The term Unayzah Formation was introduced by the author in 1982 for Permo-Carboniferous succession composed of mainly siliciclastics and minor carbonates and evaporites bracketed between the base of the main limestone of the Late Permian Khuff Formation at the top and a regional (Hercynian time-equivalent) unconformity at the base. The term was in use by oil companies and geologists in the region since then.
Great amount of data were obtained from outcrops and wells since the introduction of the formation. Such data have revealed a transgressive flooding surface representing a sequence boundary within the Unayzah Formation separating two different units; a lower mainly continental and an upper mainly shallow marine facies.
It is herein proposed that the Unayzah Formation to be divided into two members; a lower member, the Shajara member and an upper member, the Safra member. The term Shajara Member is proposed to include the lower unit, Permo-Carboniferous siliciclastics above the regional Hercynian unconformity with various older units ranging in age from Devonian to Precambrian and below the sequence boundary.
The term Safra Member is proposed to include the upper unit, Late Permian siliciclastics and minor carbonates, and evaporates, above the sequence boundary and below the base of the well defined limestone beds of the Khuff Formation. The heterogeneous shallow marine and continental porous sandstones of the two members form the Unayzah Reservoirs which host great quantities of oil, condensate, and natural gas in several oil and gas fields in the Greater Arabian Basin. These sandstones are becoming a primary exploration target and their stratigraphy has therefore attracted considerable attention.