Petroleum and Source Rock Characterization Based on C7 Star Plot Results: Examples from Egypt

Abstract

The main types of Egyptian oils/condensates have been reviewed and characterized by light-end hydrocarbon, C7-based, geochemical star plots. The technique relies on high-resolution gas chromatography of the C7 compounds, and compositional differences are evaluated by using crossplots and multivariate polar plots. C7 results of oils/condensates are presented for the three main hydrocarbon provinces in Egypt: the Gulf of Suez, the Western Desert, and the Nile Delta and nearby Mediterranean. For the Gulf of Suez, there are two main oil types, which were derived from (1) Cretaceous-Eocene carbonate-rich and (2) Miocene siliciclastic-rich source rocks. For the Western Desert, there are three distinct oil types. Two of these oil types probably originated from Middle Jurassic carbonaceous/marine shale-rich source rocks, whereas the third oil was produced from Upper Cretaceous argillaceous limestones. For the Nile Delta, there are two types of oil. The majority of the oils/condensates from this region are derived from Tertiary shale-rich source rocks, but some may have a Cretaceous-Jurassic source. The given examples show that the C7-based star plot is very effective as a discriminating tool and that the C7 results are supportive and supplementary to other, more standard, geochemical oil/condensate correlation methods. The C7 data give reliable results over a wide range of thermal maturities, but some compounds may be sensitive to evaporative fractionation, water washing, and biodegradation. Special attention is given to what extent the C7 data provide reliable information on source rock typing. For the Egyptian petroleums, it appears that marine, carbonate-rich (type II) derived oils have very different C7 compound concentrations than terrestrial, clastic (type III/II) derived oils. For any oil/condensate/ source correlation study, be it on an exploratory/regional or production/field scale, the light-end C7 data may provide extremely useful geochemical information.