A geochemical survey of both commercial and noncommercial crude oils in Australia has shown them to be predominantly paraffinic or paraffinic-naphthenic in composition. High wax contents are common.

A close stratigraphic control on crude oil type operates in the Perth and Carnarvon basins of Western Australia. The chemical character of Australian oils can be related to their sedimentary environments, which range from lacustrine to nearshore marine. Spores, pollen, and cuticles derived from land plants have been a major source for the hydrocarbons.

Apart from changing the relative proportions of the different boiling fractions, maturation has had little effect on the basic composition of the oils. Alteration of crude oil by water washing and microbial activity in the reservoir is rare but, in several oil accumulations, light fractions are thought to have been mobilized by gas invasion of the oil reservoir.

Other differences in crude oil composition can be attributed to variations in microbial activity during early diagenesis of the organic source material.