INSIGHTS ON RELATING TMAX TO VITRINITE AND SOLID BITUMEN REFLECTANCE IN HYDROUS PYROLYSIS EXPERIMENTS

Ascertaining the thermal maturity of coal or shale basins, for which measuring vitrinite (% VR_o) and solid bitumen (% BR_o) reflectance is considered the most robust analytical approach, is an important step in the exploration for potential hydrocarbons. However, the process to collect reflectance values has limitations (e.g., time-consuming, subjective to interpretation, and misidentification of macerals). Another widely used method to determine thermal maturity is by examining T_max and using the “Jarvie equation” to predict reflectance (% R_o). This study investigates whether T_max values from hydrous pyrolysis (HP) experiments can be used as a proxy to calculate % R_o by comparison to T_max-R_o relations in natural rock series.

This study utilized 8 coal and 13 shale samples that were organic-rich, of low thermal maturity, and from diverse depositional environments. The HP experiments used crushed samples heated for 72 hours at 300, 320, 340, 350, and 360°C in SwageLok mini reactors. Bulk geochemical analyses and % R_o measurements were performed on the starting materials and resulting HP experimental residues. Relational T_max-R_o equations were derived from HP residues and compared to those from natural series, specifically the Wilcox Group, the Bakken and Duvernay Formations, and the Huron Member of the Ohio Shale.

The results from the coal and shale series show that T_max and % R_o values increase with HP experiment temperature and the coal sample series show a steeper T_max increase than the shale series. Reflectance increases were greater in coals (% VR_o) than shales (% BR_o) at lower HP temperatures, though these differences decrease at higher HP temperatures such that % VR_o and % BR_o insersect at ~1.1%. Comparison of the derived equations demonstrates that the T_max-R_o relationship from HP residues cannot effectively be applied to calculate % R_o in natural series. In general, the HP-derived T_max-R_o formation-specific equations either overpredict R_o at higher T_max or underpredict R_o at lower T_max values for natural series. These results may be due to kinetic differences between vitrinite and solid bitumen maturation and how those reactions potentially differ during experimental versus natural systems, which isothermal time series maturation experiments may help elucidate.