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

D/H ratio analysis of pyrolysis-released n-alkanes from asphaltenes for correlating oils from different sources

When oil has been greatly altered by severe biodegradation, asphaltene pyrolysis is commonly conducted to obtain intact n-alkanes for carbon isotope analysis, through which oil source characterization and oil oil correlations can be performed. The ratio of hydrogen isotopes deuterium and hydrogen (D/H ratio) in the individual n-alkanes of oil maltenes has been shown to indicate differences between the depositional environments of the oil sources. The utility of the D/H ratio analysis of n-alkanes from the asphaltene pyrolysis for oil correlation need to be demonstrated. In this study, 18 asphaltene samples from different sources were collected and isothermally pyrolyzed in sealed gold tubes. Effects of the pyrolysis temperature were first illustrated for molecular ratios of alkanes and aromatic compounds and for D/H ratios of pyrolysis-yielded n-alkanes. In the case of n-alkanes in oil maltenes being unavailable for analysis after severe biodegradation, the D/H ratio range of the n-alkanes from the asphaltene pyrolysis was found to be consistent both in normal and biodegraded oils from the same source, indicating that biodegradation has had little effect on their hydrogen isotope signatures. In addition, n-alkanes yielded by asphaltenes from different sources possess clearly different D/H ratios, enabling n-alkane correlation between maltenes that are otherwise very similar. However, relatively large differences of D/H ratios can exist between n-alkanes in the asphaltene pyrolyzates and in maltenes, once the oil was altered by oil mixing formed at different maturity levels. The proposed method is thus helpful in oil oil correlation for significantly biodegraded oil and oil from a mixed origin