DIAMONDOID HYDROCARBON RATIOS - NOVEL MATURITY INDEXES FOR HIGHLY MATURE CRUDE OILS

Diamondoid hydrocarbons have been identified in oil and source rock sa mples from the Tarim, Yinggehai, Qiongdongnan and other Chinese basins . Two diamondoid hydrocarbon ratios have been used as novel high-matur ity indices to evaluate the maturation and evolution of crude oils and condensates in these Chinese basins. The two ratios, 1-MA/(1-MA + 2-M A) (%) (MAI = methyl adamantane index) and 4-MD/(1-MD + 3-MD + 4-MD) ( %) (MDI = methyl diamantane index) were found to increase with increas ing vitrinite reflectance. The initial values for the ratios are ca. 5 0 and 30%, respectively. Diamondoid hydrocarbons are products of rearr angement of polycyclic hydrocarbons and their ratios are found to be a ffected by organic input to a minor extent. Based on this study, crude oils from the Tarim Basin are generally believed to be very mature (R (o) > 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, e quivalent R(o) values of 1.6-2.0%. Gases and condensates from the Ying gehai and Qiongdongnan basins are gener ated mainly from Tertiary sour ce rocks below 4000-5000 m. (C) 1997 Elsevier Science Ltd.