Clay Minerals and Petroleum-Forming Reactions During Burial and Diagenesis

Model experiments have been carried out to investigate the effect of montmorillonite as a catalyst in promoting organic reactions of importance in hydrocarbon generation. Montmorillonite catalyst promotes the decarboxylation of fatty acids to form long-chain alkanes. It likewise promotes subsequent cracking of these alkanes to produce shorter chain alkanes with molecular distribution similar to those of petroleum. From kinetic considerations the activation energies for these two model reactions can be estimated at about 36 kcal/mole for decarboxylation and about 46.5 kcal/mole for catalytic cracking of the C21 hydrocarbon. The kinetic models are applied to a natural situation, where the geothermal gradient and rate of subsidence in a sedimentary basin permit consideration of the extent of organic transformation as a function of changing depth (temperature). A geochemical model is proposed which relates, in sequence, alkane production (by decarboxylation), maturation (cracking), and migration (water expulsion during diagenesis). The model postulates depth (temperature) zonation which is qualitatively in agreement with fatty acid and hydrocarbon molecular distributions observed when young and ancient sediments and petroleum are compared.