TERKEN, JOS. M.J. and NEIL L. FREWIN, Petroleum Development Oman, Muscat, Sultanate of Oman

Abstract: The Q Petroleum System of Oman

The Q petroleum system covers some 50,000 km² and contains 1.6 x 10⁹m³ of oil and 1,000 x 10⁹m³ of gas in place of which at least, respectively, 0.35 x 10⁹m³ and 700X10⁹m³ are recoverable (Figure 1).The Q oil is the only oil type in Oman that so far has not been linked to a source rock. Its origin has been an enigma since it was first recognised in the eighties. Although regional maps indicated the direction of the source, the exact location could not be determined. This has changed with the recent application of benzocarbazoles as indicators of relative oil migration distances.

Q oils have distinct C27-dominated sterane distributions and intermediate carbon isotope values of around -30 permille PDB and differ markedly from the other oil types in Oman. Biomarkers show its source to comprise mainly Type 1/11 structureless organic matter and that this oil is derived from Precambrian/early Cambrian Nafun and/or Ara carbonate source rock deposited in a strongly evaporitic, but not hypersaline environment. Q oil occurs mainly in the Gharif Formation in Central and North Oman, but significant amounts have also been found in the Shu'aiba Formation across the salt-structured core -of the Ghaba Salt Basin. Q gas and condensates occur predominantly in the early Paleozoic Haima Supergroup.

Regional maps of Central Oman made in the 1980s showed the NS-trending, northerly plunging noses that indicated the general direction of the Q source(s). Use of benzocarbazoles, which are polar compounds thought to partition between oil, water and mineral surfaces during oil migration, as geochemical tracer molecules, finally permitted estimation of the relative migration distances for the different accumulations and to trace the origin of the Q oil. Benzocarbazole ratios highlight a migration distance for the Q oils of up to 300 km (Figure 2). Integrating this geochemical advance with migration modelling indicates Q oil comes from two different kitchens, the smaller located in a rim basin near the western margin of the Ghaba Salt Basin. The larger source covers most of the southern part of the Fahud Salt Basin (Figure 1).

Generation modeling indicates an early charge from pre-salt Nafun and intra-salt Ara source rocks. This suggests that only the shallowest of the Ara source rocks, the post-salt Dhahaban Formation, can be considered as the source of the liquid hydrocarbons in North Oman. Burial and thermal modeling of this interval in the rim basin indicates the oil was generated over a long period in Paleozoic and Mesozoic times. In the shallower Fahud Salt Basin, oil generation took place from Jurassic until early Tertiary times. Currently only the latter has remaining generation potential, but for gas only.

Migration modeling shows that oil expelled from the rim basin into the Gharif Formation migrated to the southeast across the inverted core of the Ghaba Salt Basin towards the uplifted east flank and this direction hardly changed over time. Migration from the Fahud Salt Basin was initially in the same direction, but shifted gradually southward during the late Cretaceous and Tertiary (Figure 2). This shift in direction is linked to foreland basin development in North Oman, which caused the regional tilt to increase and its azimuth to shift to the north. The two source areas recognised give a double-lobed distribution of Q oils and have at last outlined the geographical extent of the mappable Q petroleum system. It can then thus be divided into two sub-systems:

- A large western sub-system, which extends for nearly 300 km from its proposed source.area in the southern part of the Fahud Salt basin to the northern part of the South Oman Salt Basin (Figures 1 and 3) and

- a smaller eastern sub-system thought to be sourced by the rim basin and covering the western part of the Ghaba Salt Basin (Figures 1 and 2).

Adverse timing between oil generation during Mesozoic time and migration in Tertiary time suggests that remigration of oil from breached or gas-charged Haima structures to Gharif traps has occurred in Central Oman. Consequently, significant scope for gas may remain along the deeper sections of the Q migration paths in Haima (and Gharif) reservoirs (Figure 3).

The quantity of hydrocarbons generated by the Dhahaban Formation was calculated and compared to the estimated inplace oil to determine the generation-accumulation efficiency of the Q petroleum system. The average thickness of source rock was, based on well penetrations on the east flank, assumed to be 50 meters. An interval of this thickness, with an average TOC of 5%, a post-mature source rock volume of 35.7 x 10¹⁰ ml and a difference between the original and present-day hydrogen indices of 500 mg H/g OC, would have generated in both kitchen areas 34 x 10⁹ m³ of 35° API oil. On the basis of these calculations 5% of generated hydrocarbons have accumulated in discovered traps, suggesting the Q system to be very efficient.

AAPG Search and Discovery Article #90923@1999 International Conference and Exhibition, Birmingham, England