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ABSTRACT: Paragenetic History of the Ordovician Trenton Group Carbonates, Southwestern Ontario

Ian Colquhoun, Uwe Brand

Geochemical examination of the rock matrix and cements from four oil wells within southwestern Ontario suggest various stages of diagenetic alteration and preservation of the Trenton Group carbonates. The geochemical compositions of pristine LMC brachiopods reflect the physiochemical water conditions of the ambient depositional environment. The sediments appear to be altered in the presence of seawater-dominated fluids in a relatively open diagenetic microenvironment during early burial. Conodont CAI determination suggests that the maturation levels were low and had proceeded with shallow burial temperatures of about 35-50°C.

Dolomitization of the Trenton Group carbonates is characterized by two distinct phases that proceeded at elevated temperatures. Initial dolomitization of the precursor carbonate matrix was fracture-pattern controlled and progressed in the presence of warm fluids (50-100°C) with physiochemical conditions characteristic of a progressively depleted basinal water. The dolomite matrix consists of essentially coeval Idiotopic-S and Idiotopic-E dolomite closely followed by the Xenotopic-A dolomite that predominates with depth. The second phase of dolomitization involves hydrothermal basinal fluids with temperatures of about 110-150°C. These are the postulated source for the various cements found occupying pore space, except for the lateststage calcite cement. Rock porosity was foun to be partly occluded by Idiotopic-E type dolomite. Later stage saddle dolomite, calcite, anhydrite, barite, pyrite, marcasite, and trace sphalerite cements effectively fill any remaining porosity within specific horizons. Based on cathode luminescence, formation of the different diagenetic phases probably proceeded in diagenetically open systems with chemically constant fluids.

Organic carbon and pyrite sulfur data suggest that the sediments were deposited in an essentially normal marine environment, but with the high C/S ratio (4.9) suggesting a minor meteoric influence. In general, anomalously high C/S ratios (>10) are associated with the hydrocarbon-producing pay zones of the Trenton Group carbonates.

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