Informativeness of the Phenanthrene Indicators of Organic Matter Maturity at Late Mesocatagenesis and Apocatagenesis: An Example of Material from Superdeep Hole Srednevilyuiskaya 27 in East Siberia
One of the widely used methods of assessing the maturity of organic matter based on aromatic compounds uses indexes reflecting changes in the relative concentrations of phenanthrene and its methyl-substituted derivates. A set of ratios now widely used in national and international practice (MPHI, MPI-1, MPI-1mod., PP-1, and PHI) is analyzed based on data on these compounds in terrestrial organic matter (OM) recovered by Srednevilyuskaya-27 superdeep well. Equations are derived for calculating synthetic values of vitrinite reflectance based on phenanthrene maturity indexes at meso- and apocatagenesis. Data are presented on typical changes in the relative phenanthrene concentrations and its methyl-substituted derivates under high PT parameters: the more stable behavior of 2-, 3-, and 1-methylphenanthrene than 9-methylphenanthrene. The relative concentrations of phenanthrene itself are demonstrated to broadly vary at apocatagenesis, and this factor deteriorates the maturity indexes of the phenanthrene ratios. It is necessary to select proper equations for various hydrocarbon systems and determine the boundaries of the transition from positive correlations between phenanthrene indexes with the vitrinite reflectance to negative ones. The obtained results are compared with the classic understanding of the problem.
