ABSTRACT

Using the terms bitumen and oil carelessly and failing to properly separate chemical reactions from mass transport processes can lead to models that fail when applied to natural processes. Data from both laboratory experiments and source rock measurements are used to develop a self-consistent picture of generation of C15+ extractable organic matter (EOM), lighter hydrocarbons (HC), and the fractionation that occurs during expulsion from the rock in either laboratory experiments or nature. Despite statements in the literature, hydrocarbons are generated immediately from kerogen in parallel with heavy compounds containing nitrogen, sulfur, and oxygen (NSOs) and probably with similar rate constants from kerogen and NSOs under all conditions because the same bonds are being broken to form them. Peak concentrations of C15+ EOM in the rock occur prior to peak generation of unexpelled HC because of weaker bonds being broken, not because of a sequential chemical reaction mechanism. This earlier generation of C15+ EOM relative to HC is similar for semiopen experiments, hydrous pyrolysis, and in nature, as is the fractionation of chemical types during expulsion. Properly derived activation energies for expelled oil formation from open, semiopen, and hydrous pyrolysis are similar once transport contributions are considered.

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