Mesozoic and Tertiary source rocks and crude oils from six oilfields in the Persian (Arabian) Gulf (Hendijan, BahrgansarAbouzar, Nowruz, Dorood and Foroozon) were studied using a variety of organic-geochemical techniques. Biomarker characteristics were combined with other geochemical data to identify the source rocks which generated the oil in these fields and to reconstruct their depositional environments, and also to characterize the diagenetic and catagenic processes which have occurred. The analyzed oils show a wide range of densities (19 to 39 degrees API) and high sulphur contents. They were generated by Type II-S organic matter, they are not biodegraded and their maturity level is generally low. Two main oil groups were identified from statistical analysis and can be correlated with different source rocks using age-specific biomarkers and isotope data. Group I oils include those from the Hendijan, Bahrgansar and Abouzar fields and were probably generated by a mid-Cretaceous argillaceous source rock. Group 2 oils include those from the Nowruz, Dorood and Foroozan fields, and originated from Jurassic to Early Cretaceous carbonate-rich source rocks.