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The AAPG/Datapages Combined Publications Database
CSPG Bulletin
Abstract
C.S.P.G. 1990 Convention, "Basin Perspectives"
Fracture-Related Diagenesis as a Control on Middle Ordovician Carbonate Reservoirs, Southwestern Ontario [Abstract]
ABSTRACT
One half of the producing wells in southwestern Ontario were drilled into Middle Ordovician carbonates of the Trenton and Black River groups in the Michigan Basin. Production occurs where these limestones have been fractured in the vicinity of the southwest to northeast trending Algonquin Arch.
This Middle Ordovician succession of bioclastic carbonate mudstones, wackstones, packstones and grainstones represents deposition in typical supratidal through shallow subtidal settings. Overall, these strata are interpreted as an upward-deepening sequence. As depth increased during subsequent deposition of the Blue Mountain shales, the Trenton-Black River carbonates were cemented by shallow burial, non-ferroan cements and then later by ferroan burial calcite cements. These occur as finely crystalline isopachous rims around grains, syntaxial overgrowths on echinoderm fragments, and equant sparry cements occluding remaining pore space. This latter is the most important volumetrically. Further burial resulted in minor pressure solution followed by pervasive dolomitization. Porosity developed in limestones adjacent to fractures developing in this progressively buried sequence.
Two types of porosity exist in the reservoir rocks: 1) centimetre-scale reduced fracture porosity in brecciated areas, and 2) pinpoint to vuggy porosity in grainstone interbeds. The former is facies independent and termed fracture porosity, while the latter is facies dependent and termed grainstone porosity. Both types of porosity are partially to completely occluded by later phases of saddle dolomite, sulfides, sparry calcite and anhydrite.
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ACKNOWLEDGMENTS AND ASSOCIATED FOOTNOTES
1 University of Waterloo, Waterloo N2L 3G1
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