The effect of maturation on the distribution of monoethylalkanes in Late Cretaceous sedimentary rocks and crude oils from the Gippsland Basin, Australia

Rock samples from a sedimentary sequence of Late Cretaceous age were analysed for ethylalkanes using GC MS techniques. In the less mature samples, 3- and 5-ethylalkanes were in higher abundance relative to the other isomers at odd carbon numbers from C17 to C23. In the more mature samples, this odd preference was no longer apparent. Several other low maturity samples were analysed and found to have a similar ethylalkane distribution to the shallow sample from the sedimentary sequence. A mature crude oil which was also analysed exhibited a distribution similar to the deeper sample from the sedimentary sequence. It is suggested that the initial odd preference is the result of defunctionalisation of specific natural product precursors, and that with increasing maturity this preference is diluted by input of ethylalkanes without preference. A mechanism involving acid-catalysed rearrangement of n-alkenes to form T-branched alkanes is proposed to account for the change in relative ethylalkane abundances