Ratios of low molecular weight alkylbenzenes (C0-C4) in Chinese crude oils as indicators of maturity and depositional environment

A total of 37 oils from three basins in China were analyzed using comprehensive two-dimensional gas chromatography coupled to time-of-flight mass spectrometry (GC × GC-TOFMS) to investigate the distributions of low molecular weight (LMW) alkylbenzenes (C0-C4). Thermal maturity and depositional environment exert dominant controls on the carbon number and isomer distributions of LMW alkylbenzenes, while secondary alteration, such as biodegradation or evaporative fractionation, cannot be ruled out. With increasing maturity, the relative abundance of C3 and C4 decreases while the C0-C2 alkylbenzenes increase. Some variations in isomer distributions of C4 alkylbenzenes are also attributed to maturation, as illustrated by ratios of (m- + p-)/o-propyltoluene and (3,5- + 3,4-)/(2,5- + 2,4-ethylxylene). The 1,2,3,5-/1,2,3,4-tetramethylbenzene ratio (TeMBr) is proposed to distinguish oils derived from source rocks formed under anoxic rather than oxic/dysoxic depositional conditions. The data suggest that TeMBr < 1.0 indicates anoxia, while TeMBr > 1.5 suggests an oxic/dysoxic depositional environment. Quantitative data show that the differences in TeMBr values are mainly due to variable 1,2,3,5-TeMB concentrations, whereas the range of 1,2,3,4-TeMB concentration is relatively constant. 1,2,3,5-TeMB is enriched in oils derived from source rocks deposited under oxic/dysoxic conditions, but depleted in oils from source rocks deposited under reducing conditions.