Notwithstanding extensive previous work on the origin of petroleum in the Cooper and Eromanga Basins, the relative hydrocarbon contributions by their source rocks to any given reservoir remain difficult to determine. Although oils of Jurassic and Cretaceous origins are evident locally, it is widely accepted that most oil pools are either entirely Cooper-sourced, or are of mixed source affinity. Various methods have been employed to assess the relative contributions of different source rocks to individual oil pools in basins containing more than one petroleum system. In the case of the Cooper and Eromanga Basins, these include: (1) age-specific biomarkers (Alexander et al., 1988), (2) maturity contrasts between light- and heavy-end aromatic compounds (Alexander et al., 1996), (3) n-alkane carbon isotopic profiles (Boreham and Summons, 1999) and (4) mixing curves obtained by cross-plotting aromatic source and maturity parameters (Michaelsen and McKirdy, 1999).
All these approaches are at best semi-quantitative (although the last one has a resolution of ±10%). Moreover, the methods that make use of biomarkers appear to over-estimate the contribution from less mature Eromanga source rocks, thereby under-estimating the Cooper input. A mathematical model - based on a comprehensive mass balance of light and heavy components and inspired by the work of Alexander et al. (1996) - has been devised in an attempt to better quantify the mixing ratios of discrete oils in a given reservoir. A simplified two-source model is discussed here using as examples "pure" Cooper and "pure" Eromanga oils, and the full spectrum of mixing ratios represented by the 72 oil and condensate samples analysed in this study.