Experimental organic matter maturation at 2kbar: Heat-up effect to low temperatures on vitrinite reflectance

Abstract

An experimental study was performed to evaluate the effect of heat-up to various low temperatures on vitrinite reflectance (VR) at 2 kbar employing the same previously used heat-up procedures, starting material and apparatus. Heat-up is the isobaric experimental procedure consisting of the increase in temperature of the laboratory vitrinite maturation from room temperature to the desired run temperature Tend of heat-up at which the isobaric-isothermal maturation starts. Experiments employed cold-seal pressure vessels with classical heat-up devices and were carried out on wet (water added) xylite of swamp cypress. Confined system maturation experiments were conducted at 2 kbar and involved temperatures Tend of heat-up of 200, 300 and 400 °C. Heat-up times were ~ 25 min at 200 °C, ~ 35 min at 300 °C and ~ 40 min at 400 °C. Results of this study demonstrate that heat-up has a strong influence on VR at 2 kbar for Tend of heat-up above ~ 250 °C: VR greatly increases with Tend of heat-up during the short heat-up event. This effect of heat-up points out the rapid kinetics of the initial VR increase. At 2 kbar, experimental heat-up to Tend of heat-up < ~ 250 °C does not influence VR. This lack of maturation is regarded as the result of an activation time delaying vitrinite maturation at these very low temperatures. These results are a step toward future isothermal experimental vitrinite maturations and a better kinetic formulation of VR evolution with temperature at 2 kbar. In this respect, the effect of heat-up and the activation time call for performing future series of isothermal laboratory vitrinite maturations at 2 kbar and various temperatures using an extremely immature Type III organic starting material (VR ~ 0.2%) and involving short heating times (a few minutes to a few hours). This aims to investigate the largest possible VR range, to detect the beginning of the isobaric-isothermal vitrinite maturation and to accurately constrain the rapid isobaric-isothermal kinetic evolution of the initial VR increase. Evidently, the activation time and VR gained during heat-up must be considered in the formulation of VR rate equation. Possible directions on how to surmount the effect of heat-up on VR in the kinetic analysis of the isothermal VR evolution are presented. Numerical simulations indicate that the effect of heat-up on the kinetic evolution of the isothermal VR increase diminishes with increasing heating time up to be neutralized for long heating durations. Obviously, VR previously obtained from isothermal experiments performed for long heating durations using classical heat-up procedures can be associated for kinetic studies with further VR resulting from isothermal runs conducted for short heating times in cold-seal pressure vessels equipped with rapid heat-up devices. This avoids the influence of heat-up on VR and thus the kinetic analysis of VR evolution is greatly simplified. Rapid heat-up devices are therefore of interest to carry out future laboratory organic matter maturations for short heating times.