The enormous size of the McMurray oil accumulation has made it necessary to postulate extraordinary conditions to account for its origin. Geologic opinion is divided between (1) deposition in situ of the oil source material contemporaneously with the deposition of the Lower Cretaceous McMurray sands, and (2) derivation of the oil from Paleozoic rocks below and basinward. Various features of the deposit, especially the immature character of the oil itself, give reason to regard it as an oil field in a state of arrested development.

Humic acids derived from land plants were probably the major source material of the McMurray oil. Supporting this idea are (1) the nature and quantity of the organic matter in sediments rich in such material, (2) the abundance of humic acids in the runoff waters of well vegetated areas and the adequacy thereof to account for the oil which has been formed from post-early Devonian source material, (3) the broad coincidence in the geologic time periods between the major oil resources and the coal resources of the earth, and (4) the environmental conditions and other factors relating to the deposition of the McMurray formation.

Probable steps in the transformation to oil of the organic matter entering the sea in the river waters, with special reference to the McMurray deposit, have been (1) the formation of an asphalt-like substance, perhaps essentially contemporaneous with the precipitation of the organic matter by sea water, similar to certain occurrences of asphalt-like material in sands of Recent age where waters from fresh-water swamps mingled with sea water, and (2) the solution in this asphalt-like material of bituminous plant substances (spores, pollen, resin, et cetera) in the deposit. Depth of burial of the McMurray deposit has been insufficient to result in enough pressure or rise in temperature to complete the transformation to a typical crude oil, but the supersaturation of the sand with organic material was a factor which probably had an appreciable effect in that direction.