Experimental kinetic study of organic matter maturation: an appraisal of pressure, temperature and time effects on reflectance and Raman properties of vitrinite.
Final Report Abstract
The project consisted of an experimental kinetic study of Type III organic matter (i.e. huminite/vitrinite) maturation aiming to assess the effects of time, pressure, and temperature on vitrinite reflectance (VR) and Raman signal. This project was based on the kinetic analysis of the evolution of VR and Raman signal obtained from a series of confined system laboratory maturations conducted at various pressure–temperature conditions (at 400 and 450 °C with pressures of 2, 10 and 20 kbar). Experiments were performed on xylite of swamp cypress and involved run lengths from 0 s to 50 days employing cold-seal pressure vessels and a high pressure piston-cylinder apparatus. Here are only summarized the major results of this project that are already published. The results of this project demonstrate that heating time, pressure and temperature are important variables that control VR increase and therefore the maturation of Type III organic matter. In particular, this study shows that each magnitude of VR has its own rate of increase that diminishes with increasing VR, resulting in VR increase deceleration with increasing time after rapid initial kinetics. The very rapid kinetics of the initial VR enhancement is illustrated by the strong effect of the experimental heat-up on VR that is described for the first time even for very short experiments. However, the project presents and performed the correction of the effect of heat-up on VR in the kinetic analysis of VR increase. Furthermore, this study indicates that the rate of increase of each magnitude of VR is pressure and temperature dependent. Whereas the promoting influence of temperature on VR is obvious, the role of pressure in VR increase kinetics is pointed out to be complex. However, for the first time, this study assesses the effect of pressure on VR/organic matter maturation: It depends on the magnitude of VR with respect to the key VRi = 1.34 ± 0.29% at which the transition in the effect of pressure on vitrinite maturation takes place. For VR < VRi, pressure retards vitrinite maturation because increasing pressure diminishes the kinetics of VR increase and hence hinders VR increase with increasing time. Thus, this project points out that pressure retards oil generation in sedimentary basins showing VR < VRi. Nonetheless, this retarding effect of pressure decreases with increasing VR and is neutralized when VR attains VRi. Hence, when VR = VRi, pressure practically has no impact on vitrinite maturation. For VR > VRi, the project demonstrates that pressure promotes vitrinite maturation inasmuch as increasing pressure increases the kinetics of VR increase and thus greatly augments VR increase with increasing time. Furthermore, this study indicates that the promoting effect of pressure increases with increasing VR. Obviously, this project shows that pressure promotes gas generation in sedimentary basins in which VR > VRi. This project claims that pressure promotes vitrinite maturation in geological settings at 400-450 °C because the key VRi is attained in only a few minutes to a few hours, even though pressure retards the initial VR evolution kinetics. This study shows that it is essential to consider heating time and pressure in addition to temperature in the formulation of a general equation that describes VR increase kinetics. This project indicates that models/equations that disregard the effects of heating time and pressure on VR do not accurately account for the evolution of VR in natural systems. Additionally, this study pointed out several limitations in the existing VR equations/models. Moreover, the project results in the formulation of a new rate equation that describes the increase of VR with heating time (t) as a function of pressure (P) at temperatures (T) of 400 and 450°C expressed as VR(P, T, t) = (k(P, T) t)n(P, T) where the exponent n(P, T) and the rate constant k(P, T) are a function of temperature and/or pressure. This power law kinetic formulation is regarded as a step toward a general equation that describes the evolution of VR as a function of time, pressure and temperature for Type III organic matter at diagenetic to low metamorphic temperature–low to high pressure conditions. This general P–T-dependent kinetic equation of VR increase will be a useful tool for various geosciences dealing with basin modeling, hydrocarbon exploration and metamorphic petrology and geology. This stems from the potential of this rate law to model and estimate the VR–P–T–t conditions in huminite/vitrinite-bearing sedimentary basins and very low to low temperature–low to high pressure metamorphic terranes occurring in various geodynamic settings and consequently to aid to reconstruct the geological history of sedimentary and metamorphic terranes and to improve hydrocarbon exploration in sedimentary basins. Furthermore, the project provides explicit directions on how to use such a power law equation to model VR. Additionally, the project indicates that this formalism has the advantage to be a simple and fast algorithm to simulate VR–P–T–t conditions in large-scale sedimentary basins and metamorphic terranes having complex geological histories.
Publications
- Experimental kinetic study of organic matter maturation: Time and pressure effects on vitrinite reflectance at 400 °C.
Organic Geochemistry Vol. 42. 2011, pp. 340–355.
Ronan Le Bayon, Gerhard P. Brey, W.G. Ernst, Rafael Ferreiro Mählmann
(See online at https://dx.doi.org/10.1016/j.orggeochem.2011.01.011) - Experimental organic matter maturation at 2 kbar: Heat-up effect to low temperatures on vitrinite reflectance. International Journal of Coal Geology 92. 2012, pp. 45-53.
Ronan Le Bayon, Stephan Buhre, Burkhard C. Schmidt, Rafael Ferreiro Mählmann
(See online at https://doi.org/10.1016/j.coal.2011.12.002) - Experimentally determined pressure effect on vitrinite reflectance at 450 °C. International Journal of Coal Geology, Vol.92. 2012, pp. 69-81.
Ronan Le Bayon, Christian Adam, Rafael Ferreiro Mählmann
(See online at https://doi.org/10.1016/j.coal.2011.12.007) - Laboratory organic matter maturation at high pressures: Heat-up effect on vitrinite reflectance. Swiss Journal of Geosciences, Vol.105. 2012, Issue 2, pp.171-181.
Le Bayon, R.
(See online at https://doi.org/10.1007/s00015-012-0109-1)
