Simulating natural organic matter maturation (coalification) is an important topic in organic geochemistry since experiments dating from the beginning of the century. However, because of the lack of an accurate natural reference series of coals, comparative efficiency of different simulation methods has not been tested. In this work, artificial maturation experiments were performed on raw lignites and coal extracts from a homogeneous sample set of increasing maturity (Mahakam Delta, Indonesia). In order to verify the validity of the experimental technique, the agreement between the chemical behaviours of the natural and artificial series was checked by different analytical techniques (C,H,O analysis; reflectance measurements; infrared and NMR spectroscopies; gas chromatography and mass spectrometry of hydrocarbons). (Monthioux et al., 1985, 1986; Landais et al., 1988). Thermal experiments were performed as follows: (1) in a open system, swept by an inert gas where the confinement (inverse of the dead-volume) is nil; (2) in a closed system, in sealed glass tubes where the confinement is intermediate; and (3) in a confined system, in cold-seal pressure autoclaves where the dead volume is minimal. For this last technique which was derived from hydrothermal experiments, 150 to 200 mg of coal were placed in an argon atmosphere inside a thin-walled gold tube (Landais et al., 1989). The gold tube was welded and then introduced in a cold-seal autoclave equipped to exert pressures ranging between 500 and 4000 bars (Fig. 1). Heat treatment temperatures range between 150 and 500°C for 24 hours. The H/C vs. O/C diagram of Figure 2 illustrates from a chemical standpoint the improving effect obtained by using increasing confinement conditions in the natural maturation of a lignite. Only a confined system is able to accurately duplicate the natural model, whereas other techniques quickly fall out of the range of natural coals. Numerous other parameters (evolution of infrared bands, aromaticity factor, reflectance, proportions of different classes of hydrocarbons, etc.) indicate the same conclusion. Thus, the confinement notion is an important parameter in the evolution of organic matter. In our experiments, confinement increase was due to the elevation of the effluents partial pressure. As far as a pyrolysis experiment involving the formation of free radicals, a pressure increase favours bimolecular reactions such as recombination or hydrogenation, whereas monomolecular reactions (unsaturation) are enhanced in open-system pyrolysis. Careful examination of the distribution of hydrocarbons shows that in confined pyrolysis, as well as in natural coalification, hydrogenation reactions are enhanced (Monthioux, 1988). Confined-system pyrolysis gives results comparable to those of natural maturation. Thus they can be applied to investigate the behaviour of different types of organic matter such as oxidized coals during maturation or coalification. It is shown that the behaviour of the end-member of an oxidation series of coals during maturation is roughly comparable to that of an unoxidized reference coal. Nevertheless, it was demonstrated that the hydrocarbon potential that has been lost during oxidation cannot be recovered during subsequent maturation. Artificial coalification of individual coal macerals extracted from the same parent coal also yields interesting results on their respective behaviours during coalification (Landais et al., in press).