Ediacaran–Cambrian Sirab Formation of the Al Huqf region, Sultanate of Oman

The Ediacaran–Cambrian Ara Group of the South Oman Salt Basin in the Sultanate of Oman is one of the world’s oldest petroleum systems and holds some of the most important hydrocarbon reserves in the country. However, the Ara Group salt and isolated carbonate platforms, or ‘stringers’, are known only from the subsurface and deformed fragments brought to the surface in salt-piercing domes. Thus, determining Ara source and reservoir facies architecture at high resolution is a particular problem. Here we present the results of field surveying in the Haushi-Huqf region over a number of years specifically to investigate the possibility of Ara Group equivalents being exposed in outcrop. Defined here, for the first time, is the new Sirab Formation, which we incorporate into the top of the Neoproterozoic–Cambrian Huqf Supergroup. In general, it conformably overlies the Buah Formation. However, at some localities on what were probably fault-bounded palaeo-topographic highs, the Sirab Formation rests unconformably on eroded Buah Formation or directly on the even older Shuram Formation. The Sirab Formation is overlain with marked angular unconformity by the siliciclastics of the Haima Supergroup. As such, the Sirab Formation occupies the same lithostratigraphic position as the Ara Group subsurface. We subdivide the formation into three principal members; the lower Ramayli Member, middle Shital Member and upper Aswad Member. A fourth, the Salutiyyat Member, can be recognised where the Sirab Formation lies on eroded Nafun Group palaeo-topographic highs and is probably the chrono-stratigraphic lateral equivalent at least in part of the upper Ramayli Member. The Ramayli and Shital members contain evaporite units, including halite beds, and fault- or eustatically-controlled cyclical peritidal carbonates indicating that the Al Huqf area was a shallow trough or graben during this period within a regional structural high. The middle and upper members contain significant microbial build-ups including thrombolite framestone reefs, which are the principal reservoir subsurface in the Ara ‘stringers’, and rare crinkly laminites which are the presumed source rock at depth. Whilst the precise age dates for the formation and chronostratigraphy of each member still need to be resolved, it is clear that the Sirab Formation includes exposures of litho- and bio-facies present in the Ara Group and thus could provide useful surface analogues for the petroleum geology of the South Oman Salt Basin and Central Oman High in the future.