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Geology and Tectonics of South Oman |
1 Shell Western E&P Inc., P.O. Box 991, Houston, Texas 77001, U.S.A.
3 BHP Petroleum, 35 Collins St., Melbourne, Victoria 300, Australia
2 Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK
The Ara Formation of southern Oman is a geological rarity — a late Proterozoic sedimentary sequence which contains rich hydrocarbon source rocks and large accumulations of oil. It consists of a cyclic sequence of carbonates and evaporites deposited in a restricted basin. The source rocks were deposited in relatively deep, anoxic parts of the basin; reservoir rocks, most of them dolomitic, were deposited in shallow water on adjacent shelf or ramp areas. Since the sedimentary sequence overlying the Ara Formation has been truncated by numerous unconformities, the Ara source rocks have never been buried very deeply and have had a long history of oil generation in many areas, they are still within the oil window. Thick evaporites provide a regional seal for the intraformational trapping of hydrocarbons. In addition, post-depositional basin inversion led to structural elevation of what had been the deeper parts of the basin along a broad NW dipping flank. Episodic down-dip salt removal along this flank allowed Ara oil to bypass the salt seal and to charge overlying reservoirs ranging in age from Cambrian to Tertiary.
The Ara Formation represents at least five third-order tectono-eustatic cycles of carbonate to evaporite sedimentation. Carbonates formed during periods of relatively high sea level; evaporites during sea level low stands, compounded by evaporative drawdown. The carbonates were deposited in a variety of marine to mesohaline environments; in addition to an abundance of cyanobacterial remains and framestone fabrics, they contain one of the earliest shelly faunas in the geologic record.
