Carbonates contain very little organic matter suitable for determining thermal-alteration indices visually, but commonly contain solid bituminous materials. Such bitumens form by the alteration of reservoired oil and are present as black, solid asphaltic or graphitic coatings and particles within reservoir porosity. These here are called reservoir bitumens. Their hydrogen-to-carbon atomic ratio and percent solubility in carbon disulfide can be used as an adjunct to visual kerogen ratings of maturation level; these parameters now are correlated with thermal indices in western Canada. The term "pyrobitumen" is restricted to insoluble (< 2 percent soluble in CS₂) bitumens present only in thermally altered areas of only dry-gas reservoirs.

The two primary results of this study show: (1) it is necessary to distinguish between true reservoir bitumens and other bituminous materials. Reservoir bitumens are predominantly carbon and hydrogen, whereas bituminous organic matter contains significant amounts (> 10 percent) of oxygen, nitrogen, sulfur, or ash. (2) The thermal history of a reservoir can be deduced from analyses of reservoir bitumens by using the following cut-off values:

Table

The other conclusions of this study relate to the origin of bitumens. Reservoir bitumens are formed by at least two processes: (1) thermal alteration of oils to dry gas and solid bitumen, and (2) deasphalting of oils after solution of large amounts of gas. The bitumen in the latter case is merely the precipitated asphaltenes.

The H/C atomic ratio is a sensitive indicator of increasing thermal alteration and it, together with carbon isotopes, can be used to discriminate between the alteration processes. Reservoir bitumens formed by deasphalting have isotopic ratios like the original oil, whereas those formed by thermal alteration have significantly heavier ratios because of kinetic effects in thermally induced cracking off of isotopically light methane. This loss of methane (4H for each C) explains the progressive change in both the H/C atomic ratios and the carbon isotopic ratios. The deasphalted bitumen, on the other hand, is actually a mixture of asphaltenes, NSO's, and aromatics; it is extremely reactive and readily dehydrogenates to yield H₂ in a very exothermic reaction. The selective loss f hydrogen but not of carbon explains the changes in the H/C atomic ratio and the lack of an isotopic fractionation.

In western Canada gas deasphalting apparently is more important than thermal alteration in the formation of reservoir bitumens.