The soluble extracts, as well as the hydrocarbons from cores taken between 700 and 4000 m depth in the thick homogeneous sedimentary series of the Douala Basin (Cameroon), have been analyzed by gas chromatography and mass spectrometry. They display quantitative and qualitative changes due to the effect of diagenesis with burial. The progressive evolution of this detrital organic matter has been followed from the very immature stage to the generation of oil and gas. Thermal maturation experiments in the laboratory have confirmed the most important naturally occurring changes.