Geochemical characteristics and genetic types of crude oils from a variety of geological reservoir rocks (Cretaceous–Palaeogene) in the Melut Basin, southern Sudan based on biomarker and carbon isotope composition

Abstract

This study uses physical and geochemical characteristics of crude oil samples representing the Cretaceous and Palaeogene reservoir sequence of the Melut Basin, southern Sudan to gain information about source rock properties, including organic matter characters, depositional environment and thermal maturity. The overall results show that the Melut Basin's oils have API values between 14.6° and 33.3° and indicate that most of oils are not biodegraded, except one heavy oil sample with low API gravity of 14.6°, which is likely biodegraded. The studied oils were generated from clay-rich source rocks, containing a blend of organic matter derived primarily from a mixed organic matter of aquatic and terrestrial origin and deposited in a lacustrine setting under suboxic to relatively oxic conditions as demonstrated by their biomarker and carbon isotopic compositions (d13C). The biomarker maturity indicators show that most of the Melut Basin's oils were generated in the main stage of oil generation, exhibiting a range of peak-mature to late-mature. The biomarker environmental indicators reveal that the shale source rocks of the Al-Gayger and Galhak formations contributed to these oil samples.