The correlation of vitrinite reflectance with maximum temperature in humic organic matter

Abstract

Mean random vitrinite reflectance (Rm in %) shows a strong correlation (r2=0.7, n>600) with maximum burial temperature (Tmax in °C). These data are modelled by the linear regression equation: $$\ln (Rm) = 0.078 T_{\max } - 1.2$$ Tmax and Rm were compiled from over 35 systems, rich in humic organic matter to minimize the effect of chemical composition on Rm. The thermal maturation data span a range from early diagenesis to greenschist metamorphismover a Tmax interval from about 25 – 325° C and 0.2 – 4.0 % Rm. Burial history reconstruction indicates that the functional heating duration (elapsed time as temperature increases within 15° C of Tmax) of these systems ranges from 10,000 yr to more than 10 m.y. The strong correlation of Tmax with Rm, irrespective of functional heating duration and in diverse geologic systems, indicates that increasing time at Tmax has little influence on thermal maturation of sedimentary organic matter. Instead, uncertainty in correction of borehole temperature logs, Tmax determination, and Rm measurement explains much of the remaining variability not accounted for by the regression analysis. We did not attempt to correct the measured borehole temperature to equilibrium reservoir conditions because there is no consensus on which method to use, the necessary data is often unrecorded, and predictions made from our Tmax — Rm calibration are compared to uncorrected Tmax data. We found that Tmax is difficult to determine in sedimentary environments that have cooled because of the weak thermal imprint on the rocks in low temperature systems and poorly-known burial histories. Variability in Rm measurement appears mainly due to operator or laboratory bias, increasing bireflectance with rank, and variation in diagenetic history which causes reflectance suppression.