Paleozoic hydrocarbon systems in Québec: recent data and synthesis
Denis Lavoie (GSC-Q)

Three hydrocarbon systems are known in Québec:

1. Cambrian-Ordovician in southern and eastern Québec;
2. Silurian-Devonian in Gaspésie;
3. Carboniferous-Permian in the Gulf of St. Lawrence .

Source rocks are found in Lower (TOC: 10.4%, HI: 759, Type II), Middle (TOC: 10.7%, HI: 266, Type I-II) and Upper Ordovician sequences (TOC: 14%, HI: 633, Type II). Hydrothermal dolomite (HTD) reservoirs are known in Lower and Middle/Upper Ordovician rocks. These HTD formed during shallow burial following the circulation of high-pressure, high-temperature (135-150°C) saline fluids (16-20 wt% NaCl eq). Potential reservoirs are proposed in littoral and fluvial sandstones, and in turbidites and channel sandstones of the continental slope. Reservoirs are associated with a variety of stratigraphic and tectono-diagenetic traps in the St. Lawrence Platform and along the Appalachian front. Maturation data indicate that these successions are within the gas window, with some areas being favourable for oil preservation.

Silurian-Devonian successions have little source rock potential. Data indicate that oil in Devonian reservoirs can be correlated with Ordovician source rocks. Known reservoirs are in fluvial and deltaic sandstones, and in Lower Devonian HTD associated with major fractures. Other potential targets are Lower Silurian littoral sandstones and Silurian and lowermost Devonian HTD, which formed through the action of high-temperature (180-225°C) and very saline fluids (18-26 wt% NaCl eq). These successions are associated with stratigraphic and tectono-diagenetic traps; a significant number of seismic anomalies and enhanced reflector zones are present in the Silurian successions. Maturation data indicate the system is favourable for the preservation of liquid and gaseous hydrocarbons.

With regards to Permian-Carboniferous successions, source rocks are known in Tournaisian lacustrine shales (TOC up to 30%, Types I and II). These source rocks generated the gas in the McCully field of New Brunswick. Pennsylvanian successions display numerous coal beds with strong gas potential. Known reservoirs (McCully, Stoney Creek, and East Point) consist of low-permeability Tournaisian fluvial sandstones and Pennsylvanian fluvial and deltaic sandstones. Clastic reservoirs are associated with many stratigraphic, tectonic and salt-bearing traps. Visean salt deposits form an extensive domain in the southern segment of the Gulf of St. Lawrence.