Data from recent exploratory wells suggests a new Jurassic and Paleozoic petroleum system, in addition to the well-known Silurian and Jurassic petroleum systems in Saudi Arabia. The new petroleum systems are discussed with respect to their hydrocarbon characteristics, genetic relationships and potential source rocks. The new Paleozoic oil has lacustrine characteristics and is confined to the Carboniferous-Permian Unayzah Formation. The analyzed hydrocarbons are paraffinic with high pour point and characterized by high API. The new Jurassic source rock is assumed to be Callovian-Oxfordian in age and is believed to be the lateral equivalent of the Tuwaiq Mountain/Hanifa interval. The bulk data of the analyzed Hanifa oils shows low S, Ni, V contents, and high Pr/Ph ratios which are quite different from typical Arab/Hanifa oils of eastern Saudi Arabian Basins. Biomarker data of the new Jurassic oils suggest generation from an open marine environment with continental organic matter input (similar to Rub' Al-Khali and onshore UAE) rather than a highly restricted carbonate source rock, such as the Hanifa/Tuwaiq Mountain Formation which generated the Arabian Basin oils.
The Arabian basin is one of the most prolific oil-producing regions of the world. Approximately 25% of the world's known oil reserves are found in Saudi Arabia, of which approximately two-thirds are found in Jurassic carbonate reservoirs, the remainder occurs in Paleozoic and Cretaceous reservoirs. The principal source rocks for oils in the Upper Jurassic and Cretaceous reservoirs are the carbonate of the Jurassic Hanifa and Tuwaiq Mountain Formations (Carrigan et al. 1994).The source for the hydrocarbons in the late Paleozoic reservoirs is the marine clastic shales of the basal Silurian Qusaiba Formation (Abu-Ali et al, 1991; Cole et al. 1994).Optimal depositional and tectonic conditions converged during both of these periods to promote development of two world class petroleum systems with thick, effective oil-prone source rocks, extensive reservoirs, and good seals. Data from recent exploratory wells suggest new Jurassic and Paleozoic petroleum systems, in addition to the well-known Silurian and Jurassic petroleum systems in Saudi Arabia.