Hydrothermal Dolomite Reservoirs in Eastern Canada Appalachians, Ordovician to Devonian Fold and Thrust Belts— The Hunt for Elephants

Lavoie, Denis, 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.

The Lower – Middle Paleozoic Canadian Appalachians primarily comprises two major Fold and Thrust Belts (FTB), the Ordovician Taconian FTB and the Silurian-Devonian Acadian FTB with associated shallow marine foreland ramps.

Lower Ordovician passive margin carbonates occur in the little-deformed Laurentia plat­form or as detached tectonic slices in the frontal Taconian FTB. The carbonates are locally pervasively dolomitized and porous. Abundant saddle dolomite cements are found in col­lapse platform segments. A significant oil pool has been discovered in western Newfoundland with historical gas production in southern Quebec.

Middle to Upper Ordovician foreland ramp carbonates are locally affected by early hydrothermal alteration of limestone facies with dissolution-collapse, pervasive dolomitiza­tion and saddle dolomite. These Middle Ordovician units are the most significant producers in eastern USA. Significant although sub-economic gas accumulations are reported in southern Quebec.

Lower Silurian ramp carbonates of the Acadian foreland are characterized by dissolu-tion-collapse, pervasive and saddle dolomites in Gaspé and 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 preserved in the Acadian FTB 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.