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

Diamantanes were identified in extracts from the Lower Ordovician Majiagou Formations (O1m5) of the central gas field, Shanganning Basin, China. At 3100–3800 m, corresponding to Ro 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 extracts. Changes in the MDI index in the very mature sections (Ro>2.0%) are relatively small and no linear correlation between MDI and Ro, or MDI and depth, is noted, as previously reported, suggesting the possibility that the MDI may have limited applicability in maturity determinations.