Effect of biodegradation processes on the composition and structure of asphaltenes in West Siberian oils

Abstract

NMR spectroscopy in combination with elemental analysis was used to study asphaltenes in biodegraded oils from Cenomanian pools of West Siberia. The sampling depths vary from 680 to 1800 m, formation temperatures – from 40°C to 70°C. For comparison, we used the data on asphaltenes in non-biodegraded oils of different genotypes. Given that biodegraded oils are very heavy (density: 910-950 kg/m3), they are characterized by high boiling point temperatures (145-270°C). Due to the loss of hydrocarbon components, they have higher resin and asphaltene content (9-20%) compared to non-degraded samples. Elemental analysis of asphaltenes in biodegraded and unaltered oils of different genotypes revealed an increasing trend for oxygen content in the asphaltenes from biodegraded samples, which may result from the oxidation of structural blocks of asphaltenes during microbial oxidation. It was shown that the aromaticity of the moderately biodegraded terrestrial-aquatic Novoaganskaya samples tends to increase with a decrease in asphaltene saturation, suggesting that the redistribution of structural groups of asphaltenes may be caused by biodegradation processes. High saturation of asphaltenes in strongly biodegraded Gubkinskaya and Novoportovskaya oils, along with a high degree of substitution of aromatic compounds in asphaltenes in Gubkinkaya oils (terrestrial and aquatic-terrestrial genotype) can be attributed to the formation of asphaltenes during strong biodegradation of hydrocarbon components in these oils.