Accumulation and mixing of hydrocarbons in oil fields along the Murteree Ridge, Eromanga Basin, South Australia

The Murteree Ridge is a focus for up-dip migration from two major hydrocarbon kitchens within the intracratonic Cooper (Carboniferous–Triassic) and Eromanga (Jurassic–Cretaceous) Basins of South Australia. The accumulation histories of nine oil fields along and adjacent to the ridge have been reconstructed by sequential solvent extraction and analysis of residual oils in sandstone core plugs from their stacked reservoirs. Four Cretaceous reservoir units received multiple oil charges that varied widely in source affinity, from mostly Jurassic (and/or Cretaceous) to overwhelmingly Permian in origin. The distributions of residual oil saturations in live and palaeo-columns are consistent with the existence of two compartments, with the uppermost pools (Cadna-owie, Murta) showing the highest Permian inputs. These accumulations represent the earliest escape of low-maturity Cooper-sourced oil into overlying Eromanga strata. This initial charge was displaced upwards into the shallower traps by subsequent hydrocarbon pulses. Three separate Permian-charge episodes can be recognised. The corresponding DST oils (0.6–0.7% Rc) represent either the compositional average of all charges to their respective reservoirs, or a continuation of the alternating filling pattern observed for successive charges. Oils in the Hutton (Jurassic) reservoir of the outlying Kerrina and Mudlalee Fields to the northeast appear to be mixtures of two distinct Early Permian oil families, variably co-mingled with locally derived Jurassic and possibly Cambrian hydrocarbons.