Paleozoic (Pre-Khuff Hydrocarbon Geology of the Ghawar Area, Eastern Saudi Arabia

Abstract

Saudi Aramco is conducting an exploration program to discover additional non-associated gas reserves in the Ghawar Area. The program has successfully discovered significant sweet gas and condensate reserves in the pre-Khuff siliciclastics and has further increased our understanding of the Paleozoic petroleum system. The Lower Permian Unayzah Formation is the principal pre-Khuff hydrocarbon reservoir in the Southern Ghawar Area, where it contains both oil and gas. The Unayzah consists of fluvial to marginal marine sandstones. The Devonian Jauf Formation is the principal pre-Khuff reservoir in the Northern Ghawar Area, where it hosts the recently discovered giant Hawiyah gas-condensate field. The Jauf consists of shallow marine sandstones that exhibit unusually high porosities considering the burial depths. The primary source rock for pre-Khuff hydrocarbons is the basal “hot shale” of the Lower Silurian Qalibah Formation. Maturation modeling of these shales indicates hydrocarbon generation began in the Middle Triassic (oil) and continues to the present (dry gas). Pre-Khuff hydrocarbon traps are found in simple four-way closures as well as more complex structural-stratigraphic traps on the flanks of Hercynian structures. Trap formation and modification occurred in four main phases: (1) Carboniferous (Hercynian Orogeny); (2) Early Triassic (Zagros Rifting); (3) Late Cretaceous (First or Early Alpine Orogeny); and (4) Tertiary (Second or Late Alpine Orogeny). Structures in the Ghawar Area show differences in growth histories, which have impacted the amount and type of hydrocarbons contained.