The sulphur, paraffin, resin and asphaltene contents of over 7, 100 Cenozoic, Mesozoic and Palaeozoic oils were analysed and were tentatively correlated with reservoir rock age. The data came from 90 oil-bearing basins worldwide. Oil compositions appear to show cyclical changes throughout the Phanerozoic, and these changes can be related to orogenic cycles and marine transgressions and regressions.