Hydrocarbon Characterization and Gas Potential of the Saudi Arabian Mesozoic and Paleozoic Petroleum Systems

By

 Mahdi Abu Ali¹

(1) Aramco, Dhahran, Saudi Arabia

 Saudi Arabia's Paleozoic and Mesozoic petroleum systems are discussed with respect to source rock generation potential and hydrocarbon geochemistry. Hydrocarbons found in both systems are extraordinarily distinct, suggesting the presence of an efficient seal separating the two systems. Carbon isotopes provided an excellent hydrocarbon-source rock correlation tool.

The Paleozoic petroleum system consists of the Early Silurian "hot" basal Qusaiba shales as the principal source rock. The main reservoirs are the Devonian and Permo-Carboniferous sands and carbonates. The basal Khuff clastics are the regional seal for the Permian Khuff and Unayzah reservoirs, and interbedded shales for the Devonian Jauf reservoir. Hydrocarbons found in this system include non-associated gas in Eastern Arabia and extra light oil in Central Arabia.

The Mesozoic petroleum system is made up of the Jurassic and Cretaceous sub-systems. Argillaceous carbonates of the Tuwaiq Mountain and Hanifa formations are the source rocks for the Jurassic system. Carbonates of the Arab, Manifa, Hanifa, Hadriyah and Fadhili are the main reservoirs. The pervasive Hith anhydrites are the major regional seal for the Jurassic system. Hydrocarbons found in this system are typically medium to light oil and associated gas. The Cretaceous system contains mainly two groups: light oil in the Shu'aiba carbonate reservoir of the Shaybah field, and heavy to medium oil in the Wasia clastic reservoirs of the offshore fields.

Based on basin modeling of the respective source rocks, Paleozoic hydrocarbon potential indicates gas in the east and south, and oil in Central Arabia, confirming the existing hydrocarbon finds. The Jurassic potential is oil for most of eastern Arabia and gas for southern Arabia. The potential for the Cretaceous section is mainly oil. Reconstruction of the basin's history along with hydrocarbon generation provided a regional interpretation of the oil and gas migration histories.