Some chemical and mechanistic aspects of petroleum formation

Abstract

Selected organic reactants, including n-dodecanol and stearic acid adsorbed on clays at low temperature (300 K), decomposed on heating (>400 K) to yield alkane product mixtures: thermal desorption reactions. These products showed that, while adsorbed, reactant hydrocarbon chains underwent cracking, isomerisation and hydrogen transfers. Because product distributions obtained from several different reactants were closely similar, it is concluded that these reactants extensively rearranged to form similar mixtures of surface-bonded species. From this evidence, we suggest that thermal desorption provides a realistic chemical mechanism for natural petroleum formation: it is well-known that some minerals promote kerogen breakdown. To investigate the feasibility of this mechanism in nature, the following comparisons were made. Qualitative: Our model thermal desorption reactions yielded alkanes in the C5–C8 range that are usual constituents of crude oils. Quantitative: Product yields were sufficiently large to explain the amounts of crude oil found in natural reservoirs. Kinetic: Extrapolation of our measured alkane thermal desorption rates, to lower temperatures, confirmed that long (geological) time intervals are required to accumulate the amounts of crude oils found in reservoirs. We conclude, therefore, that initial adsorption of organic compounds on suitable acid catalysts, e.g. the clay montmorillonite, with subsequent slow hydrocarbon desorption after burial, is a realistic and feasible mechanism for petroleum formation in natural sediments. Thermoanalytical rate data obtained for geochemical pyrolysis reactions are critically appraised in the context of some representative kinetic studies in the literature. Furthermore, deactivation on adsorption may preserve the organic precursors of oil from rapid, early decay.