Sediment samples from recent peaty deltaic and algal-mat lagoonal deposits were heated separately in closed systems for 1-15,000 hr at 35 degrees -550 degrees C (95 degrees -1,022 degrees F). Reaction products were monitored for quantity and isotopic composition. The products monitored were hydrogen, carbon dioxide, natural gas-range volatile hydrocarbons (C 1 -C (sub 5+) ), and oil-range hydrocarbons and nonhydrocarbons (C (sub 15+) ). Three stages of petroleum formation have been documented. The immature stage is production of C 1 -C (sub 5+) hydrocarbons but essentially no new C (sub 15+) 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 C (sub 15+) 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 C 2 -C (sub 15+) 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 C 2 -C (sub 5+) component constitutes a greater proportion of the algal-generated volatiles.--Modified journal abstract.