The influence of facies and depositional environment on the occurrence and distribution of carbazoles and benzocarbazoles in crude oils: a case study from the Gulf of Suez, Egypt

Two major hinge zones separate the Gulf of Suez (GOS) of Egypt into three tectonic sub-provinces, namely the northern, central and southern sectors. Based on bulk and molecular parameters, the crude oils from the GOS can be subdivided into three groups according to their geographical origin from the three sectors. The geochemical characteristics of the oils are consistent with inferred depositional environments of anoxic marine carbonate facies in the central sector, compared to a more oxic environment that has received an increased proportion of clastic input in the southern sector, while the northern is intermediate in character. Most of the oils cover a relatively narrow maturity range (0.86–0.98 Rc) in the peak oil window. The biomarker parameter Ts/(Ts+Tm) is controlled by variations of facies and depositional environment, but not by maturity. Concentrations of pyrrolic nitrogen compounds in the GOS crude oils are in general relatively low, but show a large range of variability. In crude oils of the northern sector a strong predominance of C4 and C5 carbazoles over the lower homologues is observed. Towards the south, the relative abundance of carbazole and the methylcarbazoles increases successively. Variations within the methylcarbazoles and C2-carbazoles are rather small and, therefore, are not useful for a straightforward classification of oil families. However, atypical patterns in some of the oils, particularly from the northern sector, give evidence for contributions from yet unidentified local sources, which might not be of general importance in the GOS petroleum province. Benzo[b]carbazole is near or even below detection limit in the GOS crude oils. The benzocarbazole a/(a+c) ratio shows a consistently positive correlation with API gravity, Pr/Ph and Ts/(Ts+Tm) and a negative correlation with the sulphur content. Based on these observations, it is inferred that facies and depositional environment of the relevant source rocks are the major factors influencing the benzocarbazole distributions. In the central sector of the GOS, in which oils are predominantly sourced from carbonates deposited under strongly reducing conditions, a/(a+c) values are mainly below 0.5. In the southern sector where source rocks were deposited under less reducing conditions and received significant clastic input, as well as in the northern sector, the a/(a+c) values are mainly above 0.5.