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Simulated maturation experiments on recent sediments from different source environments give quantitative and kinetic information on the production of petroleum and natural gas. Starting material, temperature, and time of heating play critical roles in determining quantities and types of reaction products. Sediment samples from recent peaty deltaic and algal-mat lagoonal deposits were heated separately in closed systems for 1-15,000 hr at 35°-550°C (95°-1,022°F). Reaction products were monitored for both quantity and isotopic composition. The products monitored were hydrogen, carbon dioxide, natural gas-range volatile hydrocarbons (C1-C5+), and oil-range hydrocarbons and nonhydrocarbons (C15+). Three stages of petroleum formation have been documented. The immature stage is characterized by production of C1-C5+ hydrocarbons but essentially no new C15+ components. Carbon isotope values for the initial volatile products, formed through kerogen and humic acid rearrangements, display marked kinetic effects. In the mature stage, catagenetically derived products are added to the original biologically related C15+ hydrocarbon fraction. Methane formed in this stage is produced by cleavage of methyl groups from the bitumen, and is characterized by stable carbon isotope values 15 ppt lighter than the starting material. The post-mature stage shows conversion of C2-C5+ components to isotopically heavy methane. The quantity of oil- and gas-range products formed per gram of total organic carbon is approximately the same for the peat and algal pyrolysis experiments, but the organic source facies affects the rate of generation and the product distribution. Peaty, deltaic organic matter generates hydrocarbon and nonhydrocarbon gases and extractable bitumen at lower levels of thermal stress than does algal material. The most notable product variation is that the C2-C5+ component constitutes a greater proportion of the algal-generated volatiles.