Diagenetic history of Ordovician and Devonian oil ... - Geology Ontario

ABSTRACT
Oil and gas production in Middle Ordovician Trenton and Black River strata of the
Michigan Basin occurs where the limestones have been fractured, made porous by
dissolution and locally dolomitized. Reservoir seals are provided by the overlying shales of
the Blue Mountain Formation and by non-porous limestones.
The initial alterations to the sediment include hardground development, formation of
dolomite seams, reduction of primary porosity by sparry calcite cement and chertification.
Hardgrounds are developed throughout the sequence including the Trenton-Blue Mountain
contact. The dolomite seams are located in the lower Black River Group. Primary porosity
was reduced by isopachous equant, syntaxial overgrowths and equant, initially non-ferroan
and then moderately ferroan calcite. Generally no porosity remained following sparry
calcite cementation except for rare microporosity. Cross-cutting relations between stylolites
and fractures indicate that fracturing occurred at depth following the initial development of
stylolites.
Two types of dolomite are present: 1) cap dolomite located in the upper few metres
of the Trenton Group; and 2) dolomite developed in the near vicinity of fractures. Fracturerelated
dolomite includes both replacement dolomite and saddle dolomite cement. The
fracture-related replacement dolomite is porous in grainstone interbeds and along fractures
but non-porous elsewhere. In hand sample dolomitized grainstone beds display partial
preservation of original fabric or are sucrosic with the original fabric completely obscured.
Two types of porosity are recognized in the dolomitized units: 1) fades-independent
fracture porosity; and 2) facies-controlled, pinpoint to vuggy porosity developed in
grainstone beds.
Fracture-occluding precipitates include saddle dolomite, sulphides, anhydrite,
sparry calcite, fluorite and barite. Sulphides may precede or coat saddle dolomite and are
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