Organic geochemistry of crude oils and Cretaceous source rocks in the Iranian sector of the Persian Gulf: An oil-oil and oil-source rock correlation study

The marls and argillaceous limestones of the Cretaceous Kazhdumi Formation, Ahmadi Member of the Sarvak Formation and Gurpi Formation are considered to be important source rock candidates in the Persian Gulf. To evaluate their source rock characteristics, 265 cutting samples of these rock units from 20 fields in offshore Iran were analyzed using Rock-Eval pyrolysis, organic petrography, stable carbon isotope composition and biomarker analysis. 1D basin modeling was also applied to analyze the burial and thermal history of these source rock candidates. Based on the results, the Kazhdumi and Gurpi formations, which have fair source rock potential, and the Ahmadi Member, which has fair to good source rock characteristics, were deposited in a marine reducing environment with marine organic matter as the main input. Different maturity indicators showed that the Ahmadi Member and Kazhdumi Formation are thermally immature to mature and the Gurpi Formation is immature to early mature in the study area. The results of the modeling suggested that the Kazhdumi Formation and Ahmadi Member in the southeastern parts of the study area are within the main oil window and that the Gurpi Formation has just begun to generate hydrocarbons. All three formations are thermally immature in the eastern parts. In the western parts of the Persian Gulf, the Kazhdumi Formation is early mature and the other formations are immature. The biomarker and stable carbon isotope parameters of these rock units were compared with 20 crude oil samples in the southeastern Persian Gulf. Based on the hierarchical cluster analysis (HCA), principal component analysis (PCA) and carbon isotope composition, the studied crude oils were divided into two groups: Group-I oils from Cretaceous reservoirs were probably derived from Cretaceous source rocks, while Group-II oils from Jurassic and Cretaceous reservoirs with Jurassic rocks as the expected source rocks. The performed oil-source rock correlation shows a positive correlation between the Group-I oils and the Ahmadi Member.