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 analys ed for ethylalkanes using GC-MS techniques. In the less mature samples, 3- and 5-ethylalkanes were in higher abundance relative to the other isomers a t odd carbon numbers from C-17 to C-23. In the more mature samples, this od d preference was no longer apparent. Several other low maturity samples wer e analysed and found to have a similar ethylalkane distribution to the shal low sample from the sedimentary sequence. A mature crude oil which was also analysed exhibited a distribution similar to the deeper sample from the se dimentary sequence. It is suggested that the initial odd preference is the result of defunctionalisation of specific natural product precursors, and t hat with increasing maturity this preference is diluted by input of ethylal kanes without preference. A mechanism involving acid-catalysed rearrangemen t of n-alkenes to form T-branched alkanes is proposed to account for the ch ange in relative ethylalkane abundances. (C) 1998 Elsevier Science Ltd. All rights reserved.