Hopanoid hydrocarbon maturation by an igneous intrusion

The thermal effects of an igneous intrusion (0.9 m thick dyke) upon the abundance and composition of hopanes and hopenes are reported for a suite of samples within a single sedimentary horizon (a carbonate concretion > 1.6 m in diameter) from the Jurassic Dun Caan Shale Member (Bearreraig Bay, Isle of Skye, Scotland). The conventional hopane maturity parameters show normal trends towards the dyke and through the “oil window” (approximately 22–45 cm from the dyke, i.e. 25–50% dyke thickness), with the exception of the Ts/(Ts + Tm) parameter which shows a progressive fall with increasing maturity, until the middle of the oil window when it increases markedly. Molecular abundances demonstrate that the observed changes in the hopane maturity parameters result from the combined effects of generation (presumed to be from kerogen) and degradation of individual compounds, with isomerization in the free phase being minimal or unimportant. The unexpected behaviour of Ts/(Ts + Tm) results from the minimal generation of Ts compared with Tm. Our data indicate that the Ts skeleton is not present in the kerogen, and at least under the elevated heating associated with an igneous intrusion may be derived from 18α(H)-22,29,30-trisnorneohop-13(18)-ene in the bitumen. Thermally induced modifications to hopane carbon number distributions are also reported, and show that oil-source rock correlations between samples of different maturity must consider the effects of generation/degradation of kerogen-bound hopanes.