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Sequence Stratigraphy and Chemostratigraphy of the Upper Khuff Carbonates, Ghawar Field, Saudi Arabia

Abstract

Over 15,000 ft of core and facies prediction results, using a neural network method, in 550 wells (cored and uncored) enable mapping of high frequency sequences (HFSs) and facies distribution in the upper Khuff carbonates, Ghawar Field. The lithofacies of this huge, arid epeiric ramp succession include: subaqueous -and supratidal anhydrite, tidal flat laminites, lagoonal mudstone, ooid-peloid grainstone, and subtidal off-shoal open marine mudstone. Third order sequences include the Late Permian upper Khuff C, the Early Triassic Khuff B and the Khuff A sequences, which corrrelate with global cycles. Seven HFSs make up the Changhsingian upper Khuff C. The transgressive systems tracts (TSTs) of most of the HFS are grainstone-dominated, while the highstand systems tract (HST) deposits are dolomudstone, laminite and/or paleosols. The Early Triassic Khuff B sequence is composed of four HFSs, comprised of cyclic off-shoal lime-mudstone and oolitic grainstone, commonly thrombolytic (TST) and HST grainstone and peritidal carbonate-evaporite-, and laminated and nodular anhydrites parasequences, and laminated anhydrite parasequences. The lower Khuff A sequence contains a basal laminated anhydrite and has four HFSs with TSTs of evaporite, peritidal and oolitic carbonate parasequences, and HSTs that are oolite-prone and capped by laminites with breccia marking local exposure. The upper Khuff A sequence has digitate stromatolites at the base and is dominated by peritidal carbonate parasequences with thin grainstones. The sequence stratigraphic cross sections and facies maps of systems tracts illustrates the subtle interplay between the Ghawar structure and regional paleoslopes. This is evidenced by changes in directions of progradation from one high frequency sequence to the next. Southward progradation is evident in the Khuff B, while the Khuff A shows initial northward progradation, followed by southward progradation. Carbon and oxgyen isotope profiles were obtained from cored wells across the Permian-Triassic boundary. Major global excursions are at the Wuchiapingian-Changhsingian boundary and the Permian-Triassic boundary, but several smaller excursions also appear to correlate with excursions elsewhere. The presence of the negative C-isotope excursions globally in both δ13Ccarbonate and δ13C organic as well as in deeper water sections lacking emergence surfaces, strongly supports the idea of these excursions being global phenomena related global C cycling.