Isomerization of methylphenanthrenes during catagenesis and metagenesis

Abstract

Methylphenanthrenes are one of the aromatic compounds commonly found in oils and rock extracts. Although methylphenanthrene-indices (MPI) have been proposed since the 1980's as organic maturity parameters up to high maturity level, quantitative characteristics as indicators and detailed mechanisms of isomerization have not yet been solved. Therefore, we determined a pair of preliminary kinetic parameters for methyl-shift reaction of a-isomer to )3-isomer of methylphenanthrenes, based on artificial heating experiments. The experiments were performed on Sarukawa-oils with water, clay-catalyzer and Ar-gas at a series of high temperatures (300, 350, 370, 380,400 and 450°C) for 0.5 to 40 days. The kinetic parameters obtained are 57.2kcal/mol (239kJ/ mol) for apparent activation energy and 2.23x 1012s-1 for frequency factor, respectively. Using these kinetic parameters, a computer simulation was performed on MPI 3 transformations. The results suggest that MPI 3 is sensitive to temperatures between 150 to 230°C at heating rates possible in sedimentary basin, and MPI 3 shifts towards the lower vitrinite reflectance stage when heating rates are lower. According to the results of the simulation, Tertiary mudstones from catagenetic zone show that the heating rate in Niigata district was low in the deep portion (4,2005,500m) of the Tojo-TS1 and high in the middle to deeper portion (3,200-4,400m) of the METI-Tomikura. In contrast, in Cretaceous mudstones from the metagenetic to metamorphic zone (up to about 300°C) in the Muroto district, some methylphenanthrenes survived and MPI3 decreased with increasing maturity. These results could offer key information to decipher the mechanisms of MPI transformations.