Hydrothermal dolomites in Hudson Bay platform: preliminary field and geochemical data

Abstract

Hydrothermal dolostone is one of the major producing hydrocarbon reservoirs in intracratonic basins in North America. The Hudson Platform is the largest intracratonic basin in North America but it is also the least explored. A first round of exploration took place from 1968-1985 period and resulted with the drilling of 5 dry wells in the central part of Hudson Bay and a few dry wells onshore Manitoba and Ontario. The first round of exploration occurred before the recognition of the economic significance of fault-controlled dolostone bodies. Fault-controlled porous dolostone and dolostone breccia, interpreted to be hydrothermal have been identified at two localities along the northern shore of Southampton Island (northern Hudson Bay). The dolostone breccia is partly cemented by saddle dolomite cement with up to 25% open pore space. In a well drilled near the town of Churchill in NE Manitoba, a 14 m interval of brecciated dolostone with dissolution porosity and partial infill by saddle dolomite cement was identified. In one part of this interval, dolomitization and silicification are physically associated to a vertical fracture that has been crosscut by the core. Oxygen and carbon stable isotope analyses of the dolomite cement yielded very negative ?18O ratios similar to those of coeval hydrothermal dolomite in other intracratonic basins. In the absence of microthermometric fluid inclusion data, the interpretation as being the result of precipitation from a high temperature fluid is still a working hypothesis. The delta13C ratios can be very negative, a situation commonly associated with the presence of biogenic-derived bicarbonate ions in the diagenetic fluid. The delta18O ratios of groundmass fabric-retentive dolomite have been acquired to compare with those of the saddle dolomite; the former yielded slightly more negative values than those of Upper Ordovician seawater and suggest that groundmass dolomitization likely proceeded from near marine fluid at slightly elevated temperature.