| Popis: |
Distributions of alkylated naphthalenes, phenanthrenes, and dibenzothiophenes and of selected non-alkylated (parent) tri- and tetra-aromatic compounds were determined by capillary gas chromatography in extractable aromatic hydrocarbon fractions from 46 core samples from the middle Upper Rhine Graben. Samples of Palaeogene and Jurassic ages at 250–2400 m depth were derived from four boreholes in the Rastatt and Kehl Troughs. Assessment of thermal maturity based on the Methylphenanthrene Index (MPI 1) was in agreement with mean random vitrinite reflectance (Rr) only in the borehole closest to the Graben centre, where overmature stages were reached below 2000 m. Otherwise, early thermal evolution of the organic matter inferred from Rr < 0.7% and Rock-Eval® pyrolysis data (Tmax < 440°C), was in contrast to the postmature stages indicated by the predominance of more stable, β-type 2-methyl- and 3-methylphenanthrenes (MP's) over less stable α-type 1-MP and 9-MP. The 2-MP1-MP ratios ⩾1.4 along with a predominance of phenanthrene, fluoranthene, and pyrene over alkylated tri- and tetra-aromatic hydrocarbons were attributed to pyrolytic-like polycyclic aromatic hydrocarbons probably associated with reworked organic matter. Minor impregnations of migrated oil were revealed by elevated contents of methylated phenanthrenes. Dimethylnaphthalenes (DMN's) were mostly generated at 0.8–0.9% Rr, as indicated by (2,6-DMN + 2,7-DMN)1,5-DMN ratios of 3–5, and then underwent extensive redistribution by upward migration. The exceptional predominance of 4-methyldibenzothiophene (MDBT) and dibenzothiophene together with very high 4-MDBT1-MDBT ratios of 30–50 in certain samples from the Rastatt Trough were attributed to minor impregnations of hydrothermal petroleum. |
