Geochemical assessment of upper Cretaceous crude oils from the Iranian part of the Persian Gulf Basin: Implications for thermal maturity, potential source rocks, and depositional setting

The Upper Cretaceous carbonate successions of the Sarvak Formation host giant oil reservoirs in the Persian Gulf. In this research, a total of 28 oil samples from nine oilfields located in the western, central and eastern parts of the Persian Gulf region were studied to determine the genetic relationships of oils, depositional setting of possible source rocks, thermal maturity, and source-rock ages in the Persian Gulf basin. According to the measured geochemical data, the source rocks facies vary from marine carbonates and marl/carbonates in the central and eastern oilfields to shale/carbonates in the western oilfields. The Pr/Ph ratio, steranes and terpanes suggest anoxic to dysoxic conditions of the depositional environments. The depositional environments experienced both low water stratification/low salinity and normal salinity/unstratified conditions. Evaluation of the saturated and aromatic biomarkers shows that all oil samples are mature and most of the source rocks lie within the beginning of the oil-generation window. The thermal maturity of the central oilfields is higher than that of the other samples, and has gone beyond the oil-generation stage. The C₂₈/C₂₉ steranes ratio suggest that the central oilfields of the Persian Gulf have Paleozoic and Jurassic source rocks, whereas the Sarvak reservoir in other parts of this region is sourced from Cretaceous carbonate rocks.