Abstract:
Geochemical examination of the rock matrix and cements from core
material extracted from four oil wells within southwestern Ontario suggest
various stages of diagenetic alteration and preservation of the Trenton
Group carbonates. The geochemical compositions of Middle Ordovician
(LMC) brachiopods reflect the physicochemical water conditions of the
ambient depositional environment. The sediments appear to have been
altered in the presence of mixed waters during burial in a relatively open
diagenetic microenvironment. Conodont CAl determination suggests that
the maturation levels of the Trenton Group carbonates are low and
proceeded at temperatures of about 30 - 50°C within the shallow burial
environment.
The Trenton Group carbonates are characterized by two distinct
stages of dolomitization which proceeded at elevated temperatures. Preexisting
fracture patterns, and block faulting controlled the initial
dolomitization of the precursor carbonate matrix. Dolomitization
progressed In the presence of warm fluids (60 75°C) with
physicochemical conditions characteristic of a progressively depleted
basinal water. The matrix is mostly Idiotopic-S and Idiotopic-E dolomite,
with Xenotopic-A dolomite dominating the matrix where fractures occur.
The second stage of dolomitization involved hydrothermal basinal fluid(s)
with temperatures of about 60 - 70°C. These are the postulated source for
the saddle dolomite and blocky calcite cements occurring in pore space and
fractures. Rock porosity was partly occluded by Idiotopic-E type dolomite.
Late stage saddle dolomite, calcite, anhydrite, pyrite, marcasite and minor
sphalerite and celestite cements effectively fill any remaining porosity
within specific horizons. Based on cathode luminescence, precipitation of
the different diagenetic phases probably proceeded in open diagenetic
systems from chemically homogeneous fluids. Ultraviolet fluorescence of
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the matrix and cements demonstrated that hydrocarbons were present
during the earliest formation of saddle dolomite.
Oxygen isotope values of -7.6 to -8.5 %0 (PDB), and carbon isotope
values of - 0.5 and -3.0 %0 (PDB) from the latest stage dog-tooth calcite
cement suggest that meteoric water was introduced into the system during
their formation. This is estimated to have occurred at temperatures of
about 25 - 40°C.
Specific facies associations within the Trenton Group carbonates
exhibit good hydrocarbon generating potential based on organic carbon
preservation (1-3.5%). Thermal maturation and Lopatin burial-history
evaluations suggest that hydrocarbons were generated within the Trenton
Group carbonates some time after 300 Ma .
Progressively depleted vanadium trends measured from
hydrocarbon samples within southwestern Ontario suggests its potential
use as a hydrocarbon migration indicator on local (within an oilfield) and
on regional scales.
