Isotopic characterization and genetic origin of crude oils from Gulf of Suez and western desert fields in Egypt

Stable carbon isotopes were used to asses the general characteristics of the western desert and Gulf of Suez crude oils in accordance with hydrocarbon generation, source rocks, thermal gradient and maturation level. The carbon isotopic results of all the analyzed oil samples in both areas lie in the range from -29.62 to -24.11 %. The av. σ 13C values in the Gulf of Suez reaches about -28.6% and -26.4% in western desert. It was accounted a marginal difference between the two areas by about 2.5 : 3% in carbon-13 isotope of the whole oil indicated two distinct oil types of different organic input and varies in the depositional environment. It was found that Gulf of Suez oils are dominated by marine organic matter (plankton algae) deposited in saline environment. The derived oils from the northern and central provinces of the Gulf are isotopically light, higher in sulfur content, lower in API gravity degree and have Pristane/Phytane (Pr/Ph) ratio less than or equal one (Pr/Ph = 1). In the southern province, about 0.5% isotopic enrichment was recorded in the produced oils from shallower depths, associated with gradual increment in API and maturity level as thermal gradient increase. However, low API gravity degree and less maturity of the Gulf of Suez oils could be related to the rifting temperature that forced and accelerated the expulsion rate and hydrocarbon generation prior reaching higher maturation levels. On the other hand, the produced oils from the western desert fields belong mostly to terrestrial organic debris (with minor marine fragment in some basins) deposited at deeper geological formations. It is characterized by isotopic enrichment, paraffinic waxy oils, low in sulphur content, have Pr/Ph = 1, high in API gravity and maturity level. Hydrocarbon generated from the western desert fields has been controlled by time-temperature effect in the source rocks and reservoirs where the humic organic matter are affected by high temperature over longer period of time during the burial history, where the oil kitchens are deep and thermally high that are capable for generating highly mature oil. This could enhance segregation of light hydrocarbon from the heavier oil molecules prior oil expulsion and hydrocarbon generation. These constant differences in burial history and organic matter influx specifying each area are the reason of the restricted isotopic results for each one. The deviation of oil characteristics from one environmental condition to the other reflects a mixing of those cases. This clear evidence of 13C isotopic signature implies easier specification of oil origin that could be used in other locations