The Tertiary sequence of the Labrador offshore (Karlsefni H-13 well) is formed by clayey–detrital lithologies for which thermal maturation and oil potential have been established.The kerogen of this sequence is ligneous (type III) but shows a slight tendancy towards a more sapropelic (type II) character between 2000 and 3300 m. The amorphogen, abundant throughout the borehole, is partly derived from the destruction of woody matter. The sapropelic trend of the kerogen corresponds to an increase of the organic carbon and amorphogen content. In this sequence, the best oil potential (fair to good) would be offered by the lithologies between 2000 and 3300 m, if the appropriate catagenetic zone had been reached.The amount of gas produced with respect to oil, in the 2000–3300 m interval, is a function of the thermal maturation. The oil window maturation stage (potential oil window (POW)) is reached at 3750 m. Two thermal maturation stages occurred before the POW: an upper dry gas (methane) stage (2160–2520 m) and an upper wet gas (condensate) stage (2520–3750 m). A biogenic gas zone (650–765 m) also occurs at the top of the unproductive immature zone. More than one analytical parameter must be used to differentiate these diagenetic stages. These parameters come from the physico-chemical and microscopic studies of the kerogens, organic extracts, adsorbed gases, and clay mineralogy.