Hydrothermal Dolomite Reservoirs in Eastern Canada—a Promising Newly Recognized Play in Paleozoic Carbonates

Denis Lavoie
Geological Survey of Canada, Quebec City, QC

Prolific hydrothermally altered carbonate reservoirs are known in Ordovician successions of eastern North America. However, the hydrothermal dolomite (HTD) play is underexplored in Paleozoic units of eastern Canada. Critical elements for the recognition of HTD can be regrouped in field (early fractures, breccia, dissolution and collapse) or chemical-diagenetic (pervasive dolomitisation, saddle dolomite, sulphates) evidence.

Lower Ordovician passive margin carbonates (Beekmantown and St. George groups and Romaine Formation) occur in the St. Lawrence Platform or as detached tectonic slices at the Appalachian tectonic front. The carbonates are locally pervasively dolomitized. Abundant saddle dolomite cements are found in collapse segments of brecciated/fractured units. A significant oil pool has been discovered in western Newfoundland.

Middle to Upper Ordovician foreland ramp carbonates (Black River and Trenton groups, Mingan Formation and Table Head Group) are locally affected by early hydrothermal alteration of limestone facies with dissolution-collapse, pervasive dolomitization and saddle dolomite. Significant although sub-economic gas accumulations are reported in southern Quebec.

Lower Silurian ramp carbonates (Sayabec and La Vieille formations) are characterized by dissolution-collapse, pervasive and saddle dolomites in both northern Gaspé and southern Gaspé / northern New Brunswick. Abundant bitumen is seen in open pore space and seismic flat spots are recognized at that seismostratigraphic interval in western Gaspé Peninsula.

Finally, Lower Devonian outer shelf limestones (Upper Gaspé Limestones) are locally characterized by significant saddle dolomite filled fractures and dolomitic breccias. Natural gas production in eastern Gaspé is related to an intense fracture-brecciated interval with some saddle dolomite cements.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005