. The fluvial sediments were deposited in a broad alluvial plain as rivers, shed from peripheral highs, flowed northwestward toward the center of the Arabian Plate across a localized infra-Cambrian salt basin. Salt moveme t was subdued, because the evaporites

were covered by a thick blanket of continental clastics, but some structures appear to have been paleohighs avoided by the main trunk rivers.

A dominant control on reservoir architecture was climatic variation, and this is most apparent in the distal areas to the northwest. The distal Gharif fields consist of thick (tens of meters), amalgamated, calcrete-bearing soils interbedded with laterally continuous (50 km) sequences of minor channel, coarser overbank, and lacustrine sediments that represent "wet" periods. At least one of the intervening discontinuities is preserved as a surface of fluvial incision. In the more proximal parts of the basin, the reservoirs are economically more attractive and consist of major (tens of meters thick) fluvial channel complexes vertically separated by amalgamated red-bed soils that form stacked oil columns. These reservoir-seal pairs have limited lateral extent (tens of kilometers), indicat ng an overriding autocyclic control on the proximal alluvial plain, or intrabasinal slopes influenced by residual salt movement.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)