Typical sections of the Ordovician carbonate platform sequence in the St. Lawrence Lowlands (Beekmantown, Chazy, Black River, Trenton, and Utica Groups) were sampled at Neuville, Joliette, and Montreal, in the Province of Quebec. The purpose of this work was to determine the level of thermal maturation of the carbonates using organic matter and clay studies, and to correlate it with the crystal characteristics of the microcrystalline calcite.The studied formations are supermature in the Joliette and Montreal areas, whereas they are only mature at Neuville.The numerical analysis of the petrographic data shows that some microstructures observed in the rock samples and the size and morphology of the microcrystalline calcite crystals are partly related to the thermal maturation as determined by other methods.