Biodegradation of Polycyclic Aromatic Hydrocarbons: A Case Study of the Chepaizi Uplift, Junggar Basin, NW China

Polycyclic aromatic hydrocarbons (PAHs) are important components in crude oil with extensive applications in maturity assessment, depositional environment reconstruction, and oil–oil/source rock correlation. The biodegradation of oils is common under geological conditions. Despite many studies on the biodegradation of saturated hydrocarbons, naphthalenes, and phenanthrenes, the relative susceptibilities to biodegradation of fluorene, dibenzothiophene, chrysene, pyrene and their alkylated homologues, and triaromatic steroid hydrocarbons were not well studied. In this study, a suite of oils and oil sands from the Chepaizi Uplift, Junggar Basin, were collected to investigate the fate and relative susceptibilities to the biodegradation of these PAHs under geological conditions. Gas chromatography–mass spectrometry analysis was conducted to identify the aromatic species. Based on the studied samples with different biodegradation levels, the relative susceptibilities of individual dibenzothiophene, fluorene, chrysene, and pyrene and their alkylated homologues and triaromatic steroid hydrocarbons were determined. DBT/1-MDBT, 3,7-/2,4-DMDBT, 1,2,7-+1,2,8-/2,4,6-TMDBT, 1-MF/4-MF, 1,6-DMF/B, Py/4-MPy, and C2620S TAS/C2720R TAS were proposed to assess the biodegradation. The biodegradation order is not related to thermodynamic stability, making the maturity assessment of biodegraded oils and oil sands more difficult. Moreover, most PAH-based parameters used for oil–oil/source rock correlation and depositional environment analysis are not available due to biodegradation. Maturity parameters, such as MCI1, MCI2, and MChyI2, increase with increasing biodegradation, while MDR and DMDBTI decrease with increasing biodegradation. However, the refractory C26–C28 triaromatic steroids are valid before PM 8. Therefore, using PAH-based parameters in the geochemical assessment of in-reservoir biodegraded or oil sands should be done with care, and the relative biodegradation susceptibilities of related components should be taken into consideration.