A common problem in petroleum exploration is how to predict the source of a petroleum accumulation or a surface seepage when there are no source rock data available. There is a need to be able to determine source type from petroleum properties alone, and to this end, an oil classification scheme has been devised. This is based initially upon relatively simple petroleum properties such as API gravity, sulfur content, etc., and enables classification to be made where relatively few other data are available as is often the case in frontier exploration. One property of petroleum that is useful for classification purposes is the metal content. Nickel and vanadium are the two most abundant metals in petroleum and are thought to exist largely as porphyrin complexes which in turn are mainly derived from chlorophyll precursors. It is shown that, in some oil classes, both the absolute concentration of metals and the nickel/vanadium ratio are useful oil classification parameters. Examples are given from a variety of oil classes from basins worldwide.

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