Late Jurassic Jubaila Formation Storm-Dominated Cycles of Central Arabia: Outcrop Expression of a Reservoir in the Arab Formation
Lindsay, Robert F.; Cantrell, Dave L.; Hurley, Neil F.; Al-Dhubeeb, Abdullah G.; Al-Ibrahim, Abdullah A.
The Late Jurassic (Kimmeridgian) reservoirs of the Arab Formation form the most prolific supergiant oil fields in the world. Outcrops of upper Jubaila Formation strata in central Arabia are equivalent in age, lithofacies, and reservoir architecture to some of the reservoirs of the Arab Formation in the subsurface. These outcrops were studied to improve the understanding of vertical and lateral facies relationships, cyclicity, heterogeneity, reservoir quality controls, and to constrain and condition subsurface static and dynamic models. During the Late Jurassic, Eastern Arabia, Western Iran, and Iraq formed a broad, stable, shallow marine carbonate platform that contained a series of intra-shelf basins adjacent to the Neotethys Ocean. The Gotnia Basin, to the north, was separated by the Rimthan Arch and Summan Platform from the Arabian Intershelf and Rub' al Khali basins to the south; the Arabian Shield was to the west and the Qatar-Surmeh High was to the east. Overall, the paleoclimate was arid and hot. Outcrops of the Jubaila reveal a succession of cyclic, mud-rich and grain-rich rocks. Coarse-grained grainstone to rudstone contain intraclasts, peloids, coated grains and ooids, with skeletal constitutents including bivalves, foraminifera, stromatoporoids, and corals. Mud-rich, bioturbated beds contain bivalves, some foraminifera, pellets, and peloids, and display firmground caps. Cycles are 0.5-1 m thick and separated by sharp erosional surfaces, with variable amounts of incision causing cycle thickness to vary. The lower part of the succession, showing an initial upward muddying pattern, culminates in a thick fine-grained muddy interval that was dolomitized, calcitized, and underwent Recent karst-related cave development. The upper part of the succession, showing an upward increase in grain-dominated beds, contains large-scale, chaotic, non-stratified, rudstones that contain large domed stromatoporoid and coral clasts that are 1-3 m thick. All stromatoporoids and corals were storm reworked and bored, with only one thin bed containing stromatoporoids in growth position. Swaley bedding in both mud-dominated and grain-dominated beds suggests deposition below fair-weather wave base, but within storm wave base. Storm events produced channels that were filled by debris transported from shallow water settings. The increase in grain-dominated sediment up-section suggests an upward-shoaling succession.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013