oa Ghawar Arab-D Reservoir: Widespread porosity in shoaling-upward carbonate cycles, Saudi Arabia

The Arab-D reservoir in the Ghawar field is composed of two composite sequences in outcrop and the subsurface, which are the D-Member of the Arab Formation and the upper Jubaila Formation. The upper composite sequence boundary is marked by local collapse and transported breccia into small sinkholes and is overlain by transgressive, subaqueous anhydrite. The lower composite sequence boundary is marked by transgressive, middle ramp, stromatoporoid and coral-rich cycles over ramp crest grainstone cycles. Deposition was on a broad, arid, storm-dominated carbonate ramp, which consisted of: (1) inner ramp (lagoon with localized intertidal islands and a diverse benthonic foram microfauna); (2) ramp crest shoal (skeletal-oolitic grainstones/packstones);<br>(3) proximal middle ramp (stromatoporoid and coral biostromes and mounds with sheltered areas of Cladocoropsis banks); (4) distal middle ramp (mud-rich sediment with firmgrounds overlain by storm-derived rudstone/floatstone deposits); and (5) outer ramp (mudrich sediment with sponge spicules, smaller benthonic forams and firmgrounds, with no storm-derived sediments covering the firmgrounds). The ramp-crest shoal and proximal middle ramp facies, abundant in the upper composite sequence and at the top of the lower composite sequence, contain the best reservoir quality. Limestone, the dominant lithology in the reservoir, contains a mixture of interparticle, moldic, intraparticle, and micro porosity. Dolostones, the subordinate lithology within the reservoir, form thin to thick strata-bound beds and contain moldic, intercrystal and intracrystal porosity. Fractures are locally present. Diagenetic effects include widespread dissolution, recrystallization, physical compaction, and slight cementation. Dolomitization is mostly strata-bound within transgressive mud-rich cycles and along vertically oriented fractures.