Molecular transformations of heteroatomic organic compounds in crude oils caused by biodegradation and subsequent thermal maturation: Insights from ESI FT-ICR MS

The Tarim and Junggar basins are major petroliferous sedimentary deposits in China. Some oil reservoirs in these basins experienced superimposed secondary alterations including early biodegradation followed by thermal maturation/alteration (SSA-B&M). Heteroatomic polar organic compounds in crude oil are significantly influenced by SSA-B&M, which play a vital role in understanding the evolutionary history of oil reservoir. However, research on such alterations is limited. The present study employed negative and positive ion electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI FT-ICR MS) to investigate the molecular transformations of heteroatomic organic compounds in crude oils resulting from SSA-B&M. The findings indicate that biodegradation leads to an enrichment of naphthenic acids, while subsequent cyclization, aromatization and thermal cracking reactions enhances the aromaticity of the pyrolysis products of all the oil samples with varying degree of biodegradation. Notably, highest aromaticity was observed for the crude oil that has been most severely altered by biodegradation. During early and moderate stages of biodegradation, the abundances of O2 compounds (molecules contain two oxygen atoms) representing carboxylic acids were continuously increasing. However, at severe biodegradation levels, fatty acids with 1 double bond equivalents (DBE) underwent degradation, while the naphthenic acids were further enriched. Subsequently, during maturation, O2 compounds with 1 to 5 DBE experienced a steady decline in abundance. The naphthenic acids mainly experienced aromatization, while the fatty acids were partly decarboxylized and cracked thereby forming new n-alkanes. Biodegradation led to modification in neutral nitrogen compounds with 3 to 4 condensed rings (DBE = 9–12) and long alkyl side chains, while the impact on basic nitrogen compounds was relatively minimal. After subsequent thermal maturation, both neutral and basic nitrogen compounds with long alkyl side chains underwent cracking, while those with shorter alkyl side chains and higher degrees of condensation tended to remain relatively unchanged. Finally, two new indices were defined and tested to correctly evaluate the maturation and biodegradation level for crude oils that have experienced SSA-B&M.