Strontium isotope analysis (87Sr/86Sr ratio) have been conducted on core samples representing, from top to base, the Hanifa, Araej, Izhara, Hamlah and Gulailah formations, onshore Qatar. They gave an overall age range from 157.80Ma to 233.30Ma, Oxfordian to Ladinian.
Deposition of the Gulailah and lower Hamlah formations (Ladinian-Norian) were took place in a lacustrine environment. The middle Hamlah Formation may have been deposited in shallow, normal marine environment, whilst the upper part was deposited in a hypersaline environment. A mixture of kerogen type II-III, oil and gas-prone is inferred for the Gulailah-basal Hamlah formations (TOC higher than 0.3%). Kerogen type IV, inert material is suggested for the upper Hamlah Formation. Thermal alteration index (TAI) is ranged from 3- to 3+, interpreted as peak oil to onset gas generation.
The Izhara, Araej and Hanifa formations are dominated with marine microplankton, dated early Hettangian to Oxfordian. They were deposited under shallow marine environment grading from oxic, dysoxic to anoxic shelf respectively. The organic matter is almost exclusively of marine algal origin. Izhara Formation is mature to over mature of kerogen type III-IV, gas-prone to inert material. Mature kerogen type II-III (oil and gas-prone material) is deduced for the Lower and Upper Araej, while the stylolitic limestone of the Uwainat Member has weak source. Mature kerogen type I-II, highly oil-prone, is possible for the black limestone Hanifa Formation. It is considered as the prolific petroleum source rock for the underlying and overlying reservoirs of the Uwainat (Bathonian) and the Arab “D” (Kimmeridgian) respectively.
Basins and Hydrocarbon Systems of the Gulf Region – E&P Status