Tricyclic terpane maturity parameters: response to heating by an igneous intrusion

The effects of thermal maturation upon the abundance and composition of the tricyclic terpanes have been investigated within a single sedimentary horizon within the Dun Caan Shalt Member (Isle of Skye, Scotland) intruded by a 0.9 m thick Tertiary dolerite dyke. Heating by the igneous body caused the concentrations of all 13 beta(K),14 alpha(H) and 13 alpha(H),14 alpha(H) tricyclic terpanes to increase towards or within the effective 'oil window', recording generation from abundant non-hydrocarbon precursors. such as kerogen- and asphaltene-bound components. progressive changes in tricyclic terpane composition accompany these concentration changes. An increase in the C-23 beta alpha/alpha alpha maturity parameter towards the dyke resulted from the relatively greater rate of generation of the pr isomer, combined with the earlier decline in concentration (by degradation or isomerisation) of the zu. component. The tricyclics/(tricyclics + hopanes) parameter shows a dramatic increase within the latter part of the oil window, due to the greater relative thermal stability of the tricyclic terpanes. The quantitative data demonstrate the:importance of the processes of generation (from non-hydrocarbon fractions of the sedimentary organic matter) and thermal degradation in the operation of tricyclic,terpane maturity parameters. (C) 1999 Elsevier Science Ltd. All rights reserved.