oa Mishrif Formation oil biomarkers used to assess hydrocarbon generation, migration path and accumulation sites in the Ratawi, Zubair, Rumaila North and South oil fields, southern Iraq

The reservoir rocks of the Mishrif Formation in Iraq have a porosity of between 25% and 28%. Oil analysis indicated that the source rocks have the following properties: (1) carbonate rocks inter-bedded with shale deposited in reduced marine environment with low salinity and type II kerogen. This is based on biomarkers of Carbon Preference Index (CPI), Phytane, Pristane, n-alkanes of steranes and hopanes, as well as Early Cretaceous in age based on the sterane ratio of C28/C29 and C26/C29 (C=Carbone), the presence of the dinoflagellate cyst species Pseodoceratium pelliferum, and the ratio CPI, TS/ (Ts+Tm), Pr/nC17 and Ph/nC18 biomarkers, and the brown colour of the dinoflagellate cysts and Tasmanites under light microscope. This oil analysis indicated that<br>most of the oil accumulated in the Mishrif Formation might have been sourced from the Upper Jurassic Najmah and Sargelu and the Lower Cretaceous Sulaiy formations. Palynological analysis was conducted on samples taken from cores. Based on the abundance of amorphogen palynofacies, increasing foraminiferal test linings, and the thermally mature high total organic carbon (up to 8%) it appears that the main oil expulsion was mainly from the Sulaiy Formation. This formation was deposited in a distal suboxic-anoxic basin. Moreover, pyrolysis analysis of these core rocks indicated oil and gas generation from the Sulaiy Formation. This conclusion was based on their position in the diagram of Van Krevlen, scattered remains of hydrocarbon with TOC, and the hydrogen index with Tmax. The oil was generated and expelled during the Miocene from a depth of 5,000-5,357 m and charged the Mishrif reservoir in the traps formed earlier during the Late Cretaceous Period.