U.S. DEPARTMENT OF THE INTERIOR
U.S. GEOLOGICAL SURVEY
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Open-File Report 99-50-C
Migration
of `Q' oils was thus through the Gharif section in a southerly direction
gradually moving into stratigraphically younger units and over a distance
of more than 150 km (Guit and others, 1995). Moreover, faulting during
the latest Tertiary has caused breaching of the main Khuff seal, allowing
vertical migration of Huqf oils into younger, post-Upper Cretaceous structures.
Thus, pure Huqf-type oils are found in fault-related structures where pathways
for the lateral migration of `Q' oils have been blocked. The geographic
distribution of known Huqf and `Q' oil accumulations, including admixtures
of both, is shown in figure 6.
Oil typing in northern Oman shows that over 90% of oil in place in Haushi reservoirs is derived from the `Q' source rock. The `Q' kitchen was defined using chemical "odometers" (mainly nitrogen) which provide a measure of migration distance (Richard and others, 1998a, b). Chemical odometer tracing from 18 `Q' oils suggests that the `Q' source may be located on the western margin of the Ghaba Salt Basin, in the vicinity east of Saih Rawl field (fig. 2). Seismic lines across this area show salt-filled rim synclines which possibly contain the `Q' source beds. Also, the second possible source area in the Fahud Salt Basin is marked on seismic with a high amplitude reflector at the top of the Ara Salt (Richard and others, 1998b). Both source areas are considered to have been restricted areas during Ara deposition and ideal sites for source rock preservation. The high efficiency of the main overlying seal, the Khuff Formation, has allowed for long distance migration of `Q' oils, as demonstrated by chemical odometers, and the new-field discoveries along the `Q'-migration fairway (fig. 6). Modeling of hydrocarbon generation for the top- salt Dhahaban (`Q') source-rock interval by Richard and others (1998b) indicates that `Q' oil was generated in the Ghaba Salt Basin source area in the Paleozoic and Mesozoic and peaked in the Middle Paleozoic and Triassic. In the shallower southernmost portion of th Fahud Salt Basin to the west, `Q' oil generation occurred from the Jurassic until Early Tertiary peaking twice during the Early and Late Cretaceous. Biodegraded, low API (<20°) gravity oils are produced from Haushi and Haima Group reservoirs in an area along the east-northeast flank of the Ghaba Salt Basin. Al Lamki and Terken (1996) have shown that these reservoirs are also major aquifers and are in |
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hydraulic communication.
Hydrodynamic activity in the Ghaba Salt Basin has reduced temperatures
and salinities of formation waters where recharge has occurred over the
history of the basin degrading the oils. Recharge of meteoric waters produces
a geochemical environment conducive for biodegradation by oil-reducing
bacteria in hydrocarbon-bearing reservoirs by reducing reservoir temperatures
(and geothermal gradient) and carrying oxygen and nutrients. Moreover,
areas of highly mature oils in the Ghaba Salt Basin also define the southward-migrating
`Q'-oil fairway (fig. 6) that was not affected by biodegradation.
Ghaba-Makarem Combined
Structural Assessment Unit (20140101) Expected exploration strategies and areas of new-field discoveries in this assessment unit are: 1. new-field discoveries for
deep gas/condensate (>5000 m) in clastic reservioirs of the Haima Supergroup
and possibly deeper limestone reservoirs of the Buah Formation, Huqf Supergroup,
in north-central Oman
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