LITHOSTRATIGRAPHY AND DEPOSITIONAL-ENVIRONMENTS OF THE UPPER JURASSIC ARAB-C CARBONATE AND ASSOCIATED EVAPORITES IN THE ABQAIQ FIELD, EASTERN SAUDI-ARABIA

Lithostratigraphy and depositional environment of the Arab-C carbonate in eastern Saudi Arabia were studied using cores and well logs from the Abqaiq oil field, A 275-m Upper Jurassic evaporite sequence occurs between the Sulaiy-Yamama (Thamama Group) and the Tuwaiq-Hanifa-Arab-D (Tuwaiq Group) carbonate sequences. Dolomite and limestone interbeds of 1-30 m in thickness within the evaporite sequence are continuous over long distances and have uniform thicknesses and log characteristics indicating an aggradational type of deposition. The lithologically heterogeneous Arab-C carbonate is the thickest: interbed, reaching 26-30 m in thickness. Lithology types recognized in the Arab-C carbonate are (1) ooid bioclast grainstone, (2) oolite grainstone, (3) pellet grainstone, (4) pellet packstone, (5) skeletal wackestone, (6) mudstone, (7) anhydritic limestone, and (8) dolomite. The Arab-C carbonate has been divided into seven layers based on lithology and porosity variations. Four layers (1, 3, 5, 7) are porous calcarenitic carbonates and the other three layers are nonporous. A basinal (subaqueous) rather than a sabkha-type deposition is interpreted for the evaporite sequence. Lithological and sedimentological successions in the Arab-C carbonate indicate abrupt storm-type deposition and an open-marine connection with the restricted evaporite basin. This sequence was followed by a subtidal carbonate deposition and then a gradual upward return to an evaporitic environment. As a consequence of depositional environments, a massive, very porous interval developed in the lower Arab-C carbonate sequence, and alternating porous and nonporous layers developed in the upper Arab-C.