Distribution and geochemical significance of C5 alkylated benzenes in light oils and condensates from the Tarim Basin and Beibuwan Basin

Due to the limitations of analytical methodologies, the geochemical significance of C₅ alkylated benzenes has largely been overlooked. Thirty-four light oils and condensates were collected from the Tarim Basin and Beibuwan Basin for analysis using comprehensive two-dimensional gas chromatography time-of-flight mass spectrometry (GC×GC-TOFMS). The concentrations of twelve C₅ alkylated benzenes exhibit distinct distribution patterns in light oils and condensates originating from varying sedimentary environments and organic matter sources. The concentrations of 1-E-3,4,5-TMB, 1-E-2,3,4-TMB, and 1-E-2,4,5-TMB are likely influenced by the source of organic matter, whereas those of 1-E-2,4,6-TMB, 1-E-2,3,5-TMB, and 1,2,3,4,5-PMB are potentially governed by the sedimentary environment. Based on the differential sensitivity of C₅ alkylated benzenes to sedimentary environments and organic matter sources, the l-ethyl (E)-2,4,6-trimethylbenzene (TMB)/1-E-2,3,6-TMB (C5-R1) and 1-E-2,4,6-TMB/1-E-3,4,5-TMB (C5-R2) have been proposed as geochemical indicators for distinguishing the sedimentary environments and organic matter sources of crude oils from their respective source rocks. Oils derived from their respective source rocks that formed in oxic/disyoxic sedimentary environments are characterized by relatively high C5-R1 values (greater than 0.5), while those formed under reduced sedimentary conditions exhibit relatively low C5-R1 values (less than 0.5). Oils originating from terrestrial higher plants are characterized by relatively high C5-R2 values (greater than 1.0), whereas those derived from a mixed input of lower aquatic organisms and terrestrial higher plants display relatively low C5-R2 values (less than 1.0). The C5-R1 and C5-R2 ratios are likely to remain unaffected or only minimally influenced by secondary alteration processes (evaporative fractionation, biodegradation, and thermal maturity). The C5-R1 and C5-R2 ratios can serve as supplementary parameters for identifying sedimentary environments and organic matter sources, particularly in light oils and condensates where conventional biomarkers are significantly depleted.