Online Journal for E&P Geoscientists

Al-Dukhayyil, Raed ^*1; Read, J. Fred ²; Al-Tawil, Aus ¹
(1) Saudi ARAMCO, Dhahran, Saudi Arabia. (2) Virginia Tech, Blacksburg, VA.

Over 1500 meters of core and wireline log data from 16 wells enabled mapping of high frequency sequences (HFSs) in the Permian upper Khuff C, and the Triassic Khuff B and A carbonate-evaporites. The lithofacies of this arid epeiric ramp succession include subaqueous laminated- and supratidal nodular- to massive anhydrite, tidal flat laminites, lagoonal mudstone, (with thrombolites in the Triassic), ooid-peloid grainstones, and subtidal off-shoal mudstone. These lithofacies form meter-scale parasequences, separated by exposure surfaces at sequence boundaries.

The non-reservoir uppermost Permian upper Khuff C succession, up to 105 m thick, consists of eight HFSs capped by the Permian-Triassic (P/T) unconformity. The transgressive systems tracts (TSTs) consist of grainstone-dominated parasequences, while the highstand systems tracts (HSTs) consist of dolomudstone-laminite parasequences and/or paleosols. The Triassic Khuff carbonates (up to 105 m thick), consist of Khuff B and overlying Khuff A sequences. The Khuff B sequence is made up of four HFSs. TSTs consist of off-shoal lime-mudstone and oolitic grainstone, commonly thrombolytic, parasequences whereas the HSTs consist of 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 P/T boundary in Ghawar is marked by regional exposure surface with a marked increase in total gamma ray values beneath the boundary and a significant decrease above it. The P/T boundary has a significant negative carbon excursion and the overlying thrombolytic Triassic section has an abrupt decrease in fossil and microfossil content resulting from the P/T extinction.

The study has revealed an ordered succession of HFSs that facilitate the understanding of the three dimensional distribution of lithofacies. This sequence stratigraphic framework provides insights into the distribution, prediction and exploitation of the Khuff carbonate reservoirs.