High-resolution (biomarker) geochemistry combined with other geochemical data has been used to investigate the genetic relationships of 14 crude oil samples from the Iranian sector of the Persian Gulf, and to provide information on the lithologies, palaeo-depositional environments and probable ages of the respective source rocks. Oil samples from the Resalat, Salman, Sirri A, Sirri C, Sirri D and Sirri E fields in the SE of the Gulf, and the Foroozan, Bahregansar, Hendijan, Abouzar, Doroud and Nowrouz fields in the NW of the Gulf were investigated. Results indicate that the studied oils belong to three genetic groups: (i) Group 1 oils (Resalat and Salman fields) were sourced from Jurassic or older, shaly source rocks deposited in a relatively oxic environment; (ii) Group 2 oils (Nowrouz, Doroud and Foroozan oilfields) originated from Jurassic or older carbonate-rich source rocks deposited in an anoxic environment; (iii) Group 3 oils, which, according to biomarker parameters, were probably sourced from Middle Cretaceous calcareous shales. Two subgroups are recognised: subgroup 3A oils were sampled from oilfields located in the NW of the Gulf (Hendijan, Bahregansar and Abouzar), and subgroup 3B oils came from the south (Sirri A, Sirri C, Sirri D and Sirri E fields).