Thermal maturation and potential for hydrocarbon generation and pooling are evaluated in the Upper Cambrian Grosses-Roches Formation and the Lower Ordovician Riviere Ouelle Formation of the Humber Zone in Gaspe Peninsula. Five types of organic matter (OM) analyses are used: petrography of rock thin sections and of OM concentrates, reflectance analysis, programmed pyrolysis, and combustion analysis. All analyses indicate that these successions are overmature with respect to hydrocarbon generation, but do not reach the threshold of metamorphism. Source rocks in these formations are in the dry gas zone, and the previously generated oils are now altered into various types of migrabitumen or native coke. A regular increase in maturation is observed in the Riviere Ouelle Formation (Anse du Crapaud member) from the upper condensate zone to near the end of the dry gas zone. No such gradient is observed in the Grosses-Roches Formation. Our results suggest that the upper member of the Grosses-Roches Formation may have had initial fair hydrocarbon potential (some initial TOC values up to about 2 wt%, assuming Type II kerogen) and the D2 unit of the Anse du Crapaud member may have had initial good hydrocarbon potential (some initial TOC values up to about 3 wt%, assuming Type 11 kerogen). The OM petrographic composition of these fine-grained potential source rocks shows some similarities with those of the Late Ordovician Macasty (Anticosti Island) and Lower Ordovician Green Point (Western Newfoundland) formations. The OM petrographic composition of the A to B2 units of the Anse du Crapaud member suggests former gas storage. No producible hydrocarbons remain. The recognition of Upper Cambrian to Lower Ordovician oil source rocks down-dip of potential Ordovician dolostone reservoir rocks of the Anticosti and St. Lawrence platforms opens a new perspective for hydrocarbon exploration in eastern Quebec.