The Later 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 face is distribution within this 1,500-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. 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. Later 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, ooidmore » grainstones may be greatly cemented by anhydrite. 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.« less