Khuff Platform - No modern same-scale counterpart

Khuff Anhydrite

Thick evaporitic carbonates and intercalated anhydrites of the Khuff Formation define a huge evaporitic platform (≈255 Ma) in the Arabian Gulf, with an upper section that straddles the Permo-Triassic boundary. In eastern Saudi Arabia, the Khuff Formation is divided into five units or members designated as Khuff A through E. However, throughout the Gulf region the Khuff may comprise as many as eight designated mappable subsurface units or members (KS1-7) Most of the Khuff Formation is composed of carbonates and evaporites; major siliciclastic facies increase towards the west and toward the base of the sequence, especially in Saudi Arabia. As many as six or seven grainy units are capable of acting as hydrocarbon reservoirs and correspond to depositional carbonate-anhydrite cycles where regressive anhydrites cap transgressive marine carbonates. The Khuff Formation thickens from about 260 m in southwestern Saudi Arabia to more than 915 m in the central Rub ‘al Khali Basin, 1,220 m in Oman, and 1,520 m in Iran.

Hith_basins

Major facies belts and their interrelationships in the Permian Khuff Formation of the Middle East (after Al-Jallal, 1995).

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Khuff Formation, Middle East. A) Sequences of the Khuff Formation and their equivalents in outcrop of the United Arab Emirates. Age determinations show the Middle Anhydrite was deposited in less than 0.5 million years (260-260.5 Ma). B) Palaeozoic regional cross-section, highlighting sequence stratigraphic framework of the Middle Permian to Early Triassic Formation.

Typical thicknesses for the Khuff ranges between 370 m and 490 m over most of central and eastern Arabia. The basal Khuff section consists mostly of siliciclastics throughout southern Arabia. The “basal Khuff clastics” in Saudi Arabia are made up of shale, carbonaceous shale, limestone streaks, and incised channel-fill sandstones. Individual channel-fill sandstone units of the basal Khuff clastics are as much as 18 m in thickness and can cut deeply into the underlying Unayzah Formation. In the Hazmiyah field of central Saudi Arabia, the reservoir occurs entirely within these channel sandstones in the basal Khuff. The Unayzah and basal Khuff sandstones form the principal reservoirs in central Saudi Arabia and extend north in Kuwait; however, widespread reservoir quality variations occur in these sandstone reservoirs owing to heterogeneous sedimentary facies and variable diagenetic intensity.

Over most of the rest of Saudi Arabia and west into Oman and the UAE, the Khuff is more than a 1 km thick and composed of numerous 2-10 m thick shoaling stacked dolomitised carbonate cycles or parasequences. These Late Permian dolomites are major gas reservoirs along the offshore in the Arabian Gulf north and west of Qatar. In contrast, the regionally extensive anhydrite, shale, and tight (low-permeability) carbonates of the Khuff Formation form the major regional seal for Pre-Khuff hydrocarbon accumulations in central and eastern Saudi Arabia.

Deposition of this thick carbonate succession was initiated as a ramp in the late Khazanian or early Tatarian and had evolved into a reef-rimmed evaporitic platform by the later Permian to early Triassic. Most of the cycles in the upper part of the Khuff are capable of producing gas and are variably dolomitised oolite-peloid successions, which typically grade upward into fine-grained laminites and nodular anhydrite saltern marker, pass westward via a belt of mixed detrital carbonate and siliciclastics onto the palaeohigh of the Arabian Shield. 

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