Geoporphyrins occur as mixtures of skeletal types throughout the geosphere. Although nearly all metals may be successfully chelated by porphyrins in vitro, only two metal complexes - the nickel and vanadyl - are predominant in crude oils and oil shales. The distributions of the nickel and vanadyl geoporphyrins from the same sample are usually markedly different. Such differences may be attributed to one of two factors: a) Preferential chelation of one of the two metals by different geoporphyrin species. b) The different chemistry of the nickel and vanadyl porphyrin complexes gave rise to the variation in the distributions. The results of a series of competitive metallation studies will be presented, and their role in the preferential chelation of the geoporphyrins will be discussed. The reactivity of the nickel and vanadyl porphyrins at both the β- and meso (bridge) positions will be described, and their significance in the analysis of geoporphyrin mixtures will be outlined.
