USGS Open-File Report 99-50D (Total Petroleum

U.S. DEPARTMENT OF THE INTERIOR
U.S. GEOLOGICAL SURVEY

Ghaba Salt Basin Province and Fahud Salt Basin Province, Oman -- Geological Overview and Total Petroleum Systems

By
RICHARD M. POLLASTRO

World Map with Province

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

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)
One all inclusive assessment unit, designated as the Ghaba-Makarem Combined Structural Assessment Unit, has been assigned to the North Oman Huqf/`Q' — Haushi(!) TPS of the Ghaba Salt Basin Province. The assessment unit boundary is defined and described by the TPS boundary and includes all reservoirs (Haima, Haushi, Shu'aiba, Natih, among others) and styles of structural traps (domes, anticlines, fault blocks, etc.) of the known inclusive fields (fig. 8; table 2). Thus, the Ghaba-Makarem Combined Structural Assessment Unit boundary outlines the maximum geographic extent at which similar undiscovered fields may exist in the North Oman Huqf/`Q' — Haushi(!) TPS (fig. 11).

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
     2. growth of proven gas/condensate reserves from Haima Supergroup in existing fields (field growth).
     3. salt diapir flank traps in Natih, Shu'aiba, Haushi, and Haima targets (see Faulkner, 1998)
     4. new fields in Haushi (Al Khlata and Gharif) clastics, particularly within the `Q' oil migration fair way (fig. 6).
     5. new fields in Shu'aiba and Natih limestones in low-relief, fractured `pancake' structures (large structures already drilled).
6. growth of proven oil reserves in existing fields (field growth).