Biodegradation characteristics of heavy oil and correlation with group composition and carbon isotopes

Shallow heavy oil is abundant but seriously biodegraded in Chepaizi Uplift and many biomarkers cannot be used effectively. Therefore, some new parameters and methods are used to analyze the biodegradation extent, classification, and oil-source of heavy oil samples. The acid value and group compositions are used to evaluate the biodegradation extent. Carbon isotopes are not only used to calculate the percentage of complex combined oil, but also to correlate oil-source in Chepaizi Uplift. C-13 of oil is used together with biomarkers that can bear serious degradation, such as pregnanes, tricyclic terpanes, and gammacerane. The crossplots of C-13 and biomarkers are effective correlation tools, such as the crossplot of C-13 and C-21/C-23 tricyclic terpanes. C-13 in different group compositions also show excellent oil-source correlation results. These methods to study biodegraded heavy oil are not only helpful to clarify the biodegradation extent of shallow heavy oil in an extensive area, but also effective to classify heavy oil and correlate oil-source.