Examining the relationship between Tmax and vitrinite reflectance: An empirical comparison between thermal maturity indicators

Abstract

For over 40 years Rock-eval pyrolysis data have been used to assess thermal maturity and source rock potential. There have been many published equations representing the transformation of Tmax to vitrinite reflectance (Ro) equivalence, but there has not been an attempt to investigate the empirical relationship using a large dataset. In this study, 33,732 approximately coincident measurements from 903 publications were utilized to test the applicability and practicality of Tmax, hydrogen index (HI), and production index (PI) as thermal maturity indicators. Additional sample metadata (sample type, lithology, organofacies, sample age, and other Rock-eval pyrolysis parameters) were also evaluated to ascertain if there were parameters that more frequently led to higher confidence measurements.There was a positive correlation between Tmax and Ro, but there is considerable scatter in the data. A stronger trend was revealed in the mean, median, and mode values for each Tmax versus Ro pair that can be represented by the equation Ro = (0.013 x Tmax) - 5.0 with a R2 value of 0.22, which may be an improvement upon previously published equations. There was a breakdown in the correlation between Tmax and Ro at the beginning of the dry gas window (Ro values ~1.5; Tmax ~500 °C), which was not unexpected since the S2 peak significantly decreases through the wet gas window. Tmax-derived thermal maturity estimates are therefore most reliable between the values of ~430 °C and ~500 °C. It was also observed that coals with high total organic content (TOC) and S2 values had the least amount of data scatter among the different source rock organofacies. This may be due to the higher initial S2 values and these samples having a more prominent S2 peak allowing them to yield more reliable Tmax determinations.