Relation between n-alkane distributions and effective coalification temperatures in some Permian shales |
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| Authors: | Reginald T Mathews AC Cook RB Johns |
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| Institution: | Department of Geology, University of Melbourne, Melbourne, Australia;Department of Geology, University of Wollongong, Wollongong, Australia;Department of Organic Chemistry, University of Melbourne, Melbourne, Australia |
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| Abstract: | In Permian shales of the Sydney Basin, Australia, n-alkane distributions have been compared with effective coalification temperatures (ECTs) estimated from vitrinite reflectivities. The upper, non-marine part of the section shows evidence of progressive cracking (shift of n-alkane maximum toward shorter chains and tendency to eliminate longer chains) as depth and ECT increase; but this trend is not maintained in the underlying marine section.All samples show lack of a maximum in the longer-chain n-alkane distribution. Possible reasons discussed are (i) a cracking rate of long chains greater than their formation rate; (ii) a need for higher temperatures than the rock has so far undergone to produce a new crop of long chains; or (iii) exhaustion of the straight-chain generating potential of the kerogen. Pyrolysis experiments may be effective in testing these possibilities.The linking of ECTs to alteration stages of sediment hydrocarbons opens the possibility of comparing these stages among formations which differ in age and organic and inorganic composition, and among basins of diverse geological history. |
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