Calculation of Vitrinite Reflectance from Thermal Histories: A Comparison of Some Methods

Abstract

Vitrinite reflectance values (%Ro) calculated from commonly used methods are compared with respect to time invariant temperatures and constant heating rates. Two monofunctional methods, one involving a time-temperature index to vitrinite reflectance correlation (TTI-%Ro) and the other involving a log(%Ro) to depth correlation, yield vitrinite reflectance values that are similar to those calculated by recently published Arrhenius-based methods, such as EASY%Ro. The approximate agreement between these methods supports the perception that the EASY%Ro algorithm is the most accurate method for the prediction of vitrinite reflectances throughout the range of organic maturity normally encountered. However, calibration of the e methods against vitrinite reflectance data from two basin sequences with well-documented geologic histories indicates that, although the EASY%Ro method has wide applicability, it slightly overestimates vitrinite reflectances in strata of low to medium maturity up to a %Ro value of 0.9%. The two monofunctional methods may be more accurate for prediction of vitrinite reflectances in similar sequences of low maturity. An older, but previously widely accepted TTI-%Ro correlation consistently overestimates vitrinite reflectances with respect to other methods. Underestimation of paleogeothermal gradients in the original calibration of time-temperature history to vitrinite reflectance may have introduced a systematic bias to the TTI-%Ro correlation used in this method. Also, incorporation of TAI (thermal alteration index) data and its conversion to %Ro-equivalent values may have introduced inaccuracies.