Methyl diamantane index (MDI) as a maturity parameter for Lower Palaeozoiccarbonate rocks at high maturity and overmaturity

Diamantanes were identified in extracts from the Lower Ordovician Majiagou formations (Olms) of the central gas field, Shanganning Basin, China. At 31 00 3800 m, corresponding to R-o 1.9-3.9%, the MDI [methyl diamantane index = 4-MD/(1-MD + 3-MD + 4-MD)] ranges from 40 to 65% for the source rock extr acts. Changes in the MDI index in the very mature sections (R-o > 2.0%) are relatively small and no linear correlation between MDI and R-o, or MDI and depth, is noted, as previously reported, suggesting the possibility that t he MDI may have limited applicability in maturity determinations. (C) 2000 Elsevier Science Ltd. All rights reserved.