Ghawar Arab-D: Widespread Porosity in Shoaling-Upward Carbonate Cycles, Saudi Arabia

Robert F. Lindsay, Dave L. Cantrell, Geraint W. Hughes, Thomas H. Keith, Harry W. Mueller, and S. Duffy Russell
Saudi Aramco, Dhahran, Saudi Arabia

The Arab-D reservoir in Ghawar Field (world's largest oil field) is composed of two composite sequences in outcrop and the subsurface, the D Member of the Arab Formation and the upper Jubaila Formation. The upper composite sequence boundary is marked by local collapse breccia and is overlain by transgressive subaqueous anhydrite (reservoir seal). The lower composite sequence boundary is marked by transgressive stromatoporoid/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 scattered intertidal islands and a diverse benthonic foram microfauna); 2) ramp-crest shoal (skeletal-oolitic grainstones/packstones); 3) proximal middle ramp (stromatoporoid and coral biostromes and mounds with sheltered areas of Cladocoropsis banks); 4) distal middle ramp (micritic sediments with firmgrounds overlain by storm-derived rudstone/floatstone deposits); and 5) outer ramp (micritic sediments with sponge spicules, smaller benthonic forams and firmgrounds with no storm-derived sediment covering the firmgrounds).

The ramp-crest shoal and proximal middle ramp facies, abundant in the upper composite sequence and abundant at top of the lower composite sequence, contain the best reservoir quality. Limestone, the dominant lithology in the reservoir, has a mixture of interparticle, moldic, intraparticle and micro porosity. Dolostone, mostly present as scattered, relatively thin strata-bound beds, contains moldic, intercrystal and intracrystal porosity. Fractures are locally present.

Diagenetic effects include widespread dissolution, recrystallization and physical compaction, and slight cementation. Dolomitization is mostly strata-bound within transgressive mud-rich cycles.