Organic and Carbon Isotope Geochemistry of Crude Oils from Ashrafi Field, Southern Gulf of Suez Province, Egypt: Implications for the Processes of Hydrocarbon Generation and Maturation

Abstract

Four crude oil samples representing the Miocene formations of Belayim, Kareem, and Nukhul, and the Pre-Miocene Nubia Sandstone of Ashrafi Field, offshore southern Gulf of Suez, Egypt. These crude oil samples were analyzed using a variety of organic geochemical techniques including both C7 and whole oil Gas Chromatography (GC), Gas Chromatography–Mass Spectrometry (GC–MS) in addition to stable carbon isotopes to the saturate and aromatic fractions. The organic geochemical results revealed that the Miocene and Pre-Miocene Nubia Sandstone oils generated from two different source rocks subjected to different maturation levels. The Miocene crude oil possesses geochemical properties rich in tricyclic terpanes and extended hopanes typical characteristics of oils generated from marine siliciclastic source rocks with angiosperm land plants input like Rudeis Formation as indicated from the oleanane index which exceeds 30% and low gammacerane index around 10%. Meanwhile the Pre-Miocene crude oil correlates with the marine carbonate sources of high salinity waters like Brown Limestone of Upper Cretaceous age as indicated from the oleanane index <10% and high gammacerane index >30%. The maturity parameters based on both ratios of 20S/(20S + 20R)-C29 ααα cholestane and TAS/(MAS + TAS) were found to be around 0.5 for the Miocene crude oils, meanwhile both the ratios for the Pre-Miocene crude oil were >0.5. The concluded results further support the very high maturation level of the Pre-Miocene crude oil than the Miocene crude of Ashrafi Field which concordant with the reservoir stratigraphic depths.