Abstract

Fifty-eight oil samples, extracted from cores of Lower Cretaceous sandstones and from some Devonian carbonates just below the pre-Cretaceous unconformity in eastern Alberta, show a gradational change in composition north-eastward towards the shallower part of the Western Canada Sedimentary Basin. Conventional pooled oils, also included in the study, fit into the same picture. Three stages of degradation are recognized departing from a normal oil: type “a” with a decrease in normal alkanes and an apparent increase in phytane and pristane; type “b” with the disappearance of the normal alkanes leaving only pristane and phytane; type “c” in which even the isoprenoids have disappeared leaving only other isoalkanes and cyclo-alkanes. The heavy oils, including those of the Athabasca deposits, are very similar in their cycloalkane content to the conventional Lower Cretaceous oils of the Western Canada Sedimentary Basin indicating a common origin. A study of the aromatic and thiophenic compounds reinforces that interpretation.

It is concluded that biodegradation and water washing with some possible inorganic oxidation are responsible for the progressive alteration of normal Lower Cretaceous oil migrating updip, where it encounters fresh water invading the basin from the outcrop area along the shield edge.