The integration of microscopic and chemical studies of organic matter from several offshore wells of Eastern Canada results in the definition of a maturation index. This parameter is based on the state of preservation scale, but its value is an average of all available data reflecting diagenetic evolution. The use of this maturation index apparently improves the representativity of the results obtained in the appraisal of organic metamorphism. The systematic comparison of the curves established on a regional scale by means of the maturation index, the rate of sedimentation and the temperature from well-logging, displays a conspicuous relation between maturation, time and the geological history of the section. Conversely, the present variations of the geothermal gradient do not seem to influence the diagenetic evolution of organic matter to a large extent. It appears that the beginning of the "oil window" corresponds to a degree of organic diagenesis more advanced in sediments more recently buried. This finding demonstrates that there is a compensation effect between temperature and overburden time. An obvious correlation also exists between incipient oil genesis and the diagenetic transformation of montmorillonite into illite and mixed-layered minerals. It can thus be assumed that the thermal effect causing maturation is conjugated with a mechanical effect due to the compaction of sediments resulting in the expulsion of fluids. Therefore the most favourable conditions for sediments to release liquid hydrocarbons seem to occur when this diagenetic stage (i.e. incipient "oil diagenetic" zone) has been reached.