Biodegradation levels of oils from the Chepaizi Uplift, Junggar Basin (NW China) evaluated by a full-range biodegradation index as constrained by adamantane, diamantane homologs and carboxylic acids

Biodegradation usually masks or even radically alters the original characteristics of molecular biomarkers, restricting the oil sources identification and the exploration of ultra-heavy oils (also known as oil sands, tar sands, asphalt or bitumen). The oils from the Lower Neogene Shawan Formation (N1s), Lower Cretaceous Tugulu Group (K1tg), and Upper Carboniferous Xibeikulasi Formation (C2x) in the Chepaizi Uplift, Junggar Basin (NW China) were biodegraded to different extent, consequently, many commonly used biomarker parameters became less powerful for the evaluation of the extent of biodegradation. To address this problem, the resistance of many adamantane and diamantane homologs and carboxylic acids to biodegradation was analyzed, and those with high bioresistance were selected to yield a more precise and extensive characterization of biodegradation from light to extreme. In contrast, oils from the eastern Chepaizi Uplift were generated from Permian sources, and have been subjected to heavy to extreme levels of biodegradation. In this study, fifteen oil samples obtained from the Chepaizi Uplift were analyzed by a set of mass spectrometric techniques. The results were applied to investigate the variations of commonly used biomarkers in the saturates and aromatics, to qualitatively rank oil biodegradation levels based on the PM scale and to provide a detailed assessment of the variation in carboxylic acids and adamantane and diamantane homologs in heavy oils from varying degrees of biodegradation. The results demonstrate that adamantane and diamantane homologs with various methyl groups, and carboxylic acids with various double bond equivalents (DBEs), exhibit an intact homolog series distribution and certain hypermethylation trend as biodegradation levels progress from light to extreme, which confirmed their reliability in refining the evaluation scale and characterizing the extent of biodegradation. In addition, the full-range biodegradation index (FRBI) presented here consists of the ratios of adamantane/diamantane homologs with different numbers of methyl substituents and the ratios of carboxylic acids with different DBE values, instead of the five Manco scores of 0-4 used previously, which can effectively improve the ranking precision of biodeg-radation of oils between subtle alteration changes, and universally unraveling to the full range of biodegradation, beginning from light to extreme.

Automated summary

This study analyzed fifteen oil samples obtained from the Chepaizi Uplift using mass spectrometric techniques. The results demonstrate the impact of biodegradation on biomarkers and propose a new method for evaluating biodegradation levels in oils.