EXPERIMENTAL STUDY ON THE PRESSURE DEPENDENCE OF VITRINITE MATURATION

Abstract

We conducted the first systematic experiments over a pressure range of 0.5 to 20.0 kbar, at temperatures of 200 to 350°C, and run durations of #2, #4, and #7 days to investigate the influence of pressure on vitrinite reflectance. The data indicate that applied pressure lowers vitrinite reflectance; the suppression appears to be stronger under wet than under dry conditions. In addition, high pressure favors liptinite preservation at temperatures =< 300°C, whereas at the same temperature range, yet at low pressure, liptinite is decomposed. Our data fit an empirical rate equation which expresses %Rr as a function of pressure, temperature, and time. The derived equation satisfactorily describes our experiments and enables extrapolations to geological timescales. Values of %Rr calculated for a pressure of 0.5 kbar are remarkably consistant with previous numerical pressure-independent models. A literature review shows that previous experimental data and field observations are inconsistent regarding the effect of pressure on vitrinite maturation--this and some previous studies suggest that vitrinite maturation is suppressed by increased pressure, whereas other workers regarded pressure as having little influence on the rate of coalification. We suggest that original vitrinite composition, experimental design, and natural environment (tectonic regime) play important roles regarding the effects of pressure on vitrinite maturation. Interpretation of geothermal histories of sedimentary rocks based on vitrinite reflectance data must, therefore, include evaluation of the pressure. Otherwise, vitrinite reflectance data from high-pressure terranes should be interpreted with caution if used as a quantitative paleogeothermometer.