AAPG Bulletin, V. 84, No. 7 (July 2000), P. 1041-1054.

Petroleum and Source Rock Characterization Based on C₇ Star Plot Results: Examples from Egypt¹

Hein E. Wever²

©Copyright 2000. The American Association of Petroleum Geologists. All rights reserved. 
¹Manuscript received March 23, 1998; revised manuscript received January 21, 1999; final acceptance November 15, 1999.
²StratoChem Services, 581, 5th Sector, Palestine Street, New Maadi, Cairo, Egypt; e-mail: [email protected]
The following companies are kindly thanked for permission to make use of their geochemical data: Agiba Petroleum Company, Apache Darag Company, Badr Petroleum Company, Belayim Petroleum Company, Khalda Petroleum Company, Qarun Petroleum Company, and Repsol Exploration Egipto S.A. I am grateful to Mohamed Said and Tarek El Azhary of StratoChem Services for introducing me to the petroleum geology of Egypt and for their unfailing support. This paper has benefited greatly from careful reviews by Wallace Dow, Henry Halpern, and an anonymous referee. 

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.