Paleozoic petroleum systems of Saudi Arabia: a basin modeling approach

Abstract

ABSTRACT The major Paleozoic petroleum system of Saudi Arabia is qualitatively characterized by a proven Silurian (Qusaiba Member, Qalibah Formation) source rock, Devonian (Jauf Formation), Permian and Carboniferous (Khuff and Unayzah formations) reservoirs, a laterally extensive, regional Permian seal (basal Khuff clastics and Khuff evaporites), and four-way closed Hercynian structures. Hydrocarbons found in these systems include non-associated gas in Eastern Arabia and extra light oil in Central Arabia. A basin modeling approach was used to quantify important aspects of the petroleum system. Firstly, seventeen regional wells were selected to establish a reference tool for the three-dimensional (3-D) basin model using multiple one-dimensional (1-D) models. This was accomplished by studying core material from source rocks and other lithologies for thermal maturity and kerogen quality. The major emphasis was on the Silurian section, other Paleozoic intervals and to a lesser extent on the Mesozoic cover from which only few samples were studied. Although vitrinite macerals, solid bitumen, and other vitrinite-like particles were not abundant in most of the investigated samples, enough measured data established valid maturity-depth trends allowing for calibrated models of temperature history. Sensitivity analyses for maturity support the view that thermal boundary conditions and Hercynian uplift and erosion did not greatly influence the Paleozoic petroleum systems. Secondly, a 3-D basin model was constructed using major geologic horizon maps spanning the whole stratigraphic column. This model was used to gain insight into the general maturity distribution, acquire a better control of the model boundary conditions and investigate charge, drainage, migration and filling history of the main Paleozoic reservoirs. The 3-D hydrocarbon migration simulation results qualitatively account for the present gas accumulations in the Permian-Early Triassic Khuff and Carboniferous-Permian Unayzah reservoirs in the Ghawar area. This kind of study illustrates the importance of basin modeling when used with other geologic data to describe petroleum systems. It provides a predictive exploratory tool for efficiently modeling hydrocarbon distribution from known fields. Real earth models can only be described in 3-D as pressure variations and fluid movements in the subsurface are impossible to address in 1-D and 2-D domains.