Stable Carbon Isotopic Ratio Of Individual Polycyclic Aromatic Hydrocarbons (PAHs) In Crude Oils From Different Origins

This dissertation aimed to study the geochemical characteristics of different types of oils on the basis of the stable carbon isotope compositions of individual selected aromatic compounds using a reliable and efficient separation procedure for polycyclic aromatic hydrocarbons with different benzene rings.Thus,the established procedure is applied to a set of oil samples selected from four typical types of basins,i.e.Termit Basin and Bongor Basin（Africa）,Tarim Basin,and Fushan Depression of the Beibuwan Basin（China）.The effects of different geochemical factors such as the biodegradation level,the thermal maturity and the source inputs were comprehensively discussed.The results attested that an effective separationprocedure is related to the choice of the best type of column chromatography but also to the diameter of the alumina pores.The volume and the ratio of petroleum ether and dichloromethane are inevitably related to both factors.Therefore,the type of column chromatography silica/alumina（1:1）is chosen as the best column type for good separation.The final procedure proposed in the present research attests that petroleum ether:dichloromethane mixtures of（93:7,6 ml）,（90:10,30 ml）,（75:25,20 ml）and（0:100,12 ml）are efficient for recovering mono-aromatic,di-aromatic,tri-aromatic and more than 3 ring aromatic compounds successively.As results,the procedure considerably reduced the effects of the unresolved complex mixture（UCM）on the different aromatic sub-fractions and eliminated the coelution of the majority of the aromatic compounds.Thus,it makes easier to identify the individual PAHs and realize the carbon isotope measurements.The advantages of the procedure proposed here,compared to others complicated and time-consuming procedures such as high performance liquid chromatography（HPLC）,medium pressure liquid chromatography（MPLC）,thin layer chromatography（TLC）,are that the procedure is a one-step column chromatography that recovers successfully mono-aromatic,di-aromatic,tri-aromatic and more than 3 ring aromatic sub-fractions free from each other.It is a low-cost procedure,easy to reproduce and less time-consuming.The study of the biodegradation and thermal maturity effects on the isotope compositions revealed that biodegradation has no significant effects on the carbon isotope compositions of the oil samples contrary to the thermal maturity.The result showed that the enrichment of the isotope compositions from the Tarim Basin oils with the increase of the thermal maturity attests that tthe selected Tarim oils have already reached their cracking stage.Conversely,the depletion of the isotope compositions with the rising of the maturity level of the selected oils from the Termit Basin,the Bongor Basin and the Fushan Depression revealed that these oils have not yet reached their oil cracking stage.Comparison with the results from saturated parameters,theδ¹³C values of 1,2,6-TMN（1,2,6-trimethylnaphthalene）,1,2,5-TMN（1,2,5-trimethylnaphthalene）,9-MP（9-methylphenanthrene）and 1-MP（1-methylphenanthrene）revealed that oils from the Tarim Basin are dominantly derived from marine organic matter with a minor terrestrial contribution(δ¹³C _values<-30.0‰).However,the different degrees of terrestrial contributions suggest mixing marine and terrestrial source inputs(-30.0‰>δ¹³C _values<-20.0‰)for some oil samples（*T1,*T2,*T3 and*T4）.In the Termit Basin,the isotope compositions attested that these oils are derived from a mixing of marine and terrestrial organic matter with a dominant contribution from marine source input(-30.0‰>δ¹³C _values<-20.0‰).As for the Bongor Basin and Fushan Depression,theδ¹³C values of selected aromatic isomers indicate that the selected oils are derived from mixing of organic matter with dominant aquatic organisms and minor terrestrial contribution(-30.0‰>δ¹³C _values<-20.0‰).The study of the origin of the oils revealed that the selected marine carbonate oils from Tarim Basin（NW China）have three different origins.The majority having low 9-MP/1-MP ratios（<2.0）withδ¹³C values of phenanthrene（Phen）lower than-30‰and plotting above 0 forΔ¹³C_{（9MP-Phen）}andΔ¹³C_{（1MP-Phen）}is dominant marine sourced.While the oils（*T2,*T3 and*T4）with high 9-MP/1-MP ratios（>2.0）andδ¹³C values of phenanthrene（Phen）between-30.0‰and-20.0‰,plotting below 0 forΔ¹³C_{（9MP-Phen）}andΔ¹³C_{（1MP-Phen）}are mixed oils derived from dominant terrestrial organic matter input mixed with a significant marine contribution.As for oil samples*T1（XH1）and*T9（S94）with a low 9-MP/1-MP ratio and aδ¹³C value of phenanthrene（Phen）in the range of-30.0‰to-20.0‰,plotting above 0 forΔ¹³C_{（9MP-Phen）}and below 0 forΔ¹³C_{（1MP-Phen）}assigned as mixed oils slightly dominant in marine organic input.As for the marine Termit Basin oils,the lacustrine Bongor Basin and Fushan Depression selected oils,they have a low 9-MP/1-MP ratio withδ¹³C values of phenanthrene（Phen）between-30.0‰to-20.0‰,and majorly plot above 0 forΔ¹³C_{（9MP-Phen）}and below 0 forΔ¹³C_{（1MP-Phen）}.The results suggest that the marine shale oil samples selected from Termit Basin are mixed oils with a higher marine contribution,while the Bongor Basin and Fushan Depression oils,respectively from brackish lacustrine and freshwater lacustrine shales,are mixed oils with higher contribution from aquatic organisms.