The Paleozoic petroleum system of the Central and Northern Provinces of Saudi Arabia contains one of the most prolific oil-prone source rocks of Silurian age worldwide, the Qusaiba shale of the Qalibah Formation. This source rock is responsible for charging the large, gentle structures of the Central Province with billions of barrels of light, sweet, crude oil. Organic geochemical methods such as biomarkers, organic petrography, and characterization of the source rock facies have been used to determine the most likely candidate source rock far the Paleozoic oils of Saudi Arabia, By using detailed extract and oil characterizations, the basal "hot" shales of the Qusaiba Member of the Silurian Qalibah Formation have been correlated to the oils reservoired in the Permian sands of the Central Province of Saudi Arabia. The Qusaiba shales consist of light to dark gray, elastic, marine shales and can be divided into two main zones : a low gamma-ray response and a high gamma-ray response, both having their own organic geochemical signatures. The low gamma ray zone has almost no source potential and comprises most of the Qalibah shale sequence. The shales within this sequence are light gray, have poor to moderate organic richness and little to no generative potential, and contain an organic matter assemblage deposited under mostly oxic, marine conditions. The organic matter type consists of amorphous matter, marine algae (phytoplankton), liptodetrinite, inertinite, and occasional chitinozoans and graptolites. The hot gamma-ray zone is located at the base of the Qusaiba Member of the Qalibah Formation and consists of dark gray to black, organic-rich, oil-prone, marine shales. The kerogen assemblage consists of abundant amorphous matter, marine algae, acritarchs, and abundant chitinozoans and graptolites. Carbon isotopes and sterane biomarkers resulted in an excellent correlation between the basal Qusaiba shales and the oils. No other potential source rocks correlated as well, nor did these other potential sources contain the source rock quality, or vertical and lateral regional persistence as the Silurian basal Qusaiba shales.