Pyrolytical Methods For Determining The Structure And Hydrogen Isotopic Composition Of Macromolecules And Their Applications In Studying The Genesis Of Petroleum From The Tarim Basin

Due to the growth of multiple source units and the multiple events of hydrocarbon generation and tectonic movement,the genesis of marine oil from the Tarim Basin has been one of the hot and difficult issues in the geological and geochemical studies in China for many years.Molecular and carbon isotopic compositions of soluble low-molecular fractions of oil,which were easily affected by large variations in physical properties and by secondary processes,have been thoroughly used in previous studies.To gain insights into the genesis of marine oil from the Tarim Basin,some new methods dealing with high-molecular materials in oil needed to be established and applied in related studies.This study established the pyrolytical method for hydrogen isotope analysis of n-alkane in asphaltenes and optimized conditions of the two-step thermochemolysis for characterizing geo-macromolecules.With these two progresses of analytical methods,the carbon-hydrogen isotopic compositions of n-alkanes released by the asphaltene pyrolysis were compared with those of n-alkanes in the saturated fraction of oil from the basin,and the composition and genesis of solid bitumen in the Silurian tar sands were elucidated.Four main conclusions were obtained after the study:(1)A new method used for measuring hydrogen isotope ratios of n-alkane in asphaltenes has been established: First,two different procedures of recovering n-alkanes adsorbed by 5? molecular sieve were tested with different kinds of samples;one used the solvent mixtures at relatively high temperatures and another combined the digestion by hydrofluoric acid with subsequent solvent extraction.The comparison have shown that the value of the recovery method using solvent mixtures lies in its simplicity and rapidity,and can be applied in the purification of n-alkanes and subsequent compound-specific hydrogen isotope analysis.Second,the temperature effects on the hydrogen isotopic compositions of n-alkanes in the pyrolyzates of oil asphaltenes were investigated in a low-temperature closed system,and the optimal temperature condition was thus determined.Finally,asphaltenes from distinct sources were analyzed using the new method.It was shown that the hydrogen isotope ratios of n-alkanes in the pyrolysates of asphaltene varied greatly with oil sources,suggesting that this method can be used for characterizing oil sources and performing oil-oil correlation.(2)The two-step off-line thermochemolysis for characterizing geo-macromolecules was optimized: First,with asphaltene as the experiment object,a rapid,high-temperature pyrolysis method in closed system was developed for determining the composition of geo-macromolecules.After that,flash pyrolysis,single-step and two-step off-line thermochemolysis were carried out with various types of kerogens and asphaltenes,demonstrating that the two-step off-line thermochemolysis could decipher differences in compositions of both the adsorbed and bond molecules between different kinds of geo-macromolecules.(3)The hydrogen isotopic compositions of n-alkanes released from the pyrolysis of asphaltenes from most of marine oils and the Silurian tar sands varied slightly,hardly affected by the biodegradation,suggesting a isotopic signatures of early charged oil.However,the ?13C and ?D values of n-alkanes in the pyrolysates of oil asphaltenes from the well of TD2 are significantly heavier and lighter,respectively,which is distinct from those of n-alkanes in the maltenes of oil.These results indicated that the oil from the well TD2 was derived from different sources from most of the marine crude oil in the basin.(4)Insoluble solid organic matter(ISOM)from the Silurian tar sands were analyzed by a comprehensive utilization of elemental analysis,spectroscopy analysis(including 13 C NMR,FTIR and XPS),flash pyrolysis and two-step off-line thermochemolysis.The results indicated that the faction of aromatic carbon in normally buried ISOM is different from that in thermally altered ISOM,as well as structural units attached to the condensed aromatic ring system of macromolecules.ISOM were mainly derived from asphaltenes also with some contributions from aromatic hydrocarbons and resins.The relationship of carbon isotopic signatures between ISOM and soluble,low-molecular organic matter suggested both multiple hydrocarbon charge events and alteration by biodegradation.