oa Cause, timing and distribution of GOR/CGR variations in Central Saudi Arabia and its consequences for prospectivity in the western Rub’Al-Khali Basin

Major controls of GOR/CGR (Gas/Oil and Condensate/Gas ratios) in Saudi Arabia’s petroleum systems include water washing. In central Saudi Arabia, oil and gas/ condensate in the Unayzah reservoirs exhibit variations in GOR in the range of 150 to 10,000 standard cubic ft/ barrel. Furthermore oils are extremely undersaturated with gas despite having gravities in the range of 44–47° API. Assessment of independently measured parameters used to determine water washing of crude oils has shown a strong relationship between water washing and GOR. Investigations on neotectonism affecting the Arabian Plate showed that an important denudation of the shield from the Miocene Hofuf Formation, onwards (about 12 Ma), documented by a huge fan delta southeast of Ghawar field and the thick sedimentary sequences in the foreland basin. After a Pliocene localized fore-bulge uplift phase on the eastern flanks of the Arabian Plate, the Pleistocene-Holocene exhibited an accelerating northeast tilting. This recent tectonic activity had profound repercussions on the hydrodynamics and consequently the water washing. Considering the Late Tertiary and Quaternary as the main timing for strong hydraulic flows, then the entry points of surface water to the Unayzah aquifers can be derived from the study of the drainage system on the Arabian Shield and the distribution of major Quaternary gravel deposits. On the western flank of the Rub’Al-Khali Basin, besides the Central Arabian case, two other areas were identified where severe water washing can be postulated.