Khuff Sequence KS5 outcrop-equivalents in the Oman Mountains, Sultanate of Oman: Variations to the simple “layer-cake” stratigraphy

The Khuff Formation is a major producer of non-associated gas and an exploration target in the Middle East. The Middle Permian to Lower Triassic Khuff carbonates were deposited on a gently inclined epeiric carbonate ramp, which formed on the margin of the Neo-Tethys Ocean. The formation represents a supersequence consisting of transgressive-regressive sequences KS6 to KS1 from oldest to youngest. This paper focuses on a detailed sedimentological analysis of Khuff Sequence KS5 in outcrops in Al Jabal al-Akhdar in the Oman Mountains, Sultanate of Oman. Based on the sedimentological analysis of five outcrop sections, 11 facies types were identified in KS5. These were grouped into six facies associations, which represent environments ranging from a tidal flat to offshoal. Based on the 1-D analysis of sequences and their stacking patterns, 2-D correlations were constructed on a scale of several tens of kilometers. These correlations were used to build the framework for 3-D facies models. In contrast to the typical “layer-cake”-type Upper Khuff sequences KS4 to KS1 in terms of facies associations and cyclicity prominent lateral facies association changes and thickness variations are common in KS5, which makes correlation challenging. Different correlation strategies were tested, all resulting in complex cycle and stratal geometries with cycle pinch-outs and apparent cycle set downlaps/onlaps. Due to the dynamic depositional changes throughout KS5, the lateral extent of shoal-associated grainstones is limited. The appearance of these potential reservoir bodies is mainly governed by paleogeography and stratigraphic position. The observed depositional patterns represent significant variations from a rather simple “layer-cake”-type stratigraphic architecture and are possibly due to effects of differential subsidence on a subregional scale. This study contributes to a more detailed understanding of the Lower Khuff’s reservoir distribution and continuity, which is a key to ensuring future success in Khuff exploration and the efficient recovery in producing fields.