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ABSTRACT: Petroleum of the Cooper and Eromanga Basins, Australia: A model of mixed sources

Arouri, K.R.1, X. Yu1, D. M. McKirdy1, and T. Hill2
(1) University of Adelaide, Adelaide, Australia
(2) Primary Industries and Resources, Adelaide, Australia

Secondary migration mechanisms in the Permo-Triassic Cooper and Jurassic-Cretaceous Eromanga Basins are poorly understood but can be better recognized through a comprehensive approach to more accurately estimate the relative contributions and mixing ratios of oils from all the petroleum systems involved. Conventional steroid and triterpenoid biomarkers have proved inoperative in this regard, and other qualitative (age-specific biomarkers, and MPI expulsion maturities) and semi-quantitative (isotopic mass-balance) approaches currently in use still require rigorous refinements as they may introduce bias towards the Eromanga contribution. We introduce here a new method "the mixing model" to quantify the mixing ratios of hydrocarbons in both basins, using statistical tools and computer digital imitation of various source and maturity parameters of both the light- and heavy-end hydrocarbons in crude oils. Theoretically, in a pure, marginally mature, Eromanga-sourced oil both the light-end parameters (e.g. methylethylbenzene index, MEBI) and the heavier-end parameters (e.g. methylphenanthrene index, MPI) exhibit marginal maturity signals, whereas higher maturity should be reflected in both components of pure Cooper oils. A mixed source is inferred where mixed signals for both ends occur. A simplified two-end-member model is discussed, along with examples of all the mixing spectra within both basins. Our digital imitation clearly demonstrates that the distribution pattern of the maturity data (e.g. MEBI versus MPI) of a mixed oil is not influenced only by the mixing ratio, but also by its compositional variations. The match between our digital imitation and the actual dataset is demonstratable and can be used to more precisely calculate the relative contributions of multiple sources into a hydrocarbon accumulation.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia