Abstract Gas chromatography–mass spectrometry (GC–MS) and Fourier transform ion cyclotron resonance–mass spectrometry (FTICR–MS) analyses were performed on 39 heavy oils from an onshore basin in southeastern Mexico to investigate the influence of biodegradation on whole oil compositions. Among the biodegradation resistant C14–C16 bicyclic sesquiterpanes (bicyclanes), a rearranged C15 isomer (compound X) becomes relatively enriched with increasing biodegradation levels, contrasting with a reduction in the summed C14–C16 bicyclanes concentration but consistent with an enrichment of the bicyclic naphthenic acids revealed by FTICR–MS data. The concentration ratio of compound X over all the identified C15 bicyclane isomers (DMHD ratio) is proposed as a new biodegradation parameter. This ratio correlates well with the biodegradation ratios of 3-methylchrysene/methylchrysenes, benzonaphtho[1,2-d]thiophene/benzonaphtho[2,1-d]thiophene and those based on NSO compounds. Compound X has an additional ethyl group but one less methyl group compared to the drimane structure. A lessening of steric effects through demethylation can explain the refractory nature of compound X, and this forms the basis on which possible origins of compound X are discussed. Extensive biodegradation of the studied oil samples is implied by the alteration of polyaromatic hydrocarbons and NSO compounds. This suggests the relative enrichment of compound X can be used as a new indicator for heavy oil biodegradation at Peters and Moldowan biodegradation level (PM level) 4–5, when hopanes and steranes are still intact. Changes in the distribution patterns of some NSO compound species also suggest PM level 4–5.

中文翻译：

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一种源自双环倍半萜的新型生物降解参数，用于评估中等水平的石油生物降解

摘要 对墨西哥东南部陆上盆地的 39 种重油进行了气相色谱-质谱 (GC-MS) 和傅里叶变换离子回旋共振-质谱 (FTICR-MS) 分析，以研究生物降解对整个油组成的影响。在抗生物降解的 C14-C16 双环倍半萜烷（双环烷）中，重排的 C15 异构体（化合物 X）随着生物降解水平的增加而变得相对富集，这与 C14-C16 双环烷的总浓度降低形成对比，但与双环环烷的富集一致FTICR-MS 数据显示的酸。建议将化合物 X 与所有已鉴定的 C15 双环烷异构体的浓度比（DMHD 比）作为新的生物降解参数。该比率与 3-甲基屈/甲基屈、苯并萘并[1,2-d]噻吩/苯并萘并[2,1-d]噻吩和基于NSO化合物的生物降解比率密切相关。与三甲烷结构相比，化合物 X 具有一个额外的乙基但少一个甲基。通过去甲基化减少空间效应可以解释化合物 X 的难熔性质，这构成了讨论化合物 X 可能来源的基础。多环芳烃和 NSO 化合物的改变暗示了所研究油样的广泛生物降解。这表明化合物 X 的相对富集可以用作在 Peters 和 Moldowan 生物降解水平（PM 水平）4-5 时重油生物降解的新指标，此时藿烷和甾烷仍然完好无损。