Maturation thermique des roches mères dans les bassins des basses-terres du Saint-Laurent et dans quelques buttes témoins au sud-est du Bouclier canadien

Abstract

Caradocian source rocks belonging to the Utica Group occur around and overlay the southeastern part of the Canadian Shield from the Ottawa embayment, in the southwestern part of St. Lawrence Lowlands, to the Anticosti Island basin. The kerogen of the source rocks is mainly made up of amorphous organic matter (transmitted light), resembling a solid bitumen framework in reflected light, and contains a small portion of zooclasts (graptolites and chitinozoans). The kerogen is often too mature to have preserved the structure of the original constituents, except those of graptolites and chitinozoans. Otherwise, UV fluorescent colonial alga and tasmanites are observed. In the eastern half of the St. Lawrence Lowlands, stratigraphic and tectonic burial is the main control of source rock thermal maturation. From an area located 50 km west of Quebec City, the estimated vitrinite reflectance ( R o-Std) of the source rock increases in both northeastern (Montmorency Falls) and southwestern (St. Maurice River) directions, from R o-Std of 1.2 to 1.5 and 1.7%, respectively. The source rocks of the Ottawa embayment and Montreal area (western half of the basin) are overmature due to hydrothermal and igneous activities (2.3%< R o-Std<2.8%).. In Chicoutimi and Lac Saint-Jean outliers, the source rocks show reflectances between 0.9 and 1.2%, slightly less than reflectance values in the Quebec City area ( R o-Std between 1.2 and 1.5%). On Anticosti Island, the thermal maturation is related to burial only and increases downdip of the inclined platform, from the northeast toward the southwest ( R o-Std between 0.9 and 2.1%). These observations suggest that the St. Lawrence platform and the southeastern Canadian Shield were buried by at least 3.5 km of Paleozoic strata prior to erosion to Grenville basement. This implies that most potential hydrocarbon fields are presumably also lost by erosion.