Genetic mechanism of heavy oil in the Carboniferous volcanic reservoirs of the eastern Chepaizi Uplift, Junggar Basin

A suite of crude oils from the Carboniferous volcanic reservoirs in the eastern Chepaizi Uplift, Junggar Basin, was analyzed to study the alteration of molecular compositions by severe biodegradation and the accumulation mechanisms of the crude oils. The investigation indicates that most of the crude oils of the Carboniferous reservoirs in the eastern Chepaizi Uplift are typical heavy oil with low wax and sulfur contents. The density of crude oils in the study area increases gradually from east to west, and as the burial depth increases, the density of crude oil decreases. All selected oil samples were severely degraded, with a degree of biodegradation (DOB) ranging from PM 7 to PM 9. The C21–22 steranes and diasteranes remain stable when the level of biodegradation is lower than PM 8, and they can be used as conserved “internal standards” to evaluate the biodegradation of hopanes, regular steranes, and tricyclic terpanes. However, the C21–22 steranes and diasteranes are degraded when the DOB is over PM 9. The biodegradation of tricyclic terpanes begins when the level of biodegradation reaches PM 8. Among the tricyclic terpane family, the C21 and C23 tricyclic terpanes are the most readily degraded members, while the C24-26 tricyclic terpanes seem more resistant to biodegradation. Most of the heavy oil accumulations are formed by the adjustment and remigration of paleoaccumulations due to the activity of the Hongche Fault and tectonic movement. During the remigration of hydrocarbons, biodegradation and water washing occurred in the pathways, controlling the formation of heavy oil.