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1 Clark Research Ltd, High Wycombe, UK dnclark{at}clarkresearch.ltd.uk
2 BP Exploration Operating Co. Ltd, Sunbury-on-Thames, UK
3 Qatar Petroleum, Doha, Qatar
Oil is present in the Upper Jurassic Arab A and Arab C carbonates of the Al Rayyan Field. The Arab A consists of porous limestone with thin units of tight dolomite at the top and bottom. Two reservoir intervals can be distinguished, separated by a slightly less permeable unit. The upper reservoir consists of stromatolites and cross-bedded peloidal-grainstones, whereas the lower reservoir consists predominantly of ooliticgrainstones. The less permeable unit comprises moderately cemented, cross-bedded peloidal-grainstones. Higher porosity and permeability generally occurs in the coarser grained, cross-bedded grainstones, where pores are mostly primary and intergranular; lower permeability is characteristic of grainstones with moderately developed early cement. Porosity is enhanced at some levels by the presence of leached intragranular pores, but these have little effect on permeability. The Arab C is strongly stratified and completely dolomitized. Three reservoir intervals can be recognized, separated by less permeable layers of poorer quality rock. The upper reservoir consists of partially leached and moderately cemented, cross-bedded bioclastic-peloidal-grainstones. Porosity is high because of the presence of leached intragranular pores in addition to primary intergranular pores but permeability is relatively low because of the presence of early cement. The main reservoir comprises loosely compacted and lightly cemented, cross-bedded peloidal-grainstones. Porosity is lower than that in the upper reservoir because leached intragranular pores are generally absent, but permeability is higher because cementation is relatively light. The lower reservoir is made up of peloidal-grainstones and coarse-grained bioclastic-grainstones. The upper and main reservoirs are separated by a thick interval of relatively tight, thinly bedded, strongly cemented peloidal-grainstones, whereas the main and lower reservoirs are separated only by a thin bed of tight, nodular anhydrite. The pore types and cements observed in the Arab C are similar to those of the Arab A, but dolomitization has resulted in an enhancement of permeability and capillary pressure properties compared to those of the Arab A. Thus, the overall water saturation of the Arab C is lower than that of the Arab A, and the displacement pressures and irreducible water saturations are also significantly lower than those of Arab A carbonates.
