Biochemically versatile bacteria, capable of catalyzing a good many chemical reactions, occur abundantly in recent marine sediments. Living bacteria detected in certain petroliferous formations at appreciable depths are believed to be indigenous species in a state of dynamic equilibrium with their environment. If petroleum is formed from organic matter, it is almost axiomatic that bacteria have contributed to the process, because the organic remains of both plants and animals are susceptible to microbial modification. In recent marine sediments bacteria tend to convert organic compounds into humus and other substances approaching the elementary composition of crude oil. Besides producing methane as an end-product of metabolism, certain bacteria synthesize small quantities of liquid and solid hydrocarbons as an integral part of their cell substance. Anaerobic bacteria found in sediments catalyze the oxidation of molecular hydrogen with the formation of methane, hydrogen sulfide, saturated compounds, and other substances that may occur in oil fields. Crude oil in contact with water may be slowly modified in various ways by bacterial activity. Bacteria may contribute to the migration and accumulation of oil by promoting its liberation from oil-bearing sediments.