ABSTRACT: Location of Dolomitized Grainstone Reservoirs in the Upper Ordovician Black River and Trenton Limestone Groups in Ontario
M. E. Brookfield and M. El Gadi , Land Resource Science, Guelph University, Guelph, Ontario, Canada
Dolomitized bioclastic grainstones form the main hydrocarbon reservoirs in the Ordovician of Ontario. These are controlled both by depositional environment by tectonics. Environmental position on the ramp determined where the contributing organisms lived and how they were reworked. Tectonics not only determined the configuration of the ramp and where Precambrian inliers occurred (which themselves locally control grainstone location), but also movements on faults which controlled the dolomitization.
By analogy with carbonate ramp models based on modern environments (the most applicable one being a cool Persian Gulf ramp), bioclastic grainstones occur in two main settings: in shallow shelf shoal areas around normal wave base, and in deeper shelf areas affected by contour currents. The grainstones form the tops of coarsening (and possibly shallowing) upwards cycles within a generally transgressive succession.
By analogy with tectonic models based on Cenozoic collisions (the most applicable one based on the indentation and rotation of Afghanistan by NW India), Ordovician dolomitization and reservoir formation occurred along lateral transtensional and transpressional faults related to collision of the Taconic arc with the Ordovician carbonate ramp. Differential shear can be related to the contrast between the Taconic arc indenter and the areas to the north-the dividing line roughly running from Anticosti Island down the St Lawrence into southern Ontario. The Ordovician succession in Ontario and New York is now known to be at least partially controlled by synsedimentary tectonics. Such successions, especially in the absence of a reliable chronostratigraphy, should not automatically be assumed to consist of blankets of uniform and easily correlated facies controlled by relative changes of sea level.
Search and Discovery Article #90907©2000 AAPG Eastern Section Meeting, London, Ontario, Canada