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 (T_max in °C). These data are modelled by the linear regression equation:

$$\ln (Rm) = 0.078 T_{\max } - 1.2$$

T_max 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 T_max 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 T_max) of these systems ranges from 10,000 yr to more than 10 m.y. The strong correlation of T_max with Rm, irrespective of functional heating duration and in diverse geologic systems, indicates that increasing time at T_max has little influence on thermal maturation of sedimentary organic matter. Instead, uncertainty in correction of borehole temperature logs, T_max 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 T_max — Rm calibration are compared to uncorrected T_max data. We found that T_max 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.

These studies imply that T_max controls Rm, making the relationship useful as a maximum geothermometer, but that several physico-chemical and technical factors obscure the correlation. The problems in measuring T_max and Rm, shown by the appreciable data scatter, make our calibration imprecise. However, application of this geothermometer to systems where T_max is well known shows that it yields realistic paleotemperature estimates. Other support for temperature control of Rm is documented from studies of metamorphic mineral assemblages and coal rank, and critical testing of temperature-time-rank models in sedimentary systems.