Critical review of the uncertainty of Tmax in revealing the thermal maturity of organic matter in sedimentary rocks

Abstract For more than forty years, the Rock-Eval pyrolysis technique has been widely used to rapidly assess the petroleum generating characteristics of organic matter in petroleum source rocks. Tmax, which is the temperature at which the rate of hydrocarbon generation is at its maximum during pyrolysis (25 °C/min), is used mainly as a thermal maturity parameter. This paper presents a systematic review of the numerous other factors that control Tmax. Besides including the classical literature on this topic, the review discusses revelations from recent case studies with emphasis on new developments in unconventional hydrocarbon exploration and organic-inorganic interactions. Experimental issues related to the model of Rock-Eval instrument, sample loading and the resolution and form of the pyrolysate (S2) peak(s), define the quality and reliability of Tmax data. Kerogen type sensu lato, controls the kinetics of bulk petroleum generation and thence Tmax. Thus, conversions between Tmax and the main maturity reference parameter, vitrinite reflectance, for marine (Type II) kerogen vary from shale to shale. Tmax anomalies induced by the mineral matrix effect, heavy bitumen carryover, weathering, and uranium irradiation are also discussed. On a practical note, checking the original pyrogram, comparing with adjacent samples with different organofacies, and cross-checking with other maturity proxies are helpful in identifying possible anomalies.