AN EXPERIMENTAL STUDY OF ORGANIC-INORGANIC INTERACTIONS DURING VITRINITE MATURATION

Abstract

Vitrinite reflectance is a parameter widely used to determine the extent of thermal maturation in sedimentary basins. Laboratory heating experiments were conducted to examine the role of aqueous fluid and sediment composition, temperature, and time on the development of vitrinite reflectance at elevated temperatures and pressures. Samples of organic-lean Middle Valley sediment containing dispersed vitrinite and Wyodak Coal composed almost entirely of vitrinite were heated in the presence of compositionally variable aqueous fluids at 225° to 375°C and 350 bars. Inorganic fluid composition was monitored as a function of time, and vitrinite reflectance was measured at the termination of each experiment.The rate of vitrinite maturation was influenced by inorganic fluid and vitrinite composition, in addition to time and temperature during the experiments. In particular, the rate at which vitrinite reflectance increased for a given temperature correlated positively with in situ H+ activity. This observation suggests that acid catalyzed ionic reaction mechanisms and/or catalytically active transition metals and sulfur species play an important role in organic transformations responsible for increased vitrinite reflectance. The activity of other inorganic ionic species such as Mg++, Ca++, Na+, K+, and SO4=, and the redox state of the fluid did not influence the rate of vitrinite maturation during these experiments.Results of this study demonstrate that the geochemical environment surrounding vitrinite influences the rate of maturation. The presence of water may facilitate reaction pathways that are not readily available in dry systems. Vitrinite maturation profiles that deviate from what are considered to be ''normal'' trends may reflect subsurface variations in the composition of pore fluids or the presence or absence of an aqueous phase in contact with vitrinite particles. Accordingly, kinetic models that consider organic reactions responsible for increased vitrinite reflectance in terms of time and temperature alone may not accurately account for geochemical processes occurring in natural systems.