Geochemistry of Paleozoic marine oils from the Tarim Basin, NW China. Part 4: Paleobiodegradation and oil charge mixing

•Marine oils in the Tarim Basin are severely biodegraded which alters molecular compositions of oil.•Current biodegradation level assignment is somewhat illusory and does not work in the present case.•Late charge and mixing controls physical properties and bulk compositions of oil.•Most commonly used molecular parameters were affected by biodegradation.•Peak/hump ratio can differentiate the pulse of inputs to the mixed oils. Fifty-three oil samples from Paleozoic marine reservoirs of the Tarim Basin were analyzed by GC–FID and GC–MS to elucidate the influence of biodegradation and mixing on molecular compositions. Biodegradation has a major impact on many classes of compounds but the ultimate compositions and physical properties of the oil are influenced strongly by mixing with late oil charge, subsequent to the process of biodegradation. Almost all samples display biodegradation signatures, characterized by variable size of the unresolved complex mixture (UCM) and variable amounts of 25-norhopanes (25-NHs) and 17-nortricyclic terpanes (17-NTs). The ubiquitous occurrence of n-alkanes suggests that the petroleum accumulated in these fields is mainly a mixture of previously biodegraded oils with later migrated fresh oils. The unaltered late charge of oil, which has commingled with the initial biodegraded charge, gives the wide range of oil gravities and asphaltene contents observed. Most commonly used maturity and depositional environment diagnostic parameters in the saturated hydrocarbon fraction are affected by biodegradation. Based on the intensity of 25-NHs and 17-NTs, oils from the Tabei Uplift appear to have experienced a much greater degree of biodegradation than those from the Tazhong Uplift. However, we caution against the simplistic use of molecular indicators to define a specific level of biodegradation in this case study, as it may be somewhat misleading for physical property prediction. A wide range of estimated maturity values from different components provides an important clue for multiple charge and mixing interpretations. The relationship between the absolute concentrations of compound class and peak/hump ratios of the samples can potentially, qualitatively differentiate the pulse of inputs to the mixed oils. The positive correlation between the concentrations of a given compound class and peak/hump ratios may suggest that that class of compounds is most likely derived from the late charge after biodegradation, while the negative correlatio