Keywords: 
• biodegradation

Abstract: The concentrations and the relative proportions of fluorene, dibenzofuran and dibenzothiophene and their methyl homologues(Fs,OFs and SFs) in a series of crude oils with different degrees of biodegradation from the Liaohe basin have been quantified and studied by GC—MS. The results show that fluorene, dibenzofuran and dibenzothiophene and their methyl homologuesare sensitive to biodegradation influence. In slightly biodegraded oils, the concentrations of fluorene, dibenzofuran and dibenzothiophene without alkyl side chain decreased at almost similar rate, but their relative proportions could remain constant and could be used as depositional environment indicator. In moderate or severely biodegraded oils, because of the variation of their ability to resistant biodegradation, their relative proportions have been altered and lost their original geochemical significance. However, as for their methyl homologues, the ability to resist biodegradation of methyl dibenzothiophene is more than methyl fluorene and methyl dibenzofuran, resulting in differently biodegraded rate, their relative proportions varied from slightly to severely biodegraded oils and could not be used to indicate the features of depositional environment. Although the concentrations of Fs,OFs and SFs in differently biodegraded oils were decreased during biodegradation, their rates differed significantly, their relative proportions varied with biodegradation all time. Therefore, for biodegraded oils, the cautions should be taken when their relative proportions of Fs,OFs and SFs are used to determine the depositional environment.