The Biodegradation Effect And Mechanism Of Nitrogen Compounds In Crude Oils,linpan Oilfield

Molecular composition and isotope composition were studied in this study,which included saturated hydrocarbon,aromatic hydrocarbon and nitrogen compounds in oils with differently biodegraded levels from the Linpan Oilfield,Jiyang Depression.According to the method of scaling up biodegradation levels by Peters et al.（2005）^[1],we used the ratios of Pristane and Phytane to Hopane[（Pr+Ph）/C₃₀H],from saturated hydrocarbon GC,and Phenanthrene to C₂₈ triaromtic sterane（20S）(P/C₂₈TAS),from aromatic hydrocarbon GCMS,to divide the oils to differently biodegraded levels from PM0 to PM5.Compared to C₂₈TAS,some aromatic components,such as phenanthrene（P）,fluorene（F）,dibenzofuran（DBF）,dibenzothiophene（DBT）and their homologues,are affected by biodegradation as soon as it initially happens in oils.The degradation rates of above compounds are various,and decrease with the increasing of carbon number of alkyl substitution.The isomers of above components are biodegraded in evident selectivity.Neutral nitrogen compounds,including carbazole and its homologues,benzocarbazoles and their homologues,are mainly the nitrogen compounds in crude oil with the lower molecular weight.Compared to C₅-carbazoles,carbazoles and bezocarbazoles are also affected by biodegradation as soon as it initially happens in oils,with lower degradation rate during biodegraded levels from PM0 to PM3,and higher beyond level PM3 where most of acyclic alkanes are depleted.Just as homologues of aromatic hydrocarbon,the resistibility to biodegradation of the homologues of carbazole and benzocarbazoles also increases with increasing carbon number of alkyl substitutions and rings in molecular structure.The degraded rates of four methylcarbazoles are in order as followed,2-MC>3-MC>4-MC>1-MC,dimethylcarbazole isomers as followed,partly shield>shield>exposed,and benzocarbazoles as followed,B[a]C>B[b]C>B[c]C.The higher molecular weight nitrogen compounds in crude oils are analyzed using Negative-Ion Electrospray Ionization Fourier Transform Ion Cyclotron Resonance Mass Spectrometry（NI ESI FT-ICR MS）.The N₁ class species,which have the formula C_nH_2n+zN and highest content,have the carbon number ranged from 20 to 60,and double-bond equivalent（DBE）values between 6 to 23.The relative contents of N₁species with DBE values of 9（alkyl carbazoles）and 12（alkyl benzocarbazoles）decrease gradually with the increasing of biodegradation levels,while the relative abundance of the species with DBE values of 11,13,14,15 and 16 rise with the increasing of biodegraded levels.In the levels from PM0 to PM5,the isotope compositions of bulk carbon and nitrogen of oils and groups,the compound-specific isotope of alkanes and biomarkers are little affected by biodegradation in the Linpan Oilfield,while 2‰ofΔ¹³C is observed in groups of normal alkanes of aerobic culture oils,which suggests the biodegrading process would lead to isotope fraction in some compounds.Dibenzothiophene and Carbazole have similar molecular structure,and their homologues with similar structure follow similar rules in biodegradation.This rules in biodegradation suggest that the difference of stereochemistry and the position of alkyl substituents should be the most important reasons to selective biodegradation of compounds and make an effect to biodegrading mechanism.Based on the results of actual samples from Linpan Oilfield,we proposed the possible biodegraded pathways of carbazoles with short and long-chain alkyl in oils.Short-chain alkyl substituted carbazoles,such as C₀-～C₅-carbazoles,are initially attacted by microbes at postions on the benzene rings to produce hydroxyl-carbazole or hydroxyl-alkylcarbazoles,which undergo further ring opening to yield compouds utilized by them,and the possible order of postion to be hydroxylated is as followed C-4,C-1,C-3/C-2 carbon.Since the hydrogen atoms in benzene rings are all replaced by alkyl groups,carbazoles with long-chain alkyl are biodegraded by introducing oxygen atoms into the end carbon postion of long-chain substitutions to produce hydroxyl-alklycarbazoles or carboxyl-alklycarbazoles.