Sterane isomerisation and moretane/hopane ratios in crude oils derived from Tertiary source rocks

Abstract

The extent of sterane isomerisation reactions and the moretane/hopane ratios of 234 crude oils, taken world wide, from a wide variety of source rocks of differing geological ages, have been measured. This data indicates that in 78 crude oils derived from Tertiary source rocks, sterane isomerisation reactions as determined by the 20S/(20S + 20R) ration of the C 29 5α(H), 14α(H), 17α(H) normal-steranes and the C 29 iso/(iso + normal) ratio [iso = 5 α (H), 14 β(H), 17 β(H)] are mainly incomplete and sometimes considerably so. In addition, the same crude oils have 17β(H), 21α(H)-moretane/17α(H), 21β(H)-hopane ratios which are significantly greater (predominantly in the range 0.10–0.30) than those of crude oils derived from older, mature source rocks (mainly less than 0.1). This data, for crude oils, lends support to the hypothesis, proposed by Mackenzie and McKenzie (1983) for source rock extracts, that the time/temperature constraints of sterane isomerisation reactions are such that the time available for isomerisation in Tertiary sediments is generally insufficient, despite generation of crude oil at relatively high temperatures. An alternative hypothesis is that the incomplete sterane isomerisation of Tertiary crude oils may be due to generation of these crude oils from their deltaic, land plant-containing source rocks under low heating conditions. A third hypothesis proposes that the Tertiary crude oils may have picked up the incompletely isomerised steranes from immature sediments during migration. Although possible in particular instances, such a mechanism does not appear to be generally applicable since, in that case, the phenomenon would then appear to be restricted to the Tertiary. The higher moretane/hopane ratios of the Tertiary crude oils could suggest that constraints, similar to those applying in sterane isomerisation, also operate in the conversion of moretane to 17α(H)-hopane.