As an important component of light hydrocarbon compounds, alkylbenzene compounds lack indicators to indicate the source of organic matter of light oils and condensates. Forty-one oil samples from the Tarim Basin and Beibuwan Basin were analyzed by comprehensive two-dimensional gas chromatography time-of-flight mass spectrometry (GC × GC-TOFMS). The concentration distributions of thirteen light hydrocarbon compounds with organic matter source and sedimentary environment indication were studied. There is no significant difference in the concentrations of 1-methylpropylbenzene (MPB) in all studied oils. However, the concentrations of 2-MPB in the Tarim swamp oils are higher than that in the Beibuwan lacustrine oils and Tarim marine oils. Based on the significant concentration difference of 1- and 2-MPB in all studied oils, 1-/2-MPB (MPBr) was proposed as an indicator to identify the source of organic matter in crude oils. The MPBr values greater than 1.5 indicate that the crude oil mainly comes from lower aquatic organisms, bacteria, and algae. The MPBr values greater than 1.0 and less than 1.5 indicate that crude oil was derived from the combined contributions of lower aquatic organisms, bacteria and algae, and terrestrial higher plants. The MPBr values less than 1.0 suggest that the crude oil was mainly derived from terrigenous higher plants. The MPBr values in crude oils basically are not or slightly affected by depositional environment and secondary alteration. The MPBr values can be used to infer the organic matter origin in sediments, especially for the lack of biomarkers of light oils and condensates.

A total of 45 alkylbenzenes were detected and identified in crude oils with different depositional environments and thermal maturities from the Tarim Basin, Beibuwan Basin, and Songliao Basin using comprehensive two-dimensional gas chromatography time-of-flight mass spectrometry (GC × GC-TOFMS). By analyzing the distribution characteristics of C₀–C₅ alkylbenzenes, it is found that the content of some alkylbenzenes varies greatly in crude oils. Based on the distribution characteristics of 1,2,4,5-tetramethylbenzene (TeMB) and 1,2,3,4-TeMB, the ratio of 1,2,4,5-TeMB to 1,2,3,4-TeMB is proposed to indicate the organic matter origin and depositional environment of ancient sediments. Oil samples originated mainly from lower hydrobiont, algae, bacteria and source rocks deposited under reducing/anoxic conditions have low 1,2,4,5-/1,2,3,4-TeMB values (less than 0.6), while oil samples originated mainly from terrestrial higher plants and source rocks deposited under oxic/sub-oxic conditions have higher 1,2,4,5-/1,2,3,4-TeMB values (greater than 1.0). The significant difference of 1,2,4,5-/1,2,3,4-TeMB values is controlled by 1,2,4,5-TeMB content. 1,2,4,5-TeMB content in oils derived from source rocks deposited in oxidized sedimentary environment (greater than 1.0 mg/g whole oil) is higher than that in oils from source rocks deposited in reduced sedimentary environment (less than 1.0 mg/g whole oil). 1,2,4,5-/1,2,3,4-TeMB ratio might not or slightly be affected by evaporative fractionation, biodegradation and thermal maturity. 1,2,4,5-/1,2,3,4-TeMB ratio and 1,2,4,5-TeMB content can be used as supplementary parameter for the identification of sedimentary environment and organic matter input. It should be noted that compared to the identification of organic matter sources, the 1,2,4,5-/1,2,3,4-TeMB parameter is more effective in identifying sedimentary environments.