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1 Petroleum Development Oman, Department XEM/6, P.O. Box 81, Muscat Postal Code 113, Sultanate of Oman; present address: Nederlandse Aardolie Maatschappij, Department LAN/q, P.O. Box 1, 7760AA Schoonebeek, Netherlands; j.m.j.terken{at}nam.nl
2 Petroleum Development Oman, Department XGS/2, P.O. Box 81, Muscat Postal Code 113, Sultanate of Oman
Jos J. M. Terken joined Shell in 1982 and has worked in the Netherlands, Brunei, New Zealand, and Indonesia. In 1993 he joined Petroleum Development Oman as a senior review geologist/basin modeler/seismic interpreter in the regional studies team. In close cooperation with the geochemistry group he modeled and mapped the petroleum systems in Oman. Since November 1999, he has been a senior production geologist for the Nederlandse Aardolie Maatschappij in the Netherlands. Jos received an M.Sc. cum laude in geology and sedimentology from the University of Utrecht in 1982.Neil Frewin is currently a petroleum geochemist for Petroleum Development Oman (PDO) where he leads the hydrocarbon modeling group. Prior to being posted to PDO in 1997, he worked for Shell International in the Netherlands as a research geochemist. He holds a B.Sc. (geology) from the University of Wales and a Ph.D. (geology/geochemistry) from the University of London. Neil spent a postdoctoral year researching biomarker technologies at Delft University and NIOZ, Netherlands.
The Dhahaban petroleum system, from a source rock of Cambrian age, covers some 50,000 km2 and contains 1.6 x 109 m3 of oil and 1000 x 109 m3 of gas in place of which at least, respectively, 0.35 x 109 m3 and 700 x 109 m3 are recoverable. For many years, the origin of the so-called Q oil was enigmatic and defied typing to a source rock. Integration of recent advances in geochemistry and basin modeling has now permitted us to retrace its source areas, model the generation and migration histories, and outline the areal and stratigraphic extent of the petroleum system.
The Q oil is interpreted to have been generated by top-salt source rocks (Dhahaban Formation) of the Precambrian-Cambrian Ara Group and is mainly trapped in younger reservoirs of the Permian Gharif and Cretaceous Shu'aiba formations. Gas and condensate occur in deeper Paleozoic Haima reservoirs. Relative oil migration distances estimated from geochemical tracer molecules (benzocarbazoles), together with migration modeling, indicate that the Q oil is derived from two different locations: a small source area along the western margin of the Ghaba salt basin and a large, shallower source in the Fahud salt basin. Burial and thermal history reconstructions indicate the oil was generated in several stages during the Paleozoic-earliest Tertiary, but mainly in the Mesozoic. Initially, oil from both kitchens migrated in a southeasterly direction toward the tilted east flank. Modeling shows that migration routes from the Fahud salt basin gradually shifted westward, and Q charge only reached into southern central Oman in the last 50 m.y. Temperature and salinity data suggest that besides buoyant forces, hydrodynamic fluid flow contributed to this long-distance Q oil migration.
With oil generation during the Mesozoic and migration during the Tertiary, remigration from deeper breached or gas-charged Haima traps most likely occurred. Resolving the timing of charge and actual migration geometry has identified new oil and gas prospects along migration pathways to known accumulations and on spill routes from these accumulations.
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