The effects of biodegradation on biomarker maturity indicators in sequentially biodegraded oils from Liaohe Basin,China

By aid of gas chromatogram/mass spectrometry(GC-MS) ,the distributions and the compositions of biomarkers in a set of sequentially biodegraded oils from Liaohe Basin,China,have been quantitatively analyzed,and it has been found that during the biodegradation process of crude oils,the molecular maturity parameters such as Ts/Tm,homohopane C31 22S/(22S+22R) and sterane C29 20S/(20S+20R) ratios will be affected to different extent. The results show that except homohopane C31 22S/(22S+22R) ratio,Ts/Tm ratio will decrease with increasing biodegradation,but for C29 20S/(20S+20R) ratio,it will almost remain constant in slightly and moderately biodegraded oils,and then will increase quickly in severely biodegraded oils. The main reason is that there are some differences in the ability of resistant biodegradation for different isomer of biomarkers with different stereo configuration,resulting in the fact that destroying rate by bacteria for those biomarkers with weak ability will be higher than those with strong ability in resistant biodegradation. For example,18α(H) -22,29,30-trisnorhopanes(Ts) will be destroyed more quickly than 17α(H) -22,29,30-trisnorshopanres(Tm) ,and 20R isomer is more quickly than 20S isomer for C29 sterane,resulting in the relative ratios changed with increasing biodegradation. Therefore,much more attention should be paid to the biodegradation extent of crude oils and the type of biomarker maturity indicators,when the distributions and the compositions of biomarkers in biodegraded oils are used to determine the maturity of biodegraded oils.