Ghawar Arab-D Reservoir: Widespread Porosity in Shoaling-upward Carbonate Cycles, Saudi Arabia
Robert F. Lindsay, Dave L. Cantrell, Geraint W. Hughes, Thomas H. Keith, Harry W. Mueller, III, S. Duffy Russell, 2006. "Ghawar Arab-D Reservoir: Widespread Porosity in Shoaling-upward Carbonate Cycles, Saudi Arabia", Giant Hydrocarbon Reservoirs of the World: From Rocks to Reservoir Characterization and Modeling, P. M. (Mitch) Harris, L. J. (Jim) Weber
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Ghawar field is the world's largest, most prolific field, producing 30-31° API oil from the Arab-D carbonate reservoir. The field is more than 250 km (155 mi) long, as much as 30 km (18.5 mi) wide, and has more than 300 m (1000 ft) of structural closure. The Arab-D reservoir, limestone with some dolostone horizons, stratigraphically comprises the D member of the Arab Formation and the upper part of the Jubaila Formation. Based on ammonite andbenthonic foraminiferal evidence, the reservoir formations are Upper Jurassic, Kimmeridgian, in age. The reservoir has an average thickness of more than 60 m (200 ft), an average porosity of more than 15%, and a permeability up to several darcys. The upper half of the reservoir is dominated by exceptionally high reservoir quality; the lower half contains interbeds of high and relatively lower reservoir quality. Early correlation of well logs and cores, before the advent of sequence stratigraphy, subdivided the reservoir from top to base into zones 1, 2,3, and 4. Zones 2 and 3 have been subsequently subdivided into zones 2a and 2b and zones 3a and 3b, with a more detailed zonation scheme for reservoir management.
The reservoir is composed of two composite sequences. One composite sequence is the Arab-D Member of the Arab Formation, with the upper boundary at the top of Arab-D carbonate and below the C-D evaporite, with the sequence boundary locally marked by pods of collapse breccia. The second composite sequence forms the upper part of the Jubaila Formation, for which the sequence boundary between the Arab-D Member and Jubaila Formation is located in zone 2b and is marked by a flood of slightly deeper water cycles over the more grain-dominated cycles in upper zone 3 a and lower zone 2b. Several high-frequency sequences (HFSs) have been identified, each comprising several cycle sets (parase-quence sets). Each cycle set is composed of approximately five individual carbonate cycles (parasequences), and each cycle is composed of one to three beds.
These carbonates were deposited approximately 5° south of the equator on a broad, arid, storm-dominated carbonate ramp. From upslope to downslope, the ramp consisted of the following subenvironments: (1) inner ramp; (2) ramp-crest shoal; (3) proximal middle ramp; (4) distal middle ramp; and (5) outer ramp. The inner ramp was a lagoon with localized intertidal islands composed of grainstones and packstones and a highly diverse, shallow-marine benthonic foraminiferal microfauna. The distal or seaward part of this regime consists of packstones characterized by dasyclad and encrusting algae. The ramp-crest shoal is composed of skeletal and oolitic grainstone, mud-lean packstone, and some mud-rich packstone. Skeletal sands of micritized foraminiferal tests and broken skeletal detritus also include larger fragments of transgressive and storm-derived stromatoporoids and corals. The proximal middle ramp is composed of domed and encrusting stromatoporoid-coral mounds and intermound sheltered areas dominated by branched stromatopo-roids. The distal middle ramp, deposited below fair-weather-wave base, is composed of micritic to very fine-grained sediment capped by Thalassinoides firmgrounds. These firmgrounds are overlain by storm-derived rudstone and floatstone of inner-ramp, ramp-crest, and proximal middle-ramp bioclasts. The outer ramp is composed of deeper shallow-marine deposits of micritic to very fine-grained sediment capped by Thalassinoides firmgrounds. In this setting, smaller, benthonic forami-nifera are common along with tetraxon and triaxon sponge spicules and coccoliths.
From highest to lowest reservoir quality, the lithofacies or rock types consist of (1) skeletal-oolitic grainstone, mud-lean packstone, and some mud-rich packstone; (2) stromatoporoid-red and green algae-coral rudstone and float-stone; (3) Cladocoropsis rudstone and floatstone; (4) dolomite, porous and locally extremely permeable to nonporous; (5) bivalve-coated grain-intraclast rudstone and floatstone; and (6) micritic to very fine-grained deposits.
Limestone porosity is a mixture of the following common pore types: interpar-ticle (dominant), moldic (common), intraparticle (common), and microporosity. Less common is porosity associated with Thalassinoides burrows, with vertically oriented tunnels filled by grain-rich sediment. Shelter porosity is uncommon. Do-lostone porosity, less common than the major limestone pore types, is a mixture of moldic, intercrystalline, and (least common) intracrystalline porosity. Fractures (least common) do not contribute much porosity but contribute permeability.
Diagenetic effects common within Arab-D reservoir carbonates include several dissolution events, recrystallization, and physical compaction. Cementation, episodes of dolomitization, and chemical compaction-stylolitization, although locally important, were less abundant events.
The vertical seal for the reservoir is the overlying Arab C-D anhydrite. It is more than 30 m (100 ft) thick and is composed of varvelike laminae to very thin beds of anhydrite (thicker) and carbonate or organic matter (thinner) deposited in a salina. The salina periodically shallowed upward into peritidal and intertidal settings. A few porous-permeable carbonate stringers were deposited when relative sea level rise flooded the evaporitic shelf and temporarily restarted the sub-tidal carbonate factory, whereas relative sea level fall reestablished the subtidal brine factory and precipitated more evaporites.