Field evidence suggests that conditions for the collection and retention of various hydrocarbons exert control over the composition of resident hydrocarbon mixtures. Heavy-oil deposits demonstrate that control very clearly.

Heavy oils are essentially colloidal admixtures of hydrocarbons, usually accompanied by asphaltenes, trace metals, and other organic residues. The weight of the hydrocarbon mixture depends mostly on the collective weights of its component hydrocarbons.

Tracking present-day water movements by pressure, temperature, and water chemistry through the Alberta basin and the Eastern Venezuela basin clearly leads in the direction of the very large Athabasca and Orinoco heavy-oil deposits, respectively. Significantly, those deposits occur at the downstream edge of the basinal water traffic systems, toward which large quantities of hydrocarbons and other water-sensitive materials have been continuously reworked through much of the basin history. Remarkably, much "conventional" oil and gas remain upstream in the deeper parts of both basins.