The behaviour of aromatic hydrocarbons in artificial mixtures of Permian and Jurassic end-member oils: application to in-reservoir mixing in the Eromanga Basin, Australia

Artificial mixing of two non-marine oils (one Permian and the other Jurassic) from the Cooper and Eromanga Basins, Australia was employed to assess the degree of co-mingling in reservoirs of the latter basin, and to verify estimates of mixing ratios based on a simple mass-balance approach using absolute concentrations of individual aromatic hydrocarbons. A cross-plot of the methylphenanthrene ratios 1-MP/9-MP and 2-MP/1-MP, already shown to be useful for recognising dual-sourced oil pools, has been re-calibrated against the model mixing curves (both ‘artificial’ and ‘theoretical’) obtained for experimental blends of the two end-member oils. Both curves reveal that previous attempts to identify the sources of mixed oils in this petroleum province using di- and tri-aromatic hydrocarbons seriously overestimate the Eromanga (Jurassic–Cretaceous) contribution. Samples plotting away from the model curves are likely to be mixtures of oils with maturities different to those of the end-members used in this experiment (Jurassic 0.52% Rc; Permian 1.00% Rc). The observed increase in thermal maturity (Rc) of the mixed oil as its Permian contribution increases casts doubt on the validity of distinguishing oil origins using only the maturity contrast between the Cooper and Eromanga petroleum systems.