Depositional Environments, Lithofacies Types, and Reservoir Development of the Permian Khuff Formation in Eastern Saudi Arabia.
AL-JALLAL, IBRAHIM A., Saudi Aramco, Dhahran, Saudi Arabia
The Late Permian carbonates and evaporites of the Khuff Formation contain considerable amounts of gas in Ghawar field, eastern Saudi Arabia. Sedimentary structures, textures, and grain types were used to interpret the depositional environments and to define the different lithofacies types which determine the original reservoir facies distribution within this 1500-ft-thick formation.
There are three major depositional environments: subtidal, intertidal, and sabkha. Locally, these include shoals, belts, bars, bays, lagoons, ponds, tidal channels, sabkha islands, and sabkha pans. Tidal flooding and storms are represented. These environments represent four major transgressive-regressive cycles, which conform to four Khuff units: Khuff A, B, C, and D (from youngest to oldest). Transgressive phases (subtidal), with lithofacies of grainstones and horizontally burrowed mudstones usually comprise the reservoir facies. The regressive phases (intertidal and sabkha) with lithofacies of anhydritic mudstones and anhydrites, usually seal the Khuff reservoirs.
Lateral disappearance of reservoir facies are due to deposition or diagenesis. Depositionally, the lateral change in facies may occur, for example, from a non-reservoir anhydrite and mudstone facies to a reservoir grainstone facies, or from a non-reservoir anhydritic mudstones to a reservoir burrowed-mudstone facies. Diagenetically, ooid grainstones may be greatly cemented by anhydrite. Dolomitization may preserve the original porosity in grainstones. A combination of dolomitization and leaching may create intercrystalline porosity in mudstone. Leaching creates poorly connected moldic porosity in grainstones because the resulting calcite cement plugs the interparticle porosity.
AAPG Search and Discovery Article #91007© 1991 AAPG International Conference, London, England, September 29-October 2, 1991 (2009)