Burial temperatures from coal
Burial temperatures of rocks and minerals are determined in a variety of ways. Several methods are used to infer maximum burial temperatures from organic compounds in rocks, especially oil (petroleum). Not surprisingly, several methods have also been developed to interpret maximum burial temperature of coal and, by association, surrounding strata. The methods which use coal are not generally the primary methods for absolute burial temperature determination in burial history studies, but results from coal data can be used as an approximation of burial temperature, or to support other methods.
Vitrinite reflectance is used to determine the relative amount of maturation a coal (and other organic rocks) has undergone, and is used with other parameters to assign coal rank. All researchers agree that increasing coal rank and vitrinite reflectance correspond to increasing burial temperature, but interpretations of the absolute burial temperatures that correspond to a given rank or vitrinite reflectance vary depending on the methods and models used for the calculations. This chart summarizes estimates of burial temperatures (rounded to the nearest 5 degrees) at different vitrinite reflectance and rank, as determined by different research papers on coal maturation. The chart can be used to show a general range of possible burial temperatures for different ranks of coal, or to determine which specific model to use to infer a burial temperature for a coal of given rank. Some models are only heat dependent, whereas others vary with the time the coal is exposed to high heating. For heat-time models, two examples are shown to represent the influence of time, but you can refer to the original research publications to see continuous temperature results for different time-influenced curves. You should also refer to the original publications to see the methods and models used to derive the temperatures shown. The charts are provided here to show the general range of temperatures within the earth in which coals of different rank form.