Relative Enrichment of Tricyclic Terpanes in Marine Crude Oils From Tarim Basin, NW China: Origin and Application to Genetic Petroleum Classification

ABSTRACT

The enrichment of tricyclic terpanes (TT) relative to hopanes (H) is a well‐documented phenomenon in marine‐derived crude oils from the Platform‐Basin region of the Tarim Basin, NW China. However, the genetic mechanisms remain incompletely understood and their petroleum geological implications lack systematic investigation. To investigate their enrichment mechanism, this study examines the distribution characteristics of tricyclic terpanes in crude oils from the Tazhong and Tabei areas that have been subjected to different secondary processes. The results show that the C ₂₃ TT/(C ₂₃ TT + C ₃₀ H) ratio shows an exponential positive correlation ( R ²  = 0.87) with the C ₂₈ 25‐N/C ₂₉ hopane ratio (biodegradation indicator) in biodegraded crude oils, while in Tabei crude oils this ratio exhibits a linear positive correlation with the Ts/(Ts + Tm) ratio (thermal maturity indicator), indicating both biodegradation and thermal maturity can cause tricyclic terpane enrichment relative to hopanes. In Tazhong oils, the C ₂₃ TT/(C ₂₃ TT + C ₃₀ H) ratio displays a negative correlation with the toluene/ n ‐heptane ratio (gas wash indicator), suggesting gas washing leads to relative depletion of tricyclic terpanes. This decrease may be superimposed by the effects of evaporation and migration fractionations. The diagram of C ₂₃ TT/(C ₂₃ TT + C ₃₀ H) versus Ts/(Ts + Tm) effectively distinguishes normally matured oils, severely biodegraded oils, and the oils experienced strongly evaporation distillation, showing good potential for genetic identification of variously altered oils in complex petroleum systems.