Incised Valley-Fills and Their Genetically Associated Lowstand Deltaic Sequences in the Late Permian Unayzah Reservoir of Saudi Arabia

The incised valley-fills and their genetically associated lowstand deltaic sequences were first reported by the author in the subsurface of the Upper Member of the later Late Permian Ash-Shiqqah Formation of Saudi Arabia. These very coarse- to fine-grained sandstones with excellent reservoir qualities form the uppermost part of the stratigraphically and genetically heterogeneous Permo-Carboniferous Unayzah Reservoir, and produce significant amount of sweet gas and light oil. Immense amount of data obtained from the measured sections at outcrops and from the exploration and production wells and large amount of cores with the help of palynologic studies provided valuable information on the stratigraphy, sedimentology and reservoir quality of these successions (Senalp and King, 1995; Senalp and Al-Duaiji, 1995; 2001; Senalp 2006). Their formation occurred during the opening phase of the Neo-Tethys Ocean. Stratigraphically, the Ash-Shiqqah Formation extends between the Unayzah Formation at the base and the Base Khuff D Carbonates at the top, representing the MFS P20 (260.4 Ma). It is separated from the underlying continental red bed Unayzah Formation by a thick caliche-formed crust of Pre-Ash-Shiqqah Unconformity surface. The Ash-Shiqqah Formation correlates with the Kaş Formation, and the Khuff Formation correlates with the Gomanibrik Formation of southeast Turkey.