Evaluation of a Simple Model of Vitrinite Reflectance Based on Chemical Kinetics

ABSTRACT We present a simplified version of a vitrinite maturation model, complete with sample spreadsheet, based on changes in vitrinite composition with time and temperature. The simplified model, called EASY%Ro, uses an Arrhenius first-order parallel-reaction approach with a distribution of activation energies. EASY%Ro has been calibrated to a more rigorous model of vitrinite maturation based on the chemical properties of coal vitrinite. With EASY%Ro, a profile of vitrinite reflectance vs. time can be obtained for a given stratigraphic level if the time-temperature history for that level has been estimated. When applied to multiple stratigraphic levels, EASY%Ro can be used to compute profiles of the percent of vitrinite reflectance with depth for comparison with borehole data and to optimize thermal history models. EASY%Ro can be used for vitrinite reflectance values of 0.3 to 4.5%, and for heating rates ranging from those in the laboratory (1°C/week) to those in slowly subsiding geologic basins (1°C/10 m.y.). Examples of model applications range from sedimentary rocks heated by an igneous intrusion to a variety of burial histories. Vitrinite maturation predicted by EASY%Ro is compared to other methods currently being used, such as the Lopatin time-temperature index, level of organic maturity, and other approaches using a single activation energy. Our model successfully estimates vitrinite reflectance due to thermal metamorphism of sedimentary rocks heated by igneous intrusions, geothermal fluids, and burial in a variety of basin settings.