Aromatic Hydrocarbons Geochemistry of Crude Oils from East Baghdad Oilfield, Central Iraq

Abstract Aromatic biomarkers have been analyzed for the source, depositional environment, and maturity of crude oils from Cretaceous reservoirs (Zubair, Khasib, Tanuma, and Hartha) in the East Baghdad Oilfield. Crude oil samples from these reservoirs have a wide range of API gravity. Oil in the Zubair reservoir has the highest API gravity values, whereas oil in the Hartha reservoir has the lowest. Oils from the East Baghdad Oilfield are abundant in aromatic fractions, including naphthalenes, phenanthrenes, triaromatic steroids, and dibenzothiophenes. Geochemical ratios, such as the methyldibenzothiophene distribution, DBT/P, and 9-MP/1-MP, in addition to high sulfur content, show that the oils originate from marine organic matter deposited in reducing, carbonate-rich conditions. Thermal maturity assessments using the EN ratio, triaromatic steroid cracking ratio, and equivalent vitrinite reflectance (RC) from the MPI, TNR, and MDR indices suggest an early mature to peak oil window generation stage (VR0 from 0.69 to 0.88%). The biomarker ratios reflect that the oil samples have similar organofacies with slight differences in maturity. The most likely candidate source rocks are the Middle Jurassic Sargelu Formation and the Middle Jurassic–Lower Cretaceous Chia Gara Formation. This study concludes that oils in East Baghdad were charged at two stages: early mature heavy oils in the first stage and highly mature light oils in the second one.