Pyrobitumen is a type of solid, amorphous organic matter. Pyrobitumen is mostly insoluble in carbon disulfide and other organic solvents as a result of molecular cross-linking, which renders previously soluble organic matter (i.e., bitumen) insoluble. Not all solid bitumens are pyrobitumens, in that some solid bitumens (e.g., gilsonite) are soluble in common organic solvents, including CS2, dichloromethane, and benzene-methanol mixtures.
Other related hydrocarbons 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. It should also be understood that both solubility and atomic H/C ratios form a continuum, and most solid bitumens have both soluble and insoluble components. The distinction between pyrobitumen and residual kerogen in a mature source rock is based on microscopic evidence of fluid flow within the rock fabric and is usually not determined. The terms bitumen and pyrobitumen have related definitions in the Earth's crust and in the laboratory. In geology, bitumen is the product of deposition and maturation of organic matter. The extractable organic material (EOM) in petroleum source rocks and reservoir rocks is defined as bitumen. 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. Pyrobitumen represents a significant fraction of the ultimate fate of petroleum liquids formed from kerogen during catagenesis. In the laboratory, experiments on organic-rich rocks (oil shales and petroleum source rocks), decomposition of the initially insoluble organic matter (defined as kerogen) produces gaseous and liquid products. The soluble fluid that remains in the heated rock is defined as bitumen. Upon further thermal exposure, the bitumen continues to evolve and disproportionates into pyrobitumen and more oil and gas. The terms bitumen and asphalt are often used interchangeably to describe highly viscous to solid forms of petroleum that have been used in construction since the fifth millennium B.C. Bitumen is distinct from tar, which properly describes a product formed by pyrolysis (destructive distillation) of coal or wood. Pitch recovered from petroleum by distillation is also sometimes called bitumen or asphalt.
Etymology The expression "bitumen" originated in the Sanskrit, where we find the words jatu, meaning "pitch," and jatu-krit, meaning "pitch creating", "pitch producing" (referring to coniferous or resinous trees). The Latin equivalent is claimed by some to be originally gwitu-men (pertaining to pitch), and by others, pixtumens (exuding or bubbling pitch), which was subsequently shortened to bitumen.
Definition Hunt defines bitumen as a native substance of variable color, viscosity, and volatility composed primarily of carbon and hydrogen. He further defines petroleum as a form of bitumen that is gaseous or liquid in the reservoir and can be produced through a pipe. 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). Pyrobitumen is formed by thermal decomposition and molecular cross-linking of bitumen. Pyrobitumen is distinguished from other solid bitumens extruded from early-mature kerogen-rich source rocks (e.g., gilsonite) and semi-solid bitumens in high viscosity oil sands formed by water washing and biodegradation of conventional oil (e.g., Athabasca bituminous sands), all of which are soluble in carbon disulfide.
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