Variations in chemical and stable carbon isotopic compositions of Liaohe crude oil during aerobic biodegradation simulation

Aerobic biodegradation simulations were performed on the Liaohe crude oil samples. Variations in chemical compositions of the oils and stable carbon isotopic compositions of oil fractions and individual n-alkanes were investigated by determining the degraded products in slightly to moderately biodegraded oils. The results showed that saturated hydrocarbons were preferentially degraded, and the normal alkanes were biodegraded prior to iso-alkanes. In addition, n-alkanes with lower carbon numbers were consumed prior to higher carbon number n-alkanes. No significant carbon isotopic fractionation occurred for the n-alkanes(C_(14)~C_(30)) during aerobic biodegradation. Therefore, the relative abundance and the hydrophobicity of n-alkanes should be the dominant factors controlling their degradation. That is, the n-alkanes with lower carbon numbers facilitate access to petroleum degrading microbes as substrates due to lower interfacial tension between oil and water. The aerobic biodegradation products of n-alkanes were mainly fatty acids, aliphatic alcohols and carbon dioxide. The δ~(13)C values of n-alkanes were heavier than those of other fractions in the studied Liaohe crude oil. Most of the oil-soluble fatty acids and aliphatic alcohols were combined into resin fraction while a small portion of them into asphaltene fraction through hydrogen bonds and/or ester bonds, resulting in a δ~(13)C shift to asphaltene fraction. Namely, the biodegradation products of n-alkanes might have a pull effect on δ~(13)C of asphaltenes. Whereas the water-soluble fatty acids and carbon dioxide that were dissolved into water phase would result in ~(13) C depletion of the residual oil. Therefore, if δ~(13)C of the hydrocarbons susceptible to biodegradation, such as n-alkanes, is significantly different from that of crude oil, the variation of chemical compositions of crude oil would result in significant changes in the δ~(13)C of both bulk oil and oil fractions during biodegradation.