Comparable pyrolysis experiments have been performed on rocks containing organic matter and on related kerogens which were separated from the rock by acid treatment. In some examples, hydrocarbon yields from the rocks are lower. The experimental procedure separates the lighter hydrocarbons (lower than C₁₅) from total hydrocarbons, thus showing that the decreased hydrocarbon yield from rocks as compared to kerogen is principally due to retention of the heaviest hydrocarbons. The light hydrocarbons do not seem to be reduced in quantity.

By studying mixtures of kerogens with various minerals, we infer that retention of heavy hydrocarbon products issued from the kerogen pyrolysis occurs on the mineral surfaces. With increasing temperature and time, the trapped hydrocarbons may be cracked: light hydrocarbons are released, whereas a carbon residue remains on minerals. Some of the argillaceous minerals used (illite from Le Puy, France) are particularly active whereas other minerals such as carbonates show weak activity.

Pyrolysis performed on many samples of rocks confirms these experimental assays and shows that hydrocarbon retention during pyrolysis increases with the clay content of rocks. In rocks with a low organic carbon content, these phenomena can affect the quantity of heavy hydrocarbons liberated during pyrolysis whereas the lightest hydrocarbons are little affected.

Thus, under comparable geologic conditions, certain types of source rocks would release light oil and gas.