The sedimentary rock sequence in the oilfield areas of Interior Oman ranges from the Late Proterozoic through Paleozoic, Mesozoic and Tertiary. A number of major gaps or changes in sedimentation are recognised, and these punctuate a subdivision into major rock units ranked as groups. Eleven named groups are recognised: in stratigraphic order from oldest, Huaf (Upper Proterozoic), through Haima (Lower Paleozoic), Misfar (Devonian), Haushi and Akhdar (Carboniferous to Triassic), Sahtan (Jurassic), Kahmah (Lower Cretaceous), Wasia (“Middle Cretaceous”), Aruma (Upper Cretaceous), Hadhramaut (Paleogene) to Fars (Neogene). Most of these groups are further subdivided and 35 named formations are described giving lithologic characteristics, boundaries, inferred age span and variability. These descriptions, together with reference well sections indicating wire-line log characteristics, provide a sufficient basis for consistent future use of the rock units.
Hydrocarbon occurrence within this sedimentary sequence is primarily controlled by the presence of good impermeable sealing units, particularly those of sufficient thickness to be least breached by faulting. Seven important sealing units are recognised, ranging from the evaporites of the Huaf Group to the basal shale of the Hadhramaut Group.
Commercial hydrocarbon accumulations are retained by all these seals. Reservoirs are in both carbonate and clastic units, and in South Oman are commonly in much older units than the seal, which overlines unconformably.