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Upper Khuff Formation Carbonate Evaporite Sequences across the Permian-Triassic Unconformity in Ghawar Field, Saudi Arabia

Al-Dukhayyil, Raed K.1; Al-Tawil, Aus 2; Read, James F.1
1 Geosciences, Virginia Tech, Blacksburg, VA.
2 Reservoir Characterization, Saudi Aramco, Dhahran, Saudi Arabia.

Over 4,000 feet (1200 meters) of core and wire line logs in 12 widely spaced wells are used to map High Frequency Sequences (HFS) in the subsurface Khuff Formation in Ghawar, which is divided into five informal units: D, C & B/C of Permian age, and B & A of Triassic age.

The upper Permian, dominantly non-reservoir, Khuff B/C interval (up to 100 m thick) is capped by the Permian-Triassic Unconformity. This interval is made up of up to nine upward-shallowing HFS. Storm-influenced fore-shoal/shoal facies are transgressive in each HFS and commonly shallow up into restricted lagoonal dolo-mudstone and/or “salina” subaqueous anhydrite. These are capped by microbially laminated and mud-cracked dolo-mudstone tidal flats facies and/or exposure surfaces. Supra-tidal “Sabkha” anhydrites are rare, and grainstone facies are commonly anhydrite-cemented by early reflux processes, sourced from prograding tidal flats.

The up to 43 m thick Triassic Khuff B Sequence is defined at its base by the regionally mappable Permian-Triassic Unconformity; and is made up of three upward-thinning composite sequences. Each of the first two composite sequences is made up of three HFSs with transgressive distal lime-mudstone and/or grainstone shoals and prograding High Stand Systems Tract (HST) grainstone shoals with trailing restricted facies. The third composite sequence is made up of one high-frequency sequence with transgressive tidal-flats and their fringing grainstone shoals and prograding HST peritidal cycles with sediment-displacing “sabkha” anhydrite caps.

The Khuff A Sequence (up to 40 m thick) is made up of two HFSs. In Khuff A HFS1, supra-tidal capped peritidal cycles formed the TST, whereas sub-aqueous “salina” facies developed behind prograding grainstone facies during the HST. The sequence boundary for Khuff A HFS2 (KA HFS2) is defined at the base of its initial flooding facies. The HST off KA HFS2 is made up of prograding grainstone facies on the edges of the field. In front of the prograding margin, thick subaqueous anhydrite formed in localized depressions during the Low Stand Systems Tract. The topmost Khuff carbonates are transgressive open marine lime-mudstone, possibly marking the initial TST of the overlying Sudair Formation. The presence of extensive supratidal anhyrdrite in the TST and subaqueous anhydrite locally in the HST and LST of the Khuff A suggest regional restriction of the basin during the deposition of the Khuff A sequences.

 

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009