Organic geochemistry of crude oils from the Intisar oil field (East Sirte Basin, Libya)

Four crude oil samples from the oil fields Intisar A, Intisar D and Intisar E (Sirte Basin, Libya) were investigated in order to define depositional environment, lithology, thermal maturity and geologic age of the corresponding source rocks. Saturated biomarkers (n-alkanes, isoprenoids, steranes and triterpanes) were determined using gas chromatography – mass spectrometry (GC–MS) and gas chromatography – mass spectrometry – mass spectrometry (GC–MS–MS). Aromatic hydrocarbons (phenanthrene, methylphenanthrenes, methyl-dibenzothiophenes and trimethyl-naphtalenes) were analyzed by GC–MS. The Intisar crude oils were generated from siliciclastic source rocks containing a mixture of marine and terrestrial organic matter. Maturation parameters indicated high level of thermal maturity corresponding to the main phase of oil generation. The analysis of the age-specific biomarker ratios suggested Cretaceous, most probably Lower Cretaceous age for the Intisar oils. Two source rocks for the Intisar oils were identified, similar in the precursor organic matter type, lithology, maturity and geologic age but different in the redox conditions in the environment during their deposition. The oils generated from these different sources probably migrated over different migration pathways.