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

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 correlation may indicate compounds largely inherited from the early charge before biodegradation. Some compounds show almost no correlation between their concentrations and peak/hump ratios, implying more similar intensity of charge inherited from early pulse and amalgamated from later pulses. In some extremely biodegraded cases where UCM hump can also be diminished (not in present case study), the correlation between compound class concentrations and peak/hump ratios will be more complicated.