VARIATIONS IN THE STERANE CARBON NUMBER DISTRIBUTIONS OF MARINE SOURCE ROCK DERIVED CRUDE OILS THROUGH GEOLOGICAL TIME

The analysis of the sterane data of a large set of crude oils (414) derived from marine carbonate (27) and siliciclastic source rocks (14), where influences of terrestrial or lacustrine derived organic matter can reasonably be excluded, shows that there are increases in the relative content of C//2//8 steranes and decreases in the relative content of C//2//9 steranes through geological time. There are no consistent variations in the relative content of C//2//7 steranes through time. With one major exception (a Proterozoic oil from Onman), Palaeozoic and older crude oils are thus generally characterised by strong predominances of C//2//9 steranes and low relative concentrations of C//2//8 steranes. Significantly higher proportions of C//2//8 steranes and lower proportions of C//2//9 steranes occur in oils derived from Jurassic and Upper Cretaceous source rocks. Although the technique does not give an accurate means of determining the age of the source rock of a crude oil it is possible to distinguish younger crudes derived from the Upper Cretaceous and Tertiary from Palaeozoic and older crudes.