Qualitative & Quantitative Analyses And Molecular Marker Exploration Of C₅–C₁₃ Light Hydrocarbons In Crude Oils And Source Rocks

Using comprehensive two-dimensional gas chromatography coupled to time-of-flight spectrometry?GC×GC-TOFMS?,the method for analyzing C₅–C₁₃ light hydrocarbons in crude oil was established.A total of 127 compounds were detected and identified by co-elution with standards and by comparison with standards and literature,including 48 compounds in the C₈–C₁₀ light hydrocarbons,53 compounds in the C₅–C₈ light hydrocarbons and 26 compounds in the C₁₀–C₁₃ light hydrocarbons.Retention index of those compounds were calculated under current experimental methods.A vacuum ball-milling-closed thermal desorption-helium purging-liquid nitrogen capturing-gas chromatography device was developed and the method for analyzing C₅–C₁₃ light hydrocarbons in source rocks was established by using this device.Parallel tests were performed by employing typical oil and source rock samples,which show that this device together with the analysis method can satisfy the demond for analyses of C₅–C₁₃ light hydrocarbons in source rocks.Qualitative and quantitative analyses of monoterpenoid hydrocarbons of the C₅–C₁₃ light hydrocarbons suggest that 6 compounds including 2,6-dimethyloctane?2,6-DMO?,2-methyl-3-ethylheptane?2-M-3-EH?,1,1,2,3-tetramethylcyclohexane?1,1,2,3-TeMCYC6?,trans-1-methyl-4-isopropylcyclohexane?trans-1-M-4-iPCYC6?,1-methyl-3-isopropylbenzene?1-M-3-iPB?and 1-M-4-iPB were detected in all 96 oil samples collected from 4 different sedimentary baseins in China?Tarim,Bohai Bay,Beibuwan,and Donghai Basin?.Their concentrations follow a trend of 2,6-DMO > 2-M-3-EH > 1,1,2,3-TeMCYC6 > trans-1-M-4-IPCYC6 > 1-M-3-iPB > 1-M-4-iPB.Gas chromatography-mass spectrometry?GC-MS?analyses of higher plant essential oils and their catalytic hydrogenation products indicate that the 2,6-DMO,1,1,2,3-TeMCYC6,trans-1-M-4-iPCYC6 and 1-M-4-iPB can be formed from the monoterpenoids of higher plants,however,the 2-M-3-EH and 1-M-3-i PB may have different sources or/and evolution pathways.Based on the carbon isotope compositions of whole oil and fractions,isoprenoids,steranes,hopanes distributions and geological backgrounds of the oil samples,the C₁₀ monoterpane ratio?2-M-3-EH/2,6-DMO?was established to distinguish marine oils from terrigenous oils.The quantitative data show that differences in the C₁₀ monoterpane ratio are mainly caused by variations in the concentrations of 2-M-3-EH.The C₁₀ monoterpane ratio and concentration of 2-M-3-EH may serve as new parameters for assighing the source rock depositional environment.The C₁₀ monoterpane ratio higher than 0.4 and concentration of 2-M-3-EH higher than 3.5 mg/g oil indicate a reducing condition while the C₁₀ monoterpane ratio lower than 0.3 and concentration of 2-M-3-EH lower than 2.5 mg/g oil suggest an oxic/dyoxic condition.The alkylbenzenes can be completely separated from the alkanes and cycloalkanes due to their higher molecular polarity.A toal of 43 alkylbenzene isomers were detected and identified in all samples examined.The distributions of alkylbenzenes were controlled by several factors including the organic matters incorporated into kerogens,the maturity and depositional environments.The C3-4/C₀-2 alkylbenzene,?m-+p-?/o-propyltoluene and?3,5-+3,4-?/?2,5-+2,4-ethylxylene?ratios show maturity effects to the relative abundance of alkylbenzene isomers.According to the distributions of n-alkanes,pristane and phytane,steranes,hopanes and carbon isotope compositions of whole oils and fractions as well as geological backgrounds of the oil samples,the ratio of 1,2,3,5-/1,2,3,4-tetramethylbenzene?1,2,3,5-/1,2,3,4-TeMB?is proposed to distinguish oils derived from source rocks deposited under anoxic conditions within depositional environments and those originated from source rocks deposited under oxic/dysoxic conditions.For present data,the former are found to have 1,2,3,5-/1,2,3,4-TeMB < 1.0,whereas the latter have 1,2,3,5-/1,2,3,4-TeMB >1.5.Quantitative data show that the differences in 1,2,3,5-/1,2,3,4-TeMB values are mainly due to variations of 1,2,3,5-TeMB.The 1,2,3,5-TeMB is enriched in oils derived from source rocks deposited under an oxic/dysoxic condition,but depleted in oils formed from source rock deposited under a reducing environment.This may suggest that the 1,2,3,5-/1,2,3,4-TeMB can serve as a new parameter for assigning the source rock depositional environments.