Organic geochemical analyses of 20 crude oil samples from twelve wells were performed to determine if any genetic oil families could be identified within the Eromanga-Cooper Basin area. The results have been plotted graphically to demonstrate the effects of environment of deposition, source material and thermal maturity on these various oils. The oils have been placed into 4 groups characterised by increasing maturity, with the oil groups appearing to be determined more by reservoir position relative to the basin centre, than by stratigraphic age of the reservoir. All the oils are dominantly terrestrially sourced crudes, with a minor contribution from aquatic organic matter in 3 wells only. Further to their land plant source material affinities, it seems that many of the oils were derived, at least partially, from the remnants of bacteria which reworked the land plant organic matter after sediment deposition. The oils analysed fall into 2 main types: - Type I oils (medium gravity, very waxy crudes) appear to have originated from sediments deposited in open water, mildly reducing conditions, where the organic matter responsible for imparting high molecular weight hydrocarbons to the oil was preserved; Type II oils (high gravity, low wax crudes) originated from sediments which were deposited in harsher, oxidising, restricted input sedimentary environments. Under these conditions the organic matter which would have formed waxes was apparently destroyed, resulting only in the generation of light oils tending towards condensate. Sediment acidity levels at the time of deposition appears to have had little influence on whether a Type I or Type II oil was generated, nor does any preponderance of microbial vs original organic matter in the source rock.