While the primary distinction between bitumen and pyrobitumen is solubility, the thermal processes driving the molecular cross-linking also decrease the atomic ratio of hydrogen to carbon from greater than one to less than one and ultimately to approximately one half.

Upon exposure to high regional temperatures over geological time, bitumen is converted to pyrobitumen as a result of the thermally activated reactions that drive off lighter oil and gas products and leave an insoluble, carbon-rich residue.

Hunt[3] defines bitumen as a native substance of variable color, viscosity, and volatility composed primarily of carbon and hydrogen.

Other bitumens range from very viscous (e.g., Athabasca and Venezuelan heavy oils, La Brea tar pits) to solid (e.g. gilsonite, ozocerite, grahamite, impsonite).

Archaic classification systems for classifying bitumens were constructed without the extensive knowledge of organic geochemistry developed over the past 50 years.

Although there is not a direct relationship between the classification systems in Figures 1 and 2, one kind of pyrobitumen is a subset of the post-oil solid bitumen formed by thermal degradation of kerogen and oil.

[10] More recently, the pyrobitumen retained in the source rock is considered to play an important role in the storage and production of shale gas.

[11] In oil shale retorting, pyrobitumen residue has an atomic H/C ratio of about 0.5 and is often called coke,[12] which has its analog in the production of petroleum and coal cokes by destructive distillation, although some workers, e.g., Wen and Kobylinski,[13] have inappropriately used the term pyrobitumen as the reaction intermediate between kerogen and oil to distinguish it from natural bitumen.

For humic-derived solids in the earth, an analogous position on the coal maturation pathway would place it at a minimum in the medium-volatile bituminous range (i.e., H/C<0.8, O/C<0.05, and vitrinite reflectance >1.0%).,[14][15][16] For petroleum systems, Mukhopadhyay[17] states that solid bitumen starts to form when vitrinite reflectance reaches 0.45%, i.e., the early stages of conversion of kerogen to oil and gas.

In fact, the Biomarker Guide[18] defines pyrobitumen as having an H/C ratio less than 0.5, which corresponds to vitrinite reflectance of about 2.0% and low-volatile bituminous to semi-anthracite coal rank.