2University of Regina, Department of Geology, Regina, Saskatchewan

World-class conventional hydrocarbon reservoirs are hosted by Paleozoic hydrothermal dolomites (HTD). By definition, a hydrothermal fluid has temperature at least 15ºC over that of the ambient burial fluid. If late burial allows the rock unit to be exposed to temperatures higher than those recorded by an early hydrothermal event, it is the difference of temperature at that event which is critical.

In Quebec, the carbonate units in the St. Lawrence Platform occur at multiple stratigraphic intervals from the Lower (Beekmantown Group and the Romaine Formation), the Middle (Chazy Group and Mingan Formation) and the Upper (Black River and Trenton groups) Ordovician.

Ordovician HTD occurs in the Anticosti platform, although data from passive margin (Romaine) to foreland basin (Mingan) carbonates suggests significant distinctions. Saddle and matrix-replacement dolomites of the Romaine Formation are characterized by high FI homogenization temperatures (up to 151°C), high ⁸⁷Sr/⁸⁶Sr ratios (0.709142 to 0.712862) and low Sr content (average 180 ppm). Conversely, the replacement dolomite of the Mingan Formation is characterized by lower FI homogenization temperatures (below 128°C), lower ⁸⁷Sr/⁸⁶Sr ratios (0.708815 to 0.709034) and higher Sr content (average 513 ppm). T_h of saddle and replacement dolomites are above maximum burial temperature recorded by both units, in particular in the northern part of the island (135°C and 110°C for the Romaine and Mingan, respectively), therefore, both dolomites are interpreted as hydrothermal in origin. Limited geoscience data indicate that two different fluids were responsible for the alteration of both formations, timing of the fluid pulses are currently unknown.

AAPG Search and Discovery Article #90059©2006 AAPG Eastern Section Meeting, Buffalo, New York