Abstract

The Dhahaban petroleum system, from a source rock of Cambrian age, covers some 50,000 km2 and contains 1.6 × 109 m3 of oil and 1000 × 109 m3 of gas in place of which at least, respectively, 0.35 × 109 m3 and 700 × 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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