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AAPG GEO 2010 Middle East
Geoscience Conference & Exhibition
Innovative Geoscience Solutions – Meeting Hydrocarbon Demand in Changing Times
March 7-10, 2010 – Manama, Bahrain

Ara Formation Depositional Architecture and Controls from Reservoir Development - Sultanate of Oman

Zuwena Al-Rawahi1; John Grotzinger2; Joao Rodrigues1; Sri Vaddey1; Huub Jansen1

(1) Exploration, Petroleum Development Oman, Muscat, Oman.

(2) Division of Earth and Planetary Sciences, California Institute of Technology, Los Angeles, CA.

An integrated subsurface approach to understand reservoir development of a unique Precambrian hydrocarbon system in the South Oman Salt Basin (SOSB) is presented here. New well data and recently re-processed PreSDM data were used to improve the basement fault and overburden tectonic models. Existing depositional models have been refined to identify reservoir fairways and aid in play segment risking.

The Precambrian Ara Formation comprises six (A1-A6C) carbonate-evaporite cycles, most of which are totally encased in salt. Hydrocarbons are produced from the overpressured to hydrostatically-pressured dolomite “stringer” intervals which define the focus of the study. Some of the complexities in predicting reservoir sweetspots for exploration of the Ara carbonate stringers are: 1) Slabs encased in salt can only be jump-correlated seismically to another slab hence palaogeographic reconstruction is hampered by salt halokinesis; 2) Primary reservoir quality is complicated by diagenesis and charge timing; 3) Post-charge reactivation of structures affects hydrocarbon column, -phase, and distribution of pressures.

Each stringer records a third-order sequence; Facies show substantial lateral continuity, within and between the slabs, and are dominated by thrombolitic build-ups, microbial laminites and carbonate calcarenites of the ensuing highstand phase. Subsequent lowstand evaporites are anhydrite and thick halite facies that onlap and overlap highstand reservoir carbonates indicating a tectonic control on sequence development.

Palaogeographic maps show that A1C and A2C stringers have facies distributions that differ between the Harweel and Birba areas due to the influence of antecedent topography, syndepositional salt withdrawal and basement-involved faulting. A major platform of A1C age is developed in the Birba area; this platform continued to aggrade along its edge during A2C times, forming a barrier shoal and development of isolated carbonates in restricted environments in the Birba area. In contrast, platforms and reservoirs of A2C age are best developed in the Harweel area. A3C platforms are developed atop A2C platforms. A4C reservoirs are restricted to the Birba area, where it was localized above the major platform that initiated during A1C time.
A refined risk assessment of the different stringer intervals in the SOSB could therefore be constructed in distinctive geographic areas, leading to improved ranking of future exploration targets.