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The Discovery of 25-Norhopanes in Non-Degraded Oils of Permian Reservoir in Junggar Basin, China


25-Norhopanes are interpreted as indicators of severe biodegradation, particularly when they occur as a complete homologous series. However, many oils that contain 25-norhopanes appear to be non-biodegraded or only slightly biodegraded, as indicated by a predominance of n-alkanes. Previous studies suggest that such oils were mixtures of biodegraded oil residues that were dissolved by non-degraded oil during accumulation in the reservoir. Such mixed oils are common in basins with shallow (<80 °C) reservoirs. The source rock and crude oil samples buried deeper than 3000 m in the Permian Lucaogou Formation in Junggar Basin are collected and analyzed by gas chromatography–mass spectrometry (GC–MS). The distribution of n-alkanes is completed and the 25-norhopane homologue is detected. On the basis of a hydrocarbon-generation thermal simulation experiment using kerogen in addition to GC–MS, 25-norhopane series is detected in the kerogen pyrolyzates. This result proves that the organic matter in the source rocks is capable of producing the 25-norhopane series. The presence of the 25-norhopane series in source rocks and non-biodegraded oils proves that biodegradation is not the only source of the 25-norhopane homologue series. Biomarker assemblage characteristics in the source rock indicate that the reducing deposition environment, saline environment, and the activity of anaerobic microbes may have provided the geological and geochemical conditions necessary for distribution of the 25-norhopane homologous series.