ABSTRACT

Maleimides were detected in organic solvent extracts and chromic acid oxidation products of Miocene to Pliocene sediments of the Shinjo basin and were very likely degradation products of porphyrins and/or chlorophylls. These maleimides included a series of 2-methylmaleimides with 3-n-alkyl substituents up to C₅, benzomaleimide (phthalimide) and its methyl homologs, and tetrahydrobenzomaleimide. The total concentrations of free (the organic solvent extractable) and bound (the oxidatively extractable) maleimides ranged from 0.10 to 21 and from 1.5 to 75 nmol g^-1, respectively. The most abundant compound was 2-methyl-3-ethylmaleimide both in free and bound forms in most of the sediments. The concentrations of free and bound maleimides showed different depth profiles from those of the reported extractable organic compounds (EOC) and the total organic carbon (TOC). Their ratios to TOC decreased with increasing depth of the sediments in the oil-generating zone of the sedimentary sequence and were characteristically low in the high EOC/TOC region. The molar compositions of free maleimides did not show marked dependence on the sediment depth, whereas those of the bound forms did. The depth profile of the molar ratio of bound 2, 3-dimethyl- to 2-methyl-3-ethylmaleimide showed a clearly increasing trend with depth, which agreed well with those of other widely-accepted maturity parameters.