Variations in Stable Carbon Isotopic Composition of n-Alkanes from Ordovician- and Triassic-Derived Oils in Lunnan Oilfield, Tarim Basin, NW China: Global Paleoenvironmental Constraints

Molecular stable carbon isotope technique was employed to study well-sourced crude oils collected from a single drilling well and from the entire Lunnan oilfield, Tarim Basin, NW China. The stable carbon isotopic composition of n-alkanes from crude oils showed that Ordovician-derived oils are enriched in {}+{13}C and Triassic-derived oils are depleted in {}+{13}C. This is consistent with the distribution and evolution trend of stable carbon isotope ratios in crude oils/organic matter from all over the world in geological history (Stahl, 1977; Andrusevich et al., 1998). An extensive survey of literature indicates that, except for thermal maturity, organic matter input and depositional environment, paleoenvironmental background is another key factor that affects the stable carbon isotopic composition of Ordovician- and Triassic-derived crude oils. The results showed that gas chromatographic-isotope ratio mass spectrometry (GC-C-IRMS), combining with biogeochemical evolution of organic matter in geological history, may be a powerful tool in refining oil/oil, oil/source correlations in multi-age, multi-source petroliferous basins like Tarim.