Consideration of the limitations of thermal maturity with respect to vitrinite reflectance, Tmax, and other proxies

Abstract

Many of those assessing geochemical data have taken a “cookbook” approach to interpretation. Such an approach generally does not take into consideration the limitations of the different data sets. Among these data sets are those associated with thermal maturity, which guides the interpretation of the extent of generation and hydrocarbon phase. The temperature of peak pyrolysis generation (Tmax) is commonly used to map thermal maturity directly or through a conversion to vitrinite reflectance equivalent values. Such efforts do not take into consideration analytical uncertainties or natural variability within a stratigraphic unit at any given locality. Furthermore, when presented as vitrinite reflectance equivalents, the associated error in the conversion is not considered, yet the values are presented as “absolutes.” When examining vitrinite reflectance, there are also several issues that should be considered. Individual mean values are commonly considered out of context. All thermal maturity indicators, including vitrinite reflectance, need to be placed into a geologic framework, and trends rather than discrete values should be considered. The nature of the studied samples is also significant. Whole-rock and isolated kerogen analyses yield different results, commonly because of the lack of a statistically meaningful number of individual measurements because of either low organic carbon or low concentrations of vitrinite when examining whole-rock samples. Such differences are not trivial, with final interpretation of hydrocarbon phase boundaries potentially being shifted. Bitumen reflectance shares some of the same issues as vitrinite reflectance measurement, including misidentification, the presence of multiple populations and insufficient measurements, and the possibility that environmental factors may influence the observed reflectance. Conversion issues of bitumen to vitrinite reflectance are similar to those identified for the Tmax conversion.