Isotopic compositions and biomarkers in crude oils from the Tarim Basin: Oil maturity and oil mixing

Hydrogen and carbon isotopic compositions of n-alkanes, as well as biomarkers in various types of oil from the Tarim Basin were analyzed. Data were used to discuss the occurrence and formation of marine oils from this old composite basin, of which we have limited understanding. First, hydrogen and carbon compositions of n-alkanes clearly classify the collected oils into two distinct groups, supporting the reported oil classifications based on biomarkers. Group I, the majority of oil samples, is characterized by relative enrichment in D and n-alkane δD values vary from −110‰ to −75‰. Group II includes two heavy oils and δD values of n-alkanes range from −145‰ to −142‰. Second, variations in the δ13C value of n-alkanes (2–3‰) show good correlations with the maturity indices, indicating that kinetic isotopic fractionation is one control on the δ13C value of n-alkanes in the maturation process. δD values of n-alkanes in most oils are also evidently influenced by the oil maturity. Finally, by combining the isotopic compositions with biomarkers of oils, two distinct oil mixing processes were recognized in the oils from the Tabei and Tazhong uplifts in this old composite basin. The former process is the mixing of oils from the same source at different maturity stages, while the latter involves the mixing of oils from distinct sources at different maturity stages. Biodegraded oils may contribute to the total oils, but their proportions are minor.