Maturation thermique de la matière organique dans un bassin du Paléozoïque inférieur, basses-terres du Saint-Laurent, Québec, Canada

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.

Fossil brines preserved in the St-Lawrence Lowlands, Québec, Canada as revealed by their chemistry and noble gas isotopes

“…The correlation likely describes the mixing between two sources of radiogenic gases, namely the crystalline basement and in situ production within a sedimentary reservoir. End-members representing theoretical 4 He/ 40 Ar * and 21 Ne * / 40 Ar * ratios in the upper crust and in the Ordovician dolomite are from Table 5. burial depths varying between 5.6 and 7.5 km (Heroux and Bertrand, 1991). This is agreement also with AFT thermal resetting observed in Precambrain basement rocks sampled along the faults bordering the St-Lawrence Rift System (Roden-Tice and Tremblay, 2009).…”

“…Fission-track thermochronology of the St-Lawrence Rift System suggests that the present-day thermal regime could have lasted since Late Jurassic (Roden-Tice and Tremblay, 2009). The St-Lawrence Lowlands could have reached temperatures greater than 250°C solely during (1) Devonian, when Utica shale matured, producing natural gas (Lavoie et al, 2009); or (2) briefly and locally during Cretaceous Monteregian Hills magmatic episode (Heroux and Bertrand, 1991). This suggests that 40 Ar * was introduced in the Bécancour brines sometime between these two thermal events.…”

Surface and subsurface signatures of gas seepage in the St. Lawrence Estuary (Canada): Significance to hydrocarbon exploration

“…The Utica Shales are in the dry gas zone in the Montre´al area, whereas the source rock sits in the upper part of the condensate zone in the Que´bec City area (He´roux and Bertrand, 1991). In wells, maturation positively correlates with depth, most likely suggesting a sedimentary burial control of maturation.…”

“…Most of the deformation of the frontal thrust sheets of the Appalachians has been traditionally attributed to the Middle-Late Ordovician Taconian orogeny (St-Julien and Hubert, 1975;Tremblay and Pinet, 1994;Pinet and Tremblay, 1995). The imprint of younger tectonic events, recorded by both extensional and compressional structures, is, however, increasingly documented on the basis of field (Lynch, 1998;Rocher et al, 2003), seismic (Sanford and Grant, 1990), radiochronologic (Glasmacher et al, 2003;Sasseville, 2006) and maturation (He´roux and Bertrand, 1991) data.…”

Revisiting the Appalachian structural front and offshore Anticosti Basin (northern Gulf of St. Lawrence, Canada) by integrating old and new geophysical datasets

“…Stratigraphy of the Gaspé belt based on Bourque et al (2001). The amount of eroded strata is based on studies by Héroux and Bertrand (1991), Bertrand (1987), Bertrand and Malo (2001) and Cooper et al (2001). (Fig.…”