Geochemistry of the Upper Jurassic Tuwaiq Mountain and Hanifa Formation Petroleum Source Rocks of Eastern Saudi Arabia

Abstract

Thick, regionally extensive, laminated organic-rich lime mudstone units are present within the late Jurassic Hanifa and Tuwaiq Mountain Formations. These potential source rocks were deposited during the late Callovian to early Kimmeridgian within a relatively short-lived intra-shelf basin, the Arabian basin. This basin formed on the northeastern continental shelf of the Afro-Arabian plate, in what is now eastern Saudi Arabia, as a result of relative sea level rise in combination with differential subsidence and/or local structuring within the shelf. The Arabian basin was at least partially separated from the open neo-Tethys ocean by flanking paleo-highs composed of grainstone shoal/barrier island facies. The source rocks, defined as units having TOC > 1%, have an average TOC content of about 3%, with contents as high as 13%. Total pyrolytic yield (S1 + S2 from Rock-Eval) is as high as 88 mg HC/g rock, with an average yield of 25 mg HC/g rock, indicating excellent source rock potential. Hydrogen indices of thermally immature rocks are between 600 and 800 mg HC/g TOC, which indicates an oil-prone kerogen. The organic material is dominated by lamalginite, with subordinate amounts of vitrinite and inertinite, most of which is fluorescent. Kerogen from immature rocks isolated for elemental analysis plot as type II on a van Krevelen diagram. These results show that organic-rich units within the Hanifa and Tuwaiq Mountain Formations contain type II kerogen having excellent, oil-prone source rock potential. Major oil accumulations occur in several late Jurassic carbonate reservoirs in Saudi Arabia. Representative oils from these reservoirs show very similar chromatographic and biomarker fingerprints to bitumen extracted from the Hanifa and Tuwaiq Mountain Formation source rocks, suggesting that the oils were derived from these source rocks. The similar stable carbon isotope ratios between the oils (avg. δ13C = - 26.6‰) and the kerogen (avg. δ13C = - 26.4‰) and bitumen (avg. δ13C = - 27.1‰) is also consistent with an origin from the Hanifa and Tuwaiq Mountain Formations. The thermal maturation history of the Hanifa and Tuwaiq Mountain source rocks was calculated using kinetic models. Results indicate that oil generation and expulsion began about 75 Ma B.P. in the eastern part of the basin. By 50 Ma B.P. the oil kitchens had expanded westward and oil had started filling the broad, gentle structures that had formed during the late Cretaceous. Today, the Hanifa and Tuwaiq Mountain source rocks east of the Ghawar structure have passed through the oil generation window. The source rocks in the basin center are still within the oil generation window. In the western part of the basin, the source rocks are either immature or just starting to enter the oil window.