Molecular and carbon isotopic geochemistry of crude oils and extracts from Permian source rocks in the northwestern and central Junggar Basin, China

The Junggar Basin is a major oil producing province in China. Most oil reservoirs found so far in this basin are in the Mahu sag and neighboring uplifts, northwestern Junggar Basin. A total of 78 oils and 10 Permian source rocks from the northwestern and central Junggar Basin and two oils and two Permian source rocks from the eastern Junggar Basin were analyzed by GC, GC-MS and GC-IRMS. The 78 oils can be clearly classified into two groups based on these analytical results. For group I oils, δ13C values of individual n-alkanes are relatively higher and remain stable with increasing carbon number. For group II oils, these values are relatively lower and decrease at first, and then increase with carbon number. Differences in molecular parameters can be also observed between these two groups of oils. Group I oils generally have: (1) higher Pr/n-C17 and Ph/n-C18 ratios and lower Pr/Ph ratio, and (2) higher gammacerane/(C30 hopane + gammacerane) and β-carotane/(β-carotane + C30 hopane) ratios, compared with group II oils. In addition, group I oils mainly have tricyclic terpane distribution patterns with either C20 < C21 < C23 and C20 > C21 > C23 while group II oils mainly have the pattern of C20 < C21 and C21 > C23. However, molecular parameters overlap to some extent between these two groups of oils. Group I oils correlate well with the nine source rocks of the Lower Permian Fengcheng Formation (P1f), while group II oils correlate well with the three source rocks of Middle Permian age based on carbon isotopic and molecular compositions. The occurrence of these two groups of oils in the northwestern and central Junggar Basin are consistent with facies and thickness variations in the source rocks within the Lower Permian Fengcheng Formation (P1f) and Middle Permian Lower Wuerhe Formation (P2w).