Using high-resolution sequence stratigraphy for field-scale characterization of Permo-Triassic Khuff carbonate reservoirs, Ghawar field, Saudi Arabia
Raid K. Al-Dakhil1, Ghazi A. Al-Eid2, Aus A. Al-Tawil3, Rick R. Davis4, and Shoaib M. Rawasia4.
(1) Reservoir Characterization Department, Saudi Aramco, P.O. Box 6432, Dhahran, 31311, Saudi Arabia, phone: (9663) 872-0656, [email protected], (2) Reservoir Characterization Department, Saudi Aramco, P.O. Box 6875, Dhahran, 31311, (3) Reservoir Characterization Department, Saudi Aramco, Dhahran, (4) Reservoir Characterization Department, Saudi Aramco, Dhahran
The Khuff Formation overlies the Permo-Carboniferous siliciclastic Unayzah reservoir bearing, and is overlain by the fine siliciclastics of the Triassic Sudair Formation. The Khuff C carbonate reservoir contains three high-resolution sequences; whereas the Khuff A and B carbonate reservoirs are each made up of one bearing (third-order?) sequence. Sub-tidal, high-energy, and open-marine facies, overprinted by early diagenetic fluids form high porosity reservoir compartments. Intra-formational seals are anhydritic/dolomitic carbonate coastal successions and form thick, non-reservoir intervals between the Khuff C and B, and the Khuff B and A, during longer-term possible third or second-order highstand system tracts (HST), overprinted by extreme inland climatic aridity, coincident with the assemblage of the supercontinent Pangea. Sequence and cycle-set boundaries are marked by exposure surfaces during periods of sea-level drops, which in turn have a corresponding gamma-ray signature on wire-line logs. Using these regional markers on over 15,000 ft (5,000 m) in 50 cored wells and wire line logs in 200 closely-spaced wells, allowed mapping high-resolution (fourth order?) sequences, as well as deciphering of onlap geometry of the high-frequency transgressive systems tract (TST) and prograding geometry of the high-frequency HST. Glacio-eustacy during the Permo-Triassic times, which are transitional from global ice-house to global green-house times, resulted in moderate amplitude sea-level fluctuations, giving rise to regionally mappable, high-resolution (4th-order) sequences and their component systems tracts and cycle sets. This controlled the vertical partitioning of reservoir facies within Khuff C, B, and A carbonates. The active Ghawar structure during the Permian, lead to lateral partitioning of these facies from proximal on the crest, to distal on the flanks. Furthermore, subtle highs and saddles along the crest, furthered the lateral portioning of the reservoir. Reservoir facies formed during the retrograding oolitic shoal belts of the late TST (Khuff A, B, and C) and the shallow subtidal burrowed facies of the prograding HST of each of the high-resolution sequences (Khuff C only). This high-resolution stratigraphy allowed very precise mapping of facies down to the cycle-set level at a very high resolution of 15 ft (3 m) at the scale of the Ghawar field.
Al-Dakhil, R.K., et.al., 2004, Using high-resolution sequence stratigraphy for field-scale characterization of Permo-Triassic Khuff carbonate reservoirs, Ghawar field, Saudi Arabia: GOE2004.