Discussion On Geochemical Significance Of C₃-C₅ Alkylbenzene In Crude Oil

Alkylbenzene is important component in the crude oils,especially in the light oils and condensates.In order to explore the geochemical information of alkybenzene in crude oils,a total of 62 crude oils and condensates were gathered from Tarim Basin,Beibuwan Basin and Songliao Basin.A total of 42 C₃-C₅alkylbenzene compounds were detected and identified by comprehensive two-dimensional gas chromatography coupled to time-of-flight spectrometry and comparing with mixed standards,literature,retention index and Nist 11.0.In this study,the content of total alkylbenzene is the highest in Beibuwan Basin oils,followed by that in Tarim Basin oils,and the lowest in Songliao Basin oils.The difference of total alkylbenzene is mainly based on the difference in the content of C₂-C₄alkylbenzene compounds in the crude oil.1-methyl-3-ethylbenzene substituted by interposition is more likely to be preserved than 1-methyl-2-ethylbenzene and 1-methyl-4-ethylbenzene substituted by ortho and para,while 1,2,4-trimethylbenzene is more likely to be preserved than 1,2,3-trimethylbenzene and 1,3,5-trimethylbenzene.Tert-butylbenzene,1-methyl-2-isopropylbenzene,1-methyl-2-tert-butylbenzene and 1-methyl-4-tert-butylbenzene are likely to be derived from terrestrial higher plants.1-methyl-2-isopropylbenzene in marine oil was less than 0.13 mg/g whole oil,and the content of 1-methyl-2-isopropylbenzene in terrestrial oil was more than 0.13 mg/g whole oil in Tarim Basin.The content of 1,2,4,5-tetramethylbenzene in Beibuwan Basin oils which originated from source rocks deposited under oxic/dysoxic conditions is greater than 1.0 mg/g whole oil,while that in Tarim basin and Songliao basin oils which originated from source rocks deposited under reducing conditions is less than 1.0 mg/g whole oil.According to the basic geochemical parameters of crude oil,crude oils and oil fractions carbon isotope composition characteristics,at the same time combined with alkylbenzene quantitative data,the ratios of 1-methyl-4-isoproylbenzene/1-methyl-2-isoproylbenzene?Mi PBr1?and 1-methyl-3-isoproylbenzene/1-methyl-2-isoproylbenzene?Mi PBr2?were established to distinguish sources of organic matter in crude oil.In present study,the Mi PBr1>6.0 and Mi PBr2>3.0 indicate that the organic matter of crude oil derived from lower hydrobiont,bacteria and algae,while Mi PBr1<3.0 and Mi PBr2<2.0 indicate that the organic matter of crude oil derived from higher terrestrial plants.The ratio of 1,2,4,5-/1,2,3,4-tetramethylbenzene?1,2,4,5-/1,2,3,4-Te MB?is proposed to infer oils derived from source rocks deposited under reducing or oxic/dysoxic conditions.In present study,the former are found to the 1,2,4,5-/1,2,3,4-Te MB<0.6,while the latter have 1,2,4,5-/1,2,3,4-Te MB>1.0.