Identification and Implications of Trimethyl-n-Alkylbenzenes in Marine Oils from the Deep Tarim Basin

The source of marine oils from the deep Tarim Basin is still in debate due to several alteration processes of source indicators. A series of trimethyl-alkylbenzenes has been detected in marine oils from this old, composite basin, besides the reported aryl isoprenoids with 2,3,6-trimethyl substitution (AIPs). They are characterized by regular gas chromatography elution pattern, which is similar to that of n-alkylbenzenes, and suggest a strong possibility of n-alkyl side chains. C15 trimethyl-n-alkylbenzenes were synthesized by Friedel–Crafts acylation of trimethylbenzene isomers to determine their structures. Based on the chromatography and mass spectra data and the coinjection of synthesized compounds, this series of compounds has been assigned as the 2,4,5-trimethyl-n-alkylbenzenes that coeluted with 2,3,5-trimethyl-n-alkylbenzenes, and other trimethyl-n-alkylbenzene isomers were also detected. This series of trimethyl-n-alkylbenzene (AAs) shows much higher relative abundances in light and waxy oils than in normal and heavy oils, which is opposite to the variation in relative abundances of aryl isoprenoids. The ratios of these trimethyl-n-alkylbenzenes to the aryl isoprenoids (AA/AIP ratio) generally show a good correlation with the maturity indicators for most of studied oils despite of some outliers (mainly condensates). The pyrolysis of asphaltenes has confirmed these trends. These results support an important control of thermal stress on the molecular compositions of marine oils from the deep Tarim Basin, besides other secondary alteration processes (such as oil mixing and migration fractionation, among others). These factors should be given a full consideration for the source determination of deep and ultradeep oils.