Origin, Distribution and Geochemical Significance of Isopropyltoluene Isomers in Crude Oil

With comprehensive two-dimensional gas chromatography linked to time-of-flight mass spectrometry (GC × GC-TOFMS), ten light hydrocarbon (LH) compounds were qualitatively and quantitatively studied in light hydrocarbons (LHs) components of crude oils. For significant differences in the concentrations of 3-isoproyltoluene (3-iPT), 4-isoproyltoluene (4-iPT) and 2-isoproyltoluene (2-iPT) in crude oils, and the 2-iPT probably derived mainly from similar skeleton monocyclic terpenoids via de-hydrogenation and aromatization, the ratios of (3+4)-/2-iPT (iPTr), 3-/2-iPT (iPTr1) and 4-/2-iPT (iPTr2) are proposed to distinguish the organic matter origin of crude oils. Relatively higher iPTr (>8.0), iPTr1 (>7.0) and iPTr2 (>4.0) values indicate that crude oils are sourced from the co-contribution of lower aquatic organisms, bacteria, algae, and terrestrial higher plants, whereas lower iPTr (<5.0), iPTr1 (<3.0), iPTr2 (<2.0) values suggest that crude oils originated from terrestrial higher plants. The iPTr, iPTr1, and iPTr2 values show notable distinction which is mainly controlled by 2-iPT concentrations, while the concentrations of 3-iPT and 4-iPT have similar distribution range in all studied oils. The 2-iPT depleted in marine oils from the Tarim Basin and lacustrine oils from the Beibuwan Basin is less than 0.30 mg/g LHs, whereas 2-iPT enriched in swamp oils from the Tarim Basin is greater than 0.50 mg/g LHs. The iPTr, iPTr1, and iPTr2 ratios and 2-iPT concentrations can be used to distinguish the organic matter origin of crude oils, especially for light oils and condensates with low concentrations of biomarkers.