Relating Tmax and hydrogen index to vitrinite and solid bitumen reflectance in hydrous pyrolysis residues: Comparisons to natural thermal indices

Vitrinite reflectance (VRo; %) generally is considered the most reliable technique to determine the thermal maturity of sedimentary rocks. However, it is a time-consuming process to collect reflectance (R-o; %) measurements and is subjective to the interpretation of each trained technician, who must be able to discern between vitrinite and solid bitumen and other organic matter types. Inadvertent misidentification of solid bitumen for vitrinite can lead to reports of 'suppressed' VRo, especially at lower thermal maturities (< 1.0% R-o). Programmed pyrolysis data, such as T max and hydrogen index (HI), are comparatively inexpensive and more time-efficient to obtain than R-o data and are determined by instrument settings, rather than by operator decision, and are therefore independent of operator-based training or experience bias. This study uses hydrous pyrolysis (HP) residues from various coals and shales to relate measured VRo and solid bitumen reflectance (BRo; %) values to their respective T-max and HI values and determines whether these relationships can be used as a proxy to calculate R-o in naturally matured samples. Although the estimation of R-o is not always accurate, the results demonstrate that relational equations for shales and coals derived from the T-max and HI data of HP residues can effectively calculate R-o in natural series. Approximately 60% of calculated R-o from T-max and 83% of calculated R-o from HI relational equations are within interlaboratory reproducibility limits (+/- 0.2% shale BRo; +/- 0.06% coal VRo) when compared to their respective measured R-o values from natural series. Variables that may affect accuracy of the applied relational equations include variable sedimentary organic matter composition of samples, differences of maturation reaction kinetics of the sedimentary organic matter in experimental versus natural settings, and decreasing reliability of all thermal proxy measurements at higher maturities.