Petroleum geochemistry of the Albian-Turonian Sarvak reservoir in one of the oil fields of southwest Iran

Organic geochemical investigations using thin layer chromatography-flame ionization detection (TLC-FID), Gas Chromatography (GC), Gas Chromatography-Mass Spectrometry (GC-MS), API gravimetry, elemental analysis, and isotope-ratio mass spectrometry (IR-MS) were carried out on eleven oil samples from the Sarvak reservoir in the Abadan Plain (SW Iran). Oil chemical composition, source, thermal maturity, age, lithology, and depositional environment of these oils’ source rock were determined in this study. Moreover, Sarvak oils are mainly naphthenic and paraffinic type. Their API degree between 16.2 and 20.14 and about 4.6% sulfur content indicate heavy and sulfur-rich oils. The results of the study of biomarkers, stable carbon isotope composition, trace elements, aromatic and sulfur content indicated that all oil samples are related to a marine-carbonate source rock with strongly anoxic conditions. The absence of oleanane in all oil samples, the variation of Pr/Ph versus δ13C of the whole oil, and C28/C29 steranes versus geological age proved that these oils had been produced earlier than the Late Cretaceous. Furthermore, the distribution of n-paraffins, calculation of Rc (%) from aromatic compounds, CPI (Carbon Preference Index) from gas chromatograms, and biomarker maturity indices indicated that the Sarvak oils are mature. Although the Sarvak oils are heavy, they show approximately maturity of peak oil-generative window, which represents a challenge in this study. It is guessed that the high sulfur content and low API gravity in the Sarvak reservoir oils are due to the presence of sulfur-rich organic matter (type IIS kerogen) in the source rock.