Theory and experiments suggest that catalytic reactions are involved in petroleum formation. This suggestion is not new, but has never gained wide acceptance. A model chemical system illustrates that acid-catalyzed reactions can explain several key transformations in the generation and maturation of oil. It also is shown that such reactions should occur under geologically reasonable conditions, and that naturally occurring active catalysts exist in many sediments. Catalytic principles indicate that the nature and amount of these catalysts in sediments can importantly control the fate of organic substances therein. It is suggested that under a given time-temperature history, organic matter in a sediment may or may not generate petroleum depending on the concentration and a tivity of the geocatalysts. Stated another way, sediments differ in the time-temperature histories needed to generate petroleum.