Laboratory organic matter maturation at high pressures: heat-up effect on vitrinite reflectance

Abstract

A laboratory study was conducted to assess the effect of heat-up to high diagenetic to low metamorphic temperatures on vitrinite reflectance (VR) at high pressures using the same heat-up processes, apparatus and starting material as those employed in prior experimental studies on huminite/vitrinite maturation. “Heat-up” is the isobaric increase in temperature of an organic matter maturation experiment from room temperature to the desired run temperature T ehu (T ehu = temperature at the end of heat-up). The experiments were performed on xylite of swamp cypress and used a heating rate of 50 °C/min. These confined system maturation experiments were carried out at 10 kbar and involved temperatures T ehu ranging from 175 to 450 °C. Additional experiments were conducted at pressures of 5, 20 and 25 kbar to evaluate the influence of pressure on the effect of heat-up on VR. At 10 kbar, results of this study show that heat-up does not influence VR for T ehu < ~270 °C. This absence of maturation is viewed as the result of an activation time delaying vitrinite maturation at these diagenetic to very low metamorphic temperatures. For T ehu > ~270 °C, heat-up has a significant effect on VR at 10 kbar: VR greatly increases with T ehu during the short heat-up event. This effect of heat-up on VR points out the rapid kinetics of the initial VR increase. Increasing pressure reduces VR increase gained during heat-up. Obviously, pressure retards the initial VR increase and thus controls organic matter maturation. In addition to temperature, the formulation of VR evolution rate equation must consider pressure, activation time and VR gained during heat-up.