M. Zeller1*, B. Koehrer1#, E. Adams2, M. Pöppelreiter3 and T. Aigner1

1Department of Geosciences, Sedimentary Geology, University of Tuebingen, Germany.

2 Sarawak Shell Berhad, Sarawak, Malaysia.

3 Qatar Shell Research and Technology Centre, Doha, Qatar.

*corresponding author --: present address: University of Miami, Comparative Sedimentology Laboratory, 4600 Rickenbacker Causeway, Miami, FL, 33149, USA.

#Present address: Wintershall Holding GmbH, Kassel, Germany.

The Middle Permian to Lower Triassic Khuff Formation is one of the world’s most prolific hydrocarbon reservoirs. This study is part of a research project on Khuff outcrop equivalents whose overall aim is to investigate the reservoir architecture on different scales. The present paper focuses on heterogeneities in Khuff-equivalent grainstones at the near well scale.

An outcrop in the Oman Mountains, where the Khuff-equivalent Saiq and Mahil Formations are exposed in an area 1800 m long by100 m high, was studied to map lateral and vertical depositional heterogeneities. The investigated section here represents parts of three third-order sequences: the upper part of Khuff sequence (KS) 3 (grain-dominated), the whole of KS2 (mud-dominated), and the lower part of KS1 (grain-dominated). Real-Time Kinematic GPS, satellite imagery, outcrop gamma-ray and digitized sedimentary logs were used for outcrop description and integrated into a three-dimensional digital outcrop model.

The resulting 3D facies model indicates that the apparently simple, layer-cake geometry of the grainstone reservoir facies shows pinching and swelling with a standard deviation of 12.75% of bed thickness. These thickness variations may influence volume calculations during reservoir assessments. The heterogeneities of grainstone bodies are represented by compositional (ooid, peloid or intraclast-dominated) and grain-size variations.

Muddy event beds at the cm-scale were mapped out in detail in a 200 x 40 m outcrop window and together form a dense network of potential baffles to fluid flow. The thickness of these beds varies significantly due to syndepositional erosion.

An important conclusion of this near well-scale study is that the perceived simple, layer-cake Khuff-equivalent succession in the study area is in fact heterogeneous in mud-rich deposits, while grain-rich deposits extend over a wide area with some thickness variations. These findings can be directly applied in Khuff subsurface correlation and modelling attempts

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