Why does slow underground heating produce a lighter oil from oil shale?

When oil shale is processed underground (in situ), the rock is heated very slowly because it acts as an excellent insulator, a process that can take months or even years. This slow heating allows pyrolytic reactions to occur at significantly lower temperatures—roughly 325–400 °C (620–750 °F)—compared to the rapid, high-temperature heating (480–550 °C) used in surface retorts.1

Under these conditions of slow, lower-temperature heating, the chemical bonds of the kerogen macromolecules are broken down in a way that favors the production of a lighter oil and a higher gas-to-oil ratio. This process is a form of thermal cracking, where heavy hydrocarbon molecules are broken into smaller, lighter ones. By maintaining these specific thermal conditions underground, the resulting shale oil is lighter than that typically produced by the rapid heating methods used in surface facilities.12