Online Journal for E&P Geoscientists

Mousa, Yusuf H.^*1; Read, J. Fred ²
(1) RCD, Saudi Aramco, Dhahran, Saudi Arabia. (2) Virginia Tech University, Blaksburg, VA.

The Upper Jurassic Arab-C member represents one of the major reservoir intervals in the Abqaiq field of Saudi Arabia. Lithofacies distribution has a strong influence on reservoir quality and reservoir development strategies. Based on extensive core studies, facies and sequence stratigraphic frameworks were constructed to provide a basis for 3D geocellular models and the development plans.

Arab-C carbonates and the overlying anhydrite in Abqaiq field are part of an overall shallowing upward, 3rd order composite sequence that was deposited during the late Jurassic period. This sequence is composed of four higher frequency carbonate sequences (HFS-1, HFS-2, HFS-3, and HFS-4), followed by two evaporate dominated higher frequency sequences (HFS-5 and HFS-6). HFS-1, the basal unit of the Arab-C member unconformably overlies the Arab-D evaporates, and commences with a basal transgressive lag unit composed of a mixture of rip-up interclastic carbonates and associated thinly bedded ooid grainstone and pellet packstone/wackestone. Overlying these, are cyclic ooid grainstone with cerithic gastropods, probably representing deposits of a high energy shoreline such as beach ridges and shoals. HFS-2 consists of algal laminated wackestone with anhydrite nodules. HFS-1 and HFS-2 together represent the transgressive deposits of the Arab-C composite sequence and the top of HFS-2 makes the turn-around in deposition (MFS) that subdivides the Arab-C into a “lower coarse” section and an “upper fine” section. Thrombolites also mark the earliest highstand deposition at the base of HFS-3, which together with HFS-4 represent progressively more restricted “muddy” lagoonal and tidal flat facies of early highstand deposits.

The uppermost highstand deposits of the Arab-C composite sequence (HFS-5 and HFS-6) are evaporite dominated, reflecting a severe basin-wide restriction. The lower part of HFS-5 has a thick carbonate-anhydrite section composed of a peloid packstone interval capped by dololaminites, and/or non-displacive sub-aqueous and displacive anhydrite, which is overlain by a relatively thick (up to 30 feet) dominantly anhydrite unit that forms the master seal to the Arab-C unit. HFS-6 is made up of subaqueous and displacive anhydrite layers interbedded with thin transgressive restricted mudstone or dololaminite layers (1-4 ft thick).