Evidence for synthetic generation of methylphenanthrenes in sediments

All isomers of methylphenanthrene are formed when phenanthrene is reacted with methane and/or long chain n-alkanes in the presence of coals or kaolinite in closed steel reactors in the range 230-360 °C. In particular, the 3-methylphenanthrene isomer, for which a convincing degradative precursor-controlled source has not been described, becomes increasingly prominent at higher experimental temperature due to the 4 → 3 methyl shift promoted by steric interaction at the 4- and 5-positions. Therefore, a geosynthetic origin for the high concentrations of 3-methylphenanthrene in mature carbonaceous sediments is indicated. Furthermore, in a broader sense, the data demonstrate the need to consider synthetic, in addition to degradative, reactions in maturational processes in sediments.