Online Journal for E&P Geoscientists

Obermaier, Michael ^*1; Aigner, Thomas ¹
(1) Department of Geosciences, Sedimentary Geology, University of Tuebingen, Tuebingen, Germany.

Whereas the Permian-Triassic Khuff formation is world famous for its hydrocarbon accumulations in the Middle East, the overlying Sudair and Jilh Formations are mostly top seal or baffle units. In contrast, outcrops in the Jabal Akhdar area of the Oman Mountains (where the Sudair and Jilh Formations are referred to as Middle and Upper Mahil members) consist mainly of dolomites with only occasional thin shale intercalations and reveal potential reservoir facies. While the Sudair is dominated by grainy carbonates of a subtidal shoal setting, the overlying Jilh indicates a restricted supratidal marsh to peritidal/lagoon environment.

Three potential reservoir facies could be determined:

(1) Shoal associated well-sorted oolitic/peloidal grainstones

(2) Peritidal cross-bedded intraclastic grain-/rudstones

(3) Peritidal microbial laminites.

Despite difficulties in age determination preliminary sequence stratigraphic interpretations subdivide the Sudair Formation into one 2nd order and three 3rd order transgressive-regressive sequences. The Jilh Formation includes almost three complete 2nd order and seven 3rd order sequences.

The distribution of reservoir and seal units in this shallow marine epeiric system is largely dictated by sea level fluctuations and therefore can be predicted by sequence stratigraphic analyses. In the Sudair and Jilh potential reservoir facies occur sequence stratigraphically around 3rd order maximum flooding zones (shoal facies), and during early 2nd order transgressions (peritidal facies). However, up to tens of meters thick intraformational seals comprising mudstones and shale beds develop preferentially around 2nd order sequence boundaries.

Correlations indicate these large-scale sequences to be persistent over 100s of kilometers with thickness and facies changes mainly controlled by the occurrence of very subtle paleo-highs and -lows. Further, a regional erosional truncation at top Jilh (base Jurassic Mafraq) strongly determines the reservoir thickness and extent into the upper Jilh Formation.

3D static facies models on the scale of the Oman Mountains show a paleogeographic deepening trend towards the Northeast accompanied by an increasing reservoir potential. Additionally, increasing erosion at top Jilh towards the SE reduces the probability of reservoirs in the Jilh Formation.