The chemical character of the crude oils produced throughout four major fields and several minor fields in western Canada has been studied. The variations in nickel and vanadium content appear systematic and are undoubtedly a key to the accumulation history of the oils. The metal variations run parallel for the most part to variations in sulphur, resins, and asphaltene content of the oil as well as to the gravity, and are apparently sensitive indicators of crude-oil alteration. The Pembina field is the most interesting because the magnitude of the variations from point to point in the field are very large—about ten-fold for the metals. Much smaller variations are noted for the other fields, and this may be a result of the smaller sizes of those fields. If the data are interpreted in the light of probable adsorption alteration during accumulation and probable source conditions, the Pembina-Cardium oil apparently accumulated in the Cardium sands in a direction away from the present updip edge at the northeast; the Redwater oil, from downdip toward the south; the Viking Joffre oil, from downdip at the southwest; and the Lloydminster oil pools, from very local source rocks.