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

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Dolomite Seams
Rare seams of dolomite up to l cm thick are located in the Gull River Formation.
The dolomite crystals are hypidiotopic, up to 40 Jim in size and are enclosed within
insoluble debris. No isotope analyses were performed Examination under backscatter
indicates no zonation within individual crystals (Plate 2d). Microprobe analyses indicate
that this dolomite contains no iron and is non-stoichiometric (Ca/Fe+Mg+Mn s 1.2 and
1.3). There are no isotopic analyses for the dolomite seams.
Sparry Calcite Cements
The earliest interparticle cements occur as equant isopachous sparry rims around
brachiopods, bryozoans, corals and trilobites and as syntaxial overgrowths on echinoderm
fragments. This is followed by equant sparry cement partially occluding remaining pore
space (Plate 3a). Some grainstones are completely occluded by sparry calcite cement,
whereas others have minor pore space remaining. Interparticle pores reach up to 250 }im in
size and subhedral isopachous sparry calcite crystals protrude into the pore space.
Potassium fenicyanide stain indicates that sparry calcite cement is non-ferroan followed by
ferroan sparry calcite. According to microprobe analyses the ferroan sparry calcite has a
very low iron content of 0.25 to 0.35 mol efo FeCOs. This slightly ferroan phase is more
significant towards the top of the Trenton Group and decreases in abundance downsection
toward the base of the Black River Group, where this slightly ferroan phase of sparry
calcite is absent In fact, near the top of the Trenton Group there are examples where the
syntaxial overgrowths are entirely composed of the slightly ferroan phase, whereas
isopachous rims are non-ferroan. Examination of the OGS-82-2 Harwich 25-EECR core
reveals that the slightly ferroan phase appears and disappears several times throughout the
lower half of the core.