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The Confusion About Thermal Maturity With Respect to Vitrinite Reflectance, Tmax and Other Proxies

Abstract

There has been clear recognition of the role of geochemistry in the evaluation of unconventional plays. Many of those now assessing the geochemical data have taken a “cookbook” approach to interpretation. Such an approach does not take into consideration the limitations of the datasets. This work will evaluate the issues that exist with three commonly used thermal maturity indicators: Rock-Eval pyrolysis, vitrinite reflectance, and bitumen reflectance. Tmax derived from pyrolysis is now commonly used to map thermal maturity directly or through a conversion to vitrinite reflectance equivalent values. Such efforts do not take into consideration analytical issues or the natural variability within a stratigraphic unit at a given locality. Furthermore, when presented as vitrinite reflectance equivalents the associated error in the conversion in not being considered yet the values are presented as absolute values. In addition, when utilizing these data to map thermal maturity zones representing oil, condensate, and gas the differences in kerogen types and typically not being considered. When examining vitrinite reflectance there are also several issues that should be considered. Individual mean values are often considered out of context. All thermal maturity indicators, including vitrinite reflectance, to be correctly interpreted, need to be placed into a geologic framework. The nature of the studied samples is also a significant issue. Whole rock and isolated kerogen analyses could yield quite different results, often because of the lack of a statistically meaningful number of individual measurements either because of low organic carbon or low concentrations of vitrinite. Such differences are not trivial with final interpretation of phase boundaries being shifted in position by multiple counties. Bitumen reflectance, commonly reported as vitrinite reflectance equivalent, shares some of the same issues as direct vitrinite measurement, but is also impacted by a lack of agreement on the conversion and the possibility that environmental factors may influence the observed reflectance and conversion. In summary, it is clear that the “simple” maturity measurements that are finding their way into the general geologist’s toolbox are not as simple and require a skilled interpreter to accurately assess.