USE OF THE METHYLPHENANTHRENE INDEX TO CHARACTERISE EXPULSION OF COOPER AND EROMANGA BASIN OILS

Abstract

The methylphenanthrene index (MPI) molecular maturity parameter is available for over 100 Cooper and Eromanga Basin oils. Oil maturity data define the threshold and range of expulsion maturity for source rocks and can be used to determine oil-source affinity. Mapping of this maturity range for all potential source rocks identifies areas of greatest oil potential.

Cooper and Eromanga oils were expelled over a wide maturity range commencing at 0.6 per cent calculated vitrinite reflectance equivalent in some parts of the basin. Oil occurrence and expulsion maturity are controlled by variations in source quality such that no single expulsion threshold can be applied basin-wide. The full oil potential of the basin will only be realised by selective drilling of prospects with access to source rocks in the 0.60-0.95 per cent vitrinite reflectance range.

The timing of oil expulsion is determined by using oil maturity data to calibrate thermal modelling of basin depocentres. Peak expulsion occurred during the Cretaceous and therefore prospects with pre-Tertiary structural growth are favoured.

Structural embayments with thick Permian section at the southern margin of the Cooper Basin, plus the flanks of the Patchawarra and Nappamerri troughs, are highly prospective in terms of oil source potential and will be further evaluated by drilling in 1990.