GEOCHEMICAL CHARACTERIZATION OF A BIODEGRADED CRUDE OIL, ASSRAN FIELD, CENTRAL GULF OF SUEZ

A. H. Hegazi^1* and M. Sh. El-Gayar¹

¹ Chemistry Department, Faculty of Science, Alexandria University, Alexandria, Egypt.

*Corresponding author, ahegazih@yahoo.com

A crude oil sample from the Assran field in the Central Gulf of Suez (Egypt) was analysed geochemically and characterized in terms of a variety of source and maturity dependent biomarkers. Biodegradation was indicated by increasing concentration ratios of Pr/n-C₁₇ and Ph/n-C₁₈. However, biodegradation was only slight as GC-MS analyses of the saturate and aromatic fractions showed that hopanes, steranes, aromatic steroids and polycyclic aromatic compounds including sulphur heterocycles remained intact. The sterane and hopane distributions showed a predominance of C₂₇ steranes, a low diasterane index, an abundance of gammacerane, a high homohopane index and an oleanane index < 0.2. The results indicate that the Assran-10 crude oil was derived from a marine carbonate source deposited in a highly reducing saline environment with a high bacterial contribution, consistent with the Upper Cretaceous Brown Limestone or Lower Eocene Thebes Formation containing Type IIS kerogen. Maturity parameters based on changes in the stereochemistry at chirality centres in hopane and sterane nuclei, such as C₃₀ ba/(ba+ab) and C₃₁ 22S/(22S+22R) hopanes and C₂₉ bb/(bb+aa) and C₂₉ 20S/(20S+20R) steranes, together with triaromatic sterane cracking ratios, indicate that the oil sample was marginally mature. The results also suggest that  biodegradation is probably due to sulphate-reducing anaerobic bacteria.

Key words: biodegradation, biomarkers, polycyclic aromatic hydrocarbons, Gulf of Suez, Assran field, Egypt.

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