An enhanced presence of tricyclic terpanes in the Triassic oils from the tight conglomerate reservoir in the Mahu Sag, Junggar Basin, NW China

A major exploration breakthrough has been made in the Mahu Sag, Junggar Basin. However, the origin of oils from Triassic Baikouquan Formation (T1b) remains debated. In this study, 20 oils and 44 cores were selected to investigate their distributions of tricyclic terpanes (TTs). The results show that the relative abundance of TTs in oils is higher than those of hopanes due to a high maturity. Permian Wuerhe Formation (P2w) cores are dominated by C20TT. Permian Fengcheng Formation (P1f) samples, predominated by C23TT, are mature to high-mature, while Permian Jiamuhe Formation (P1j) samples, having the same abundance of C21TT and C23TT, are in the over-mature stage. Maturity may be a likely possible reason for the different occurrences of C19-C23TTs between P1f and P1j as their organic matter source and depositional environment were similar. High gammacerane and C29 sterane occurred in T1b oils, P1f and P1j cores, while those in P2w are little, indicating P2w had a minor contribution for T1b oils. The delta 13C values of oils and P1f are similar and lower than those of P2w and P1j source rocks. Therefore, it is preliminarily speculated that the oils may be sourced from the deep buried P1f in the over-mature stage.