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Oil collected from Tarim oilfield was poured with different proportions into oil collected from Shengli oilfield. The mixed oil samples were placed in an artificial climate chamber to simulate a 160-days weathering process and reveal long-term weathering effects on conventional biomarker diagnostic indicators of n-alkanes in these mixed oil samples. The results show that only CPI2 in all common characteristic ratios of the experimental oils are suitable as the characteristic ratios for oil identification in long-term weathering processes (160d). The fact that a common diagnostic ratio of an oil sample after a serious weathering process changed more shapely than its original oil, maybe manifest that the original oil sample is a one with mixed source.

A set of sequentially weathered oils residue in sediments collected from Dalian Bay in different time after “7-16” oil spill accident, were analyzed by Gas chromatography–mass spectrometry (GC-MS) and Gas chromatography–isotope ratio mass spectrometry (GC-IRMS) to analyze the weathering process and evaluated the possibilities of GC-IRMS as a correlation tool in oil spill identification. Although some temporal variation was observed, no significant temporal shifts in the δ13C values for n-alkanes were measured in moderated weathered oils. The standard deviations of δ13C values of individual n-alkanes varied from 0.07% to 0.20%, which showed that the weathering has no significant effect on values of individual n-alkanes over 120 days. The results suggest that stable carbon isotope profile of n-alkanes can be a useful tool for tracing the source of an oil spill, particularly for the weathered oils absence of sterane and terpane biomarkers.