Diamondoid hydrocarbon ratios: novel maturity indices for highly mature crude oils

Diamondoid hydrocarbons have been identified in oil and source rock samples from the Tarim, Yinggehai, Qiongdongnan and other Chinese basins. Two diamondoid hydrocarbon ratios have been used as novel high-maturity indices to evaluate the maturation and evolution of crude oils and condensates in these Chinese basins. The two ratios, 1-MA/(1-MA + 2-MA) (%) (MAI = methyl adamantane index) and 4-MD/(1-MD + 3-MD + 4-MD) (%) (MDI = methyl diamantane index) were found to increase with increasing vitrinite reflectance. The initial values for the ratios are ca. 50 and 30%, respectively. Diamondoid hydrocarbons are products of rearrangement of polycyclic hydrocarbons and their ratios are found to be affected by organic input to a minor extent. Based on this study, crude oils from the Tarim Basin are generally believed to be very mature (Ro > 1.1%). The Cambrian-Ordovician source beds are the major source rocks of crude oils and natural gases in this basin. Condensates from the Tarim, Yinggehai and Qiongdongnan Basins have higher maturities, equivalent to Ro values of 1.6–2.0%. Gases and condensates from the Yinggehai and Qiongdongnan basins are generated mainly from Tertiary source rocks below 4000–5000 m.