Hydrocarbon Systems of Central, Eastern and Southern Saudi Arabia

By

Abdelghayoum Ahmed¹, Mahdi Abu Ali¹, J. Michael Moldowan²

(1) Aramco, Dhahran, Saudi Arabia (2) Biomarker Technology, Sebastopol, CA

 Hydrocarbons from Paleozoic and Mesozoic petroleum systems are assessed with respect to their source rocks potential and genetic relationships. Paleozoic petroleum system source rocks principally include Early Silurian basal Qusaiba hot shales. Mesozoic petroleum systems consist of Jurassic and Cretaceous systems. Argillaceous carbonates of Tuwaiq/Hanifa formations are the principal Upper Jurassic source rocks in eastern and southern Arabia. Middle to Upper Jurassic source units are confined to northern Arabia area. Cretaceous source system comprises of Wasia formation in southern Arabia and both Wasia and Sulaiy for offshore Arabia.

Paleozoic source rock potential indicates gas in the east and south, and oil in central Arabia, confirming the existing hydrocarbon finds. Hydrocarbons found in this system include non-associated gas in eastern Arabia and extra light oil in central Arabia. The Jurassic source potential is oil for most of eastern Arabia and gas for southern Arabia. Hydrocarbons found in this system are medium to light oil and associated gas. The source rock potential for the Cretaceous section is mainly oil. Hydrocarbons include light oil in southern Arabia and medium to light oil in the offshore areas.

Diamondoids, biomarkers and isotope data suggest that the Paleozoic oils are uncracked to lightly cracked in central area and southern Ghawar accumulations but are extensively cracked in north Ghawar field. In addition, some of the analyzed Paleozoic hydrocarbons show mixed origin as indicated by the presence of biomarkers and diamondoids. The possible hydrocarbon mixing from both Mesozoic and Paleozoic systems demonstrates migration complexity despite assumed effective seals.