THE EFFECTS OF OVERPRESSURE, LITHOLOGY, CHEMISTRY AND HEATING RATE ON VITRINITE REFLECTANCE EVOLUTION, AND ITS RELATIONSHIP WITH OIL GENERATION

Abstract

Overpressure has been regarded by several authors to retard vitrinite reflectance evolution. The previously presented data regarding overpressure, however, are inconclusive because the covariation between burial rate, fluid pressures and heating rate has been overlooked. The present results show that the thermal effect of overpressure in shales has no important influence on the observed relationship between temperature and reflectance. The pressure acting on vitrinite and other solid organic matter is mainly lithostatic, and the influence of overpressure on the confining pressure of vitrinite is negligible. The apparent relationship between vitrinite reflectance and lithology is closely linked with important differences in the chemistry of vitrinites. Systematic differences in the reflectance of vitrinites in coals and interbedded shales are associated with variations in the concentration of lignin-derived products in the source organic matter, and their state of biochemical/chemical modifications governed by the depositional conditions. Vitrinite reflectance is related to both the transformation ratio and the molecular structural organisation of the vitrinite. In immature vitrinites subjected to intermediate to high heating rates, the ultrafine textural heterogeneity is enhanced, retarding the reflectance increase, compared to those heated at low rates, for the same change in transformation ratio. There exists a small but systematic difference between the heating-rate dependency of vitrinite reflectance versus heating-rate dependency of oil generation.

To reflect the slow evolution of reflectance through the zone of extensive oil generation, kinetic models for predicting reflectance employ wide activation energy distributions. Due to this intrinsic property, it is simple to calibrate or predict reflectance, but the use of reflectance for predicting oil generation or calibration of thermal models has a low precision, with a wide range of solutions with equally good fits.