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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

Paradigm Change: Seal Turns into Reservoir - An Outcrop and Modeling Study of a Sudair Formation Equivalent in the Oman Mountains (Jebel Al Akhdar Area)

Michael Obermaier1; Christoph Schneider1; Thomas Aigner1; Michele Claps2; Claus von Winterfeld2

(1) Institute of Geosciences, University of Tubingen, Tubingen, Germany.

(2) Petroleum Development Oman, Muscat, Oman.

The deposition of evaporites and shales over most of the Arabian platform during the Lower Triassic provides seals for hydrocarbon accumulations in the underlying Khuff Formation. Our work on well-exposed outcrops of the Mahil Formation in the Oman Mountains (a time-equivalent unit to the Sudair Formation in the subsurface) reveals abundant potential reservoir units. A major aim of the ongoing study is to find relationships between shoal thickness and lateral extent as well as to unravel the overall geometries of shoal reservoir bodies in low-accommodation settings.
Sedimentological analyses of several sections in the region of Jebel Al Akhdar yielded 3 facies associations with a total of 12 facies types of a mostly backshoal to shoal setting. Except for few meters thick exposure-related layers at the top of one of the investigated sections, no occurrences of any sabkha-associates were detected. An integration of facies types in stacking patterns revealed three basic cycle motifs.

Potential seal units only occur in the detrital-rich backshoal cycle type in the lowest Middle Mahil Formation and consist of laminated claystones and bioturbated mud-/wackestones. The remaining major part of the about 260 m thick formation is mainly built by dolomitic backshoal to shoal cycles with a high reservoir potential. Shoal deposits consist either of oolitic- or peloidal-rich grainstones that strongly determine reservoir quality.
Sequence stratigraphic correlations based on litho-, chemo-, and biostratigraphy show subtle pinch-outs and facies changes on the scale of tens of kilometers. Thicker grainstone bodies are laterally more extensive and are correlatable within 5th order cycles on a field scale.

A field-scale static reservoir model was created using the outcrop sections as pseudo-well logs. Correlating the sections on a north-south transect indicated a deepening trend to the North by a shift from a mainly backshoal-associated setting around the center of the Jebel Al Akhdar anticline to more shoal-associated facies types at the northern flank of the anticline. This proximal-distal trend and the overall cyclicity could be well reproduced with stratigraphic forward modeling using the software Dionisos, suggesting that relative sealevel changes form major controls on the depositional architecture.