Geochemical variations among eastern Saudi Arabian paleozoic condensates related to different source kitchen areas

Large reserves of gas and condensate are found in Paleozoic reservoirs (Khuff, Unayzah and Jauf) in the Ghawar area of eastern Saudi Arabia. The source rock for these hydrocarbons is the basal organic-rich “hot shale” of the Qusaiba member of the Silurian Qalibah Formation. This is the same prolific source rock that generated the sweet light crude oil and condensate found in the Unayzah reservoirs in central Saudi Arabia. Thermal maturity modeling of the source rock identifies two kitchen areas: to the east and west of the Ghawar structure. The west kitchen was well into the oil window by 210 Ma, while the east kitchen was just entering the oil window. Large areas of both kitchens were in the wet gas window by 125 Ma and by 90 Ma most of the area north of Ghawar was generating dry gas. In eastern Saudi Arabia, Paleozoic oil and condensate are found in early-formed structures (Carboniferous and early Triassic growth), whereas late Cretaceous and Tertiary structures contain only dry gas. The geochemistry of the oils and condensates indicates two groups, each generated from a different hydrocarbon kitchen, suggesting that the two kitchens differ slightly in organic facies. Within each group, variations among hydrocarbons are related to the thermal maturity of the source rock during generation. Within the Khuff reservoir, gas souring is caused by thermochemical sulfate reduction.