An experimental study of organic-inorganic interactions during vitrinite maturation

Vitrinite reflectance is a parameter widely used to determine the extent of thermal maturation in sedimentary basins. Laboratory heating experiments w ere conducted to examine the role of aqueous fluid and sediment composition , temperature, and time on the development of vitrinite reflectance at elev ated temperatures and pressures. Samples of organic-lean Middle Valley sedi ment containing dispersed vitrinite and Wyodak Coal composed almost entirel y of vitrinite were heated in the presence of compositionally variable aque ous fluids at 225 degrees to 375 degrees C and 350 bars. Inorganic fluid co mposition was monitored as a function of time, and vitrinite reflectance wa s measured at the termination of each experiment. The rate of vitrinite maturation was influenced by inorganic fluid and vitr inite composition, in addition to time and temperature during the experimen ts. In particular, the rate at which vitrinite reflectance increased for a given temperature correlated positively with in situ H+ activity. This obse rvation suggests that acid catalyzed ionic reaction mechanisms and/or catal ytically 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 surround ing 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 por e fluids or the presence or absence of an aqueous phase in contact with vit rinite particles. Accordingly, kinetic models that consider organic reactio ns responsible for increased vitrinite reflectance in terms of time and tem perature alone may not accurately account for geochemical processes occurri ng in natural systems. Copyright (C) 2000 Elsevier Science Ltd.