The chemical composition of hydrocarbon fractions of artificially matured (pyrolyzed) Green River oil shale were studied by optical (visible near-infrared, vis-NIR) spectroscopy. The shale samples were pyrolyzed to several maturities, allowing the compositions of the hydrocarbon fractions to be analyzed as a function of maturity. Oil (the hydrocarbon fraction volatile at pyrolysis conditions but liquid at room temperature), bitumen (the hydrocarbon fraction nonvolatile at pyrolysis conditions and soluble in organic solvent), and asphaltene (the fraction of bitumen that is insoluble in heptane) were all found to show an exponential increase in optical absorption with increasing optical frequency. A similar exponential increase has been observed in other hydrocarbon mixtures and is described by the Urbach tail formulation. The optical properties of the oil are found not to change with maturity, likely because oil is released by volatilization throughout the maturation process, thus producing a distilled product. Bitumen and asphaltenes, however, show strong trends with maturity, including the formation of larger chromophores during maturation. This result differs from naturally occurring crude oils and asphaltenes, which show remarkably similar Urbach tail slopes over a large range of American Petroleum Institute gravity. This difference between the naturally occurring and laboratory produced products can be explained as result of the laboratory produced bitumen and asphaltenes being retained in the source rock during semiopen pyrolysis, while natural oils are expelled.