The Study On The Effect Of Biodegradation On The Chemical Compositions Of Liaohe Crude Oils

Biodegradation is one of the most common secondary alterations in the reservoirs and a principal process forming most of world’s heavy crude oils. The polar components such as resins and asphaltenes in crude oils increase with progressive consumption of hydrocarbons and generation of polar compounds during biodegradation, resulting in an increase in oil density, acidity and viscosity, with negative economic consequences. Meanwhile, the polar components in crude oils are also continuously altered by microorganism during biodegradation. It has taken a substantial research effort to work out the biodegradation process of petroleum hydrocarbons. However, biodegradation processes and mechanisms of polar compounds are only poorly understood due to their complex composition, strong polarity and low volatility and ineffective separations/detection by routine instrumental analysis.It is a common occurrence for biodegradation of crude oils in most of faulted-block reservoirs in the Liaohe basin, NE China. It has been proposed that the heavy oils were generated by biodegradation of normal gravity oils sourced by an identical source rock. Thus it is well suited for the study on the biodegradation process and mechanism of crude oils. In this thesis, tar sand bitumens from the Lengdong oilfield in the Liaohe basin characterized with a natural biodegradation sequence were selected as research objects. A combination of gas chromatography-mass spectrometry(GC-MS) and negative-ion electrospray(ESI) Fourier transform ion cyclotron resonance mass spectrometry(FT-ICR MS) were employed in the analysis of saturated hydrocarbons, resins and asphaltenes in crude oil, together with flash pyrolysis coupled with gas chromatography(Py-GC). The variations in chemical compositions and molecular structures of these oil fractions were expected to reveal the biodegradation processes and mechanisms of crude oils. The results showed that 1) the saturate fractions in Liaohe crude oils gradually decrease while the resin and asphaltene fractions relatively increase with increasing biodegradation. The general order of bio-resistance of various biomarker compound classes to biodegradation mostly follows the sequence as n-alkanes < acyclic isoprenoids < steranes < hopanes < diasteranes < aromatic steroids. 2) The maltene fractions were found to mainly contain N1, N1O1, N1O2, N1O3, O1, O2, O3 and O4 classes, while the asphaltene fractions mainly contain N1, N2O1, N1O1, N1O2, N1O3, N1O4, O2, O3, O4 and O5 classes. These species identified by FT-ICR MS in asphaltene fractions are likely to be chemisorbed/co-precipitated compounds, or the species precipitated due to high polarity during de-asphaltene process. 3) The results also indicate that microorganisms alter the distribution of acids and nitrogen-containing compounds by selective removal and preservation of certain classes of compounds according to their susceptibility to biodegradation. For example, O2 and N1O2 classes increase significantly while N1 and N1O1 classes decrease with biodegradation.There is an on-going debate about whether asphaltenes can be anaerobically biodegraded in reservoir rocks due to their complex macromolecular structure. The compositions of alkyl moieties within the asphaltene molecular structure were characterized by on-line flash pyrolysis-gas chromatography(Py-GC), on-line thermally assisted hydrolysis and methylation using tetramethylammonium hydroxide(THM-GC), and Py-GC combined with selective chemical degradation using alkaline hydrolysis. Furthermore, the effect of biodegradation on the molecular composition and structure of asphaltenes were explored. The results indicate that 1) alkyl moieties bonded to asphaltene macromolecules are dominated by linear structures with chain lengths up to C27, mainly bonded through C-C and ether(thioether) bonds but less so through ester and hydrogen bonds(accounting for approximately 1.4–6.0%). 2) Most n-fatty acids and linear aliphatic alcohols moieties were altered at slight–moderate biodegradation stages, while linear alkyl moieties attached to the asphaltene core through C-C and ether(thioether) bonds are likely altered at heavy–severe biodegradation stages. 3) The results also suggest that variations in the chemical compositions and molecular structure of asphaltenes during biodegradation may not only be attributed to the alteration of linear alkyl moieties, but also the replacement of linear alkyl moieties bonded to the asphaltene structure by hydrogen and ester bonds with other biodegradation products.To corroborate the biodegradation process of crude oils occurred in the reservoir, aerobic biodegradation simulations were performed on Liaohe crude oil samples. Variations in chemical compositions of the oils were studied and stable carbon isotopic compositions and fractionation were investigated for both individual n-alkanes and oil fractions during slight and moderate biodegradation. The results showed that saturated hydrocarbons were preferentially degraded. The normal alkanes were biodegraded prior to their isomeric counterparts. In addition, n-alkanes with lower carbon number were consumed prior to those with higher carbon number. No significant carbon isotopic fractionation occurred for the n-alkanes(C14–C30) during aerobic biodegradation. The variation of chemical compositions of crude oil might result in significant changes in the δ13C of both bulk oil and oil fractions during biodegradation. The distribution of compounds in oils from biodegradation simulated experiment is similar to that of natural biodegraded oils at slight to moderate biodegradation degrees. It indicates that aerobic biodegradation simulations of crude oils may to some extent reflect the process of in-reservoir biodegradation.