Middle Ordovician Trenton strata in southern Ontario are represented by a generally transgressive sequence that reflects a wide spectrum of carbonate environments from tidal flat, through lagoon and shoal, into deeper shelf carbonates. The Trenton conformably overlies the shallow water carbonates of the Black River and is unconformably overlain by the gray-black noncalcareous shales of the Blue Mountain Formation.

Virtually all Ordovician production in Ontario is associated with structural deformation related to rejuvenation of a Precambrian fracture framework triggered by orogenic events in the nearby Appalachian orogene. The reservoirs are characterized by the replacement of original bioclastic limestone beds by more or less discontinuous lenses of fine to medium-grained, light to medium-brown crystalline dolostone. Pools generally are linear, following the trend of the associated fracture.

Six of the 18 known Ordovician pools in Ontario are located in Essex County. A detailed study of the geology and reservoirs confirmed the close association of fracturing, dolomitization, and hydrocarbon entrapment. Representative samples of well cuttings from 20 wells were analyzed by XRD (x-ray defraction) to determine calcite-dolomite ratios. As expected, low ratios were present in the producing reservoirs. Partially dolomitized zones were revealed in wells in close proximity to fractures. Formation water originating in the underlying Cambrian sandstones was probably the main dolomitizing agent as it migrated up through the fracture. Dolomitization enhanced already existing porosity within the bioclastic zones.

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