Novel parameters derived from alkylchrysenes to differentiate severe biodegradation influence on molecular compositions in crude oils

Solvent extracted bitumens from oil sands in the Athabasca region, West Canada and Liaohe Basin, NE China and oils from the Bongor Basin, SW Chad have been geochemically characterized to investigate biodegradation influence on molecular compositions. All samples in each case history were derived from the same source rock systems and have a similar maturity level. The variations in molecular compositions are merely caused by variable levels of biodegradation. GC-MS analysis results based on overall saturated and aromatic hydrocarbon distributions illustrated that biodegradation influences on Athabasca oil sands were less severe than these in Liaohe oil sands and Bongor oils but biodegradation level assignment is inconclusive and confusion. Novel biodegradation parameters based on methylchrysene (MCh) isomer distributions and relative abundance between alkylchrysenes and alkylphenanthrenes were proposed to differentiate biodegradation influence at heavy to severe biodegradation levels. The 3-MCh is proved to be the most stable methylchrysene isomer and the ratio of 3-MCh/Sigma MCh can serve as a novel biodegradation parameter. Meanwhile, alkylchrysenes show much higher ability to resist biodegradation than alkylphenanthrenes and the ratio of C0-1Ch/C2P was proposed as another novel biodegradation parameter. While the nature of organic source input may exert some impacts on the initial values of these novel parameters, continuous increases in these two parameters with intensified biodegradation levels may provide a supplemental tool for biodegradation influence assessment especially when biodegradation approaches to severe levels.