Alkyl and polynuclear aromatic thiophenes were detected in sedimentary rocks of a stratigraphic sequence from Miocene to Pliocene in Shinjo basin, Japan. C 25 Highly branched isoprenoid (HBI) thiophenes, dibenzothiophene, C 1 -and C 2 -alkyldibenzothiophenes, and benzo[b]naphto[1,2-d]thiophene were rather predominantly present. The C 25 HBI thiophenes consist of three isomers including a pair of diastereomers. They were present in sediments from the Nakawatari to the upper part of the Kusanagi

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... rt of the Kusanagi Formations and absent at the middle and lower parts of the Formation. The depth profiles of their concentrations do not coincide with those of the C 25 HBI alkane concentration and bulk sulfur content in the same sediment samples. It is postulated that the C 25 HBI thiophenes and the C 25 HBI alkane were produced by the reaction of C 25 HBI alkadiene and C 25 HBI polyene, respectively, with reduced sulfur during early diagenesis. The variation in composition of the C 25 HBI thiophene isomers was observed in the sediments, suggesting that they were derived from different kinds of C 25 HBI alkadienes. The concentrations of the polynuclear aromatic thiophenes showed an increasing trend downwards the upper to middle Kusanagi Formation. Among these thiophenes dibenzothiophene and benzo[b]naphto[1,2-d]thiophene concentrations varied from 10 to 730 and 14 to 210 pmol g -1 , respectively. The depth profiles of their concentrations were different from that of the C 25 HBI thiophenes. It is likely that the polynuclear aromatic thiophene formation started from different precursors followed by different processes from those of the C 25 HBI thiophenes.