The essential elements of chromatography are defined as (1) a mixture which becomes separated, (2) the stationary phase (a medium, usually fine-grained, with large surface area), and (3) the mobile phase (a fluid that carries the mixture with it over or through the stationary phase during separation). One or more of such specific processes as ion-exchange, partition, diffusion, and absorption may participate in a chromatographic separation, which, however, is more complex than any one of them.

The permeability of rock involved in petroleum migration ranges from a maximum in uncemented uniformly graded sandstone to a minimum in shaly rocks. At the lower limits of permeability, the petroleum must flow past or through fine-grained rocks of great surface area. Within more permeable strata some fine-grained material is encountered. Petroleum colloids probably also act as chromatographic stationary phases.

In the laboratory, when mixed hydrocarbons are passed over fine-grained materials, the paraffins always advance at a greater rate than the cycloparaffins which, in turn, move faster than the aromatics. In addition trace metals are retained selectively in a certain order on the stationary phases.

In natural occurrences of petroleum in multiple pools, it is commonly reported that the upper pools are paraffin-rich, the intermediate pools cycloparaffin-rich, and the lower pools are richer in aromatics. In these cases chromatographic separation during migration is offered as an alternative to hypotheses of separate origins for the pools. Chromatographic separation of trace metals in petroleum fields is reviewed.

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