Naphthalenes in Neogene sediments of the Shinjo basin, Japan

Naphthalene and its methyl homologs were found in Miocene to Pliocene sedim ents of the Shinjo basin. The total concentrations of naphthalenes ranged f rom 26 to 5,700 ng g(-1) from the top to the bottom sediments of an about 1 ,600 m thick sequence. Naphthalenes were mainly distributed in the Kusanagi Formation and their depth profile showed a characteristic feature, indicat ing a concentration peak at the onset of oil-generation layer. The ratio of 2 to 1-methylnaphthatene as well as the relative abundance of dimethylnaph thalene isomers, except for 1,8-isomer, showed no depth dependence. Therefo re, it seems that no significant isomerizations of the naphthalene homologs had occurred during the burial diagenesis around and above the oil-generat ing zone in the sediments. However, as to the least stable 1,8-dimethylnaph thalene isomer, the negative logarithmic change of the relative abundance o f the isomer (Alexander et al., 1984) was observed showing a trend of incre asing with depth.