A Prediction Model of Vitrinite Reflectance for Suppression of Organic-Matter Maturation by Overpressure

Abstract

Vitrinite reflectance is the most commonly used index in the study of source rock evaluation, diagenetic stage division, basin simulation, and diagenesis. Because data on vitrinite reflectance are limited, many prediction models have been proposed to address this problem. However, these models may fail to predict vitrinite reflectance effectively in overpressured basins because overpressure can suppress organic-matter maturation. Overpressure appears to reduce the degree of disorder in the hydrocarbon generation reaction system and to increase the mechanical work against the force of pore fluid that occurs as the volume of the activated complex increases, thereby reducing the preexponential factor and increasing the activation energy of the hydrocarbon generation reaction; therefore, organic-matter maturation is slowed. In this article, the reasons for the abnormally low vitrinite reflectance in the overpressured system were analyzed, which revealed that overpressure is interpreted as significantly suppressing organic-matter maturation. A new prediction model for vitrinite reflectance, A-Ea-Ro, is proposed that accounts for the suppression of overpressure in organic-matter maturation based on data from wells in the Fukang Sag in the Junggar Basin, northwestern China. The overpressure is modeled by introducing a preexponential factor and activation energy in this model. Based on measured drilling and geochemical data and simulated data, the overpressure controlling factors were optimized by a nonlinear constrained programming method and assigned 151.23 MPa and 30.65 kJ·mol−1·MPa−1, respectively. Prediction models including A-Ea-Ro were applied to wells Fu10 and Mu7 of Fukang Sag. The reflectance values predicted by the model A-Ea-Ro, applied to wells Fu10 and Mu7, were in good agreement with the measured values. This model provides a closer match between predicted and observed vitrinite reflectance values for this study area.