Seal turns into reservoir; Sudair equivalents in outcrops, Al Jabal al-Akhdar, Sultanate of Oman

The Lower Triassic Sudair Formation represents the deposits of an epeiric carbonate-evaporite-siliciclastic platform. It is widely known as top seal for the underlying Khuff reservoir in some of the largest hydrocarbon accumulations in the world (e.g. the North Dome and the Ghawar field). Towards the seaward edge of the platform however, in northeast Oman for example, the sealing anhydrites and shales pinch out. Dolomite layers turn increasingly grainy, forming up to 8 m-thick individual grainstone bodies. Thus, on a regional scale, a top seal turns laterally into a potential reservoir. This paper outlines facies, depositional environment and stratigraphic architecture of the Middle Mahil Member, the outcrop equivalent of the Sudair Formation in northeast Oman. The Middle Mahil, some 260 m thick, is made up of just a few facies types: argillaceous, muddy, microbially-laminated, graded, and cross-bedded dolomites. These represent tidal flat, backshoal and shoal environments. Facies are arranged in regular sequences; the smallest units being 2-5 m thick cycles. A clear hierarchical organization of sequences is apparent with bundling of 4-8 cycles into cycle sets, and 2-3 cycle sets forming depositional sequences. The lower third of the Middle Mahil is dominated by rather muddy textures, the middle part is grain-dominated and the upper part again is muddier with a higher percentage of microbial laminites. The maximum grainstone thickness is observed in the middle of the Middle Mahil, at the interpreted zone of maximum flooding. Overall the Middle Mahil shows "layer-cake" type architecture over distances of 4-8 km. Grainstone layers are laterally continuous at this scale. Pinching and swelling geometries of grainstones are widespread. Grainstones gradually increase in thickness towards the northeast, the direction of the seaward platform edge. Mapping of Middle Mahil grainstones suggest excellent reservoir potential in such platform margin settings. The observations from Oman provide a calibration point to explorationists targeting the Lower Triassic carbonate sequence in the Middle East.