F. O. Meyer and *G.W. Hughes
Carbonate Research Consulting, 15450 Archer Terrace, Elbert, CO. 80106, USA, *Saudi Aramco, Box 5000, Geological R&D, X-4800, Dhahran 31311, Saudi Arabia Aramco Geological Research and Development, New Technology Division
Khuff Formation, C and B members, are stacked Late Permian to Early Triassic age carbonate gas reservoirs in Ghawar Field, Saudi Arabia. With challenging facies architectures, an understanding of how fluids accumulate in and pass through these reservoirs would benefit greatly from a high-resolution sequence stratigraphic investigation with a multidisciplinary (sedimentology, micropaleontology, and petrophysical) approach. Forming the interpretation foundations for this study is a total of 5,860 feet of core, 9,867 core plugs, and 3,200 thin sections from 18 wells. Khuff-C and –B reservoirs represent shoal water carbonate facies accumulations around antecedent and syndepositional structural topography in a mid-shelf setting. Observed vertical facies and textural patterns document a repetitive evolution in Khuff sedimentation from subtidal open shelf marine accumulations to prograding grainstone shoal complexes and associated tidal flat deposits. Corrosive and dolomitizing fluids that passed through the strata after their deposition add another level of complexity, which also has an intimate link to the localized topographic features and to sea level cycles having three different rhythms. Asymmetric cycle sets, which represent small-scale rises in relative sea level, divide the reservoir-hosting Khuff succession into thin correlation units less than 15 feet thick. Bundles of cycle sets (high frequency sequences), define sea level changes of an intermediate frequency that divide the reservoir into 13 time stratigraphic packages, 18-149 feet thick. The interval from top of the Khuff-D anhydrite up to the intra Khuff-B Permo-Triassic (P/T) boundary comprise Tatarian high frequency sequences (HFS) KHSP1.1, HSFP1.2, HSFP1.P3, HSFP1.P4, HSFP1. HSFP2.1, HSFP2.2 HSFP2.3, HSFP3.1and HSFP3.2. Induan HFST1.1, HSFT1.2, and HSFT1.3 make up the interval from the P/T boundary to the top of the Khuff Formation. Punctuating every HFS is a regionally correlative break in deposition, expressed by paleosol, karst, or other exposure horizons. Up to 30 feet of erosional relief characterizes the P/T boundary in the study area. Stacks of HFS (composite sequences), which define long-term rises in relative sea level, divide the cycle stack into four large-scale repetitive depositional episodes. Bounding the composite sequences are unconformities KCSBP1, KCSBP2, KCSBP3, KCSBT1 and the top of the Khuff.