ABSTRACT

Forty five mono and 12 bicyclic alkanes were detected in a sedimentary sequence from Miocene to Pliocene in the Shinjo basin, northeastern Japan. The monocyclic alkanes included polyalkylcyclohexanes (C₉, C₁₀, C₁₁, and C₁₃), a series of n-alkylcyclohexanes (C₉–C₂₆) and a series of n-alkylcyclopentanes (C₁₂–C₂₆). The bicyclic alkanes were from C₉ to C₁₂, including cis- and trans-hydrindanes, cis- and trans-decalins, and methyl- and dimethyldecalins. These cyclic alkanes were detected at a level of 0.01–1 nmol g^-1. Concentrations of 2-alkyl-1, 1, 3-trimethylcyclohexanes (C₁₁ and C₁₃), a major portion of n-alkylcyclohexanes (C₁₁–C₂₆), all n-alkylcyclopentanes, and methyl- and dimethyldecalins showed a maximum in the lower part of the sequence which has been known to be an oil generation zone. n-Alkane (C₁₅–C₃₈) concentration as well as that of pristane and phytane also showed a maximum in the lower part. Concentrations of 1, 1, 3-trimethyl-, and 1, 1, 2, 3-tetramethylcyclohexanes, a minor portion of n-alkylcyclohexanes (C₉ and C₁₀), cis-hydrindane, cis- and trans-decalin showed a maximum in the middle part of the sequence. Only trans-hydrindane showed a maximum concentration in the upper part of the sequence, as did perylene. The molecular distributions of series of n-alkylcyclohexanes (C₁₅–C₂₆) and n-alkylcyclopentanes (C₁₅–C₂₆) showed a predominance in a range from C₁₅ to C₁₈ in the upper part of the sequence, whereas that of n-alkanes did from C₂₇–C₃₁ in the part. However, the distributions of the three series became relatively smooth in the lower part, reflecting the further progress of diagenetic maturation resulting from the oil generation in the part.