Abstract

The organic matter (OM) sampled in 15 oil exploration wells and 102 outcrops of Cambrian-Ordovician rocks in the St. Lawrence Lowlands consists of zooclasts (chitinozoans, graptolites, and scolecodonts) and solid bitumen (mainly pyrobitumen). The reflectance of pyrobitumen, transformed in vitrinite-equivalent (Ro-Std-B), indicates that the upper part of the platform sequence is mature (condensate zone) in the Québec area, but overmature in the Montréal area.The platform is divided into three domains based on optical texture of OM and types of bitumen: domain 1, south of Montréal, contains a highly reflecting and coked pyrobitumen showing alteration rims; domain 3, east of Trois-Rivières, contains low-reflecting, late solid bitumen commonly associated with oil impregnations; domain 2, located midway, contains a pyrobitumen with moderate reflectance and, locally, fine mosaic texture. The thermal maturation in the autochthonous sequences of the St. Lawrence Lowlands and in the Appalachian allochthons increases from the north-east toward the south-west and in the direction of the Appalachian belt. In wells, the gradient of Ro-Std-B with depth decrease from the autochthonous zone toward the Appalachian belt, and is inversely related with thickness of the sequences. Isoreflectance values parallel the outline of the Chambly–Fortierville Syncline in the central and eastern parts of the basin. Consequently, thermal maturation predates folding. Reflectance jumps observed between the Lowlands and the first Appalachian overthrusts, and observed when crossing Logan's Line, demonstrate that the maximum burial of Appalachian sequences predates the tectonic transport. The Ro-Std-B in allochthonous zones shows higher values in the St-Francis River cross-section than in structural equivalent units of the Québec area. Therefore the increase of thermal maturation observed from the northeast toward the southwest, in the St. Lawrence Lowlands, is also developed in the Appalachian allochthonous units. A post-Taconic regional thermal event explains this similarity in both autochthonous and allochthonous sequences, with the sequences of the Connecticut Valley – Gaspé Synclinorium being the most thermally mature.Zones of highest thermal maturation, locally observed in the Montréal area, are explained by (i) hydrothermal activity (Ro-Std = 3–4%), accounted for by sulfate and sulfide mineralization (Ba, Zn, Pb) and by (ii) contact metamorphism, related to alkaline intrusions (Ro-Std-B = 13%). The contact metamorphism is restricted to aureoles less than 5 km wide around the Monteregian alkaline intrusions, but the hydrothermal alteration, apparently not related to contact metamorphism, covers an area of 10 km around mineralized domains.

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