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ABSTRACT: Burial Dedolomitization, Middle Ordovician Carbonates, Southwestern Ontario

K. Middleton, M. Coniglio, S. K. Frape

The Middle Ordovician in the subsurface of southwestern Ontario consists of the Trenton-Black River groups. In this transgressive succession of bioclastic carbonates, hydrocarbon production occurs where normally nonporous limestones have been fractured, solution enhanced, and dolomitized. Stylolite-fracture relationships indicate syncompactional fracturing. Migration of solutions along fractures and adjacent grainstone beds resulted in two types of pore space: Solution-enhanced fracture porosity and pinpoint to vuggy porosity in grainstone beds. Dolomitization of these limestones in the vicinity of fractures produced massive replacive dolomite and saddle dolomite cement partially occluding porosity. Adjacent limestone contains disseminated dolomite crystals. Both fracture related dolomite types are nonluminescent whereas the disseminated dolomite displays an intricate luminescent zonation. Following saddle dolomite precipitation, the remaining pore space was partially to completely occluded by sulfides, sparry calcite, and anhydrite.

Dolomite-late sparry calcite relationships include: (1) concomitant replacement by calcite of zones within saddle dolomite, massive replacive dolomite or disseminated dolomite, (2) corroded saddle dolomite crystal faces, overlain by calcite, and (3) unaltered saddle dolomite terminations abutting sparry calcite. Dedolomitization of saddle dolomite, in the first relationship above, varies between partial replacement of less stable ferroan zones to nearly complete replacement of the entire crystal. The replacement calcite acquires the same sweeping extinction as the saddle dolomite. The ^dgr18O values of the fracture-related massive replacive dolomite, saddle dolomite, and sparry calcite vary between -8.0 and -10.4 per mil (PDB) and the ^dgr13C values vary between .5 and -0.2 per mil for dolomite and -0.8 and -2.9 per mil for sparry calcite. The similarity in isotopic signature for fracture-related dolomite and late calcite indicate dolomitization and dedolomitization occurred under comparable burial conditions. The common association of dolomite and calcite with late stage anhydrite suggests that fluctuations in the calcium sulfate saturation state of the brines may be responsible for the replacement of dolomite by calcite.

AAPG Search and Discovery Article #90998 AAPG Eastern Section Meeting, London, Ontario, Canada, September 10-12, 1990