EXPERIMENTAL INVESTIGATIONS ON VITRINITE REFLECTANCE: TOWARD A TOOL TO MODEL MATURATION AND METAMORPHIC CONDITIONS IN LOW TEMPERATURE METASEDIMENTARY TERRANES

We carried out laboratory rate studies to elucidate and quantify the effects of time (t), temperature (T) and pressure (P) on vitrinite reflectance (VR). A series of confined system maturation experiments was conducted at various pressures and temperatures. Experiments were performed on xylite of swamp cypress and involved run lengths from 0 seconds to dozens of days.

Our experimental results demonstrate temperature and heating time to be important variables that promote VR increase and therefore the maturation of Type III organic material. VR increases with time at each investigated pressure. Despite rapid initial kinetics, the increase in VR decelerates with time at each pressure. When VR < ~1.2-1.5%, increasing pressure reduces the rate of VR increase and hence retards the initial VR enhancement with time. The retarding effect of pressure on VR increase diminishes with enhancing VR. The retardation of VR increase is insignificant for geological maturation at T ≥ 300 °C because a VR of ~1.2-1.5% is attained in only a few hours or days. When VR > ~1.2-1.5%, increasing pressure counteracts the deceleration of VR increase with time and thus greatly enhances the increase in VR with time. The strong effect of the experimental heat-up on VR is obvious even for very short experiments and must be corrected in kinetic analysis. The evolution of VR with heating time, temperature and pressure from an initial VR of 0% is well described at the investigated experimental P–T conditions by our new power law rate equation

VR(P, T, t) = (k(P, T) t)^{n(P, T)},

where the exponent n(P, T) and the rate constant k(P, T) depend on P and T. We regard this preliminary kinetic formulation as a step toward a general equation describing VR evolution as a function of time, pressure and temperature for Type III organic matter. This VR rate equation will be a useful tool to model VR in sedimentary basins and to estimate the P–T–t conditions in metamorphic terranes occurring in various tectonic settings (e.g., exhumed subducted terranes, collided terranes in orogenic wedges). This will aid to gain insight into geodynamic evolution of sedimentary and metamorphic terranes and to improve hydrocarbon generation modeling in sedimentary basins.