The hydrodenitrogenation (HDN) reactions of fractionated shale oils were carried out over Co-Mo/Al2O3 hydrotreating catalyst in a trickle bed reactor, and causes for reduced HDN kinetics rates as molecular weight of shale oil increases were investigated. Results showed that competitive adsorption either from high molecular weight molecules or from polar heteroatoms could not fully account for the reduced kinetics in the system studied. Likewise, the selection of a catalyst with large average pore radius (similar to 60 Angstrom) resulted in high effectiveness factors and diffusional resistance was minimal. The loss in intrinsic reaction rate was found to follow a power law function of molecular weight. Models were hypothesized to account for observed results. A "disk-shadow" model simulating the availability of surface sites to large and planer molecules comes closest to agreeing with the observed kinetics. We, therefore, conclude that geometric effects based on size and shape play an important, and perhaps dominant, role in governing intrinsic kinetic rates when catalytically hydroprocessing large molecules.