Distribution and evolution of tricyclic terpanes in lacustrine carbonates

Abundant and peculiar pairs of C20 to C25 tricyclic terpanes were discovered in the Tertiary lacustrine carbonates of the Biyang Depression. Though all organic matter contained in the entire carbonate section are of type I kerogen, the distribution of the tricyclic terpanes varies regularly with the burial depth of samples. The relative concentration of 13α(H), 14α(H) isomers in the paired tricyclic terpanes declines sharply with increasing burial depth. On the basis of this variation, new tricyclic terpane maturity parameters for immature and early mature carbonate source rocks are proposed.