New alkylbenzene parameters to identify organic matter sources for source rocks of light oils and condensates from the Tarim Basin and Beibuwan Basin

Abstract

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.