Kerogen maturity is achieved when a kerogen is subjected to high temperatures over long periods of time. Catagenesis and Metagenesis, which are stages of thermal decomposition reactions, result in the breaking off of smaller molecules, and leave behind a more resistant kerogen residue. These small molecules eventually transform into petroleum and natural gas. Thus, the term Catagenesis usually refers to the stage in which kerogen decomposition yields oil and wet gas, while Metagensis refers to the stage in which dry gas is generated.