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

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DOLOMITIZATION (PREVIOUS WORK)
The same types of dolomite are found throughout the Michigan Basin. These
include: 1) cap dolomite, 2) fracture-related replacement dolomite, and 3) fractureoccluding
saddle dolomite cement. In addition the Middle Ordovician sequence in the
extreme south and west portion of the Michigan Basin is massively dolomitized ("regional
dolomite*1). The diagenesis in Michigan, Indiana and Ohio has been described by Shaw,
1975; Taylor and Sibley, 1986; Keith, 1986,1988b; Budai and Wilson, 1988; DeHaas and
Jones, 1988; Fara and Keith, 1988; Haefner, et al., 1988; and Prouty, 1988. In
southwestern Ontario the same dolomite types have recently been investigated by Bailey
Geological Services Ltd. and Cochrane (1983); Dollar (1988); Trevail (1988a, 1988b);
Colquoun and Brand (1989a, 1989b); and Middleton et al. (1989,1990a, 1990b).
In a study of dolomitization of Ordovician strata in Michigan, Taylor and Sibley
(1986) provided convincing evidence to explain dolomitization of the cap via magnesiumand
iron-rich waters expelled during dewatering and illitization of the overlying Utica
Shale. The source of the brines responsible for fracture-related dolomitization is still in
question. Sanford et al. (1985) suggested the source of the dolomitizing solutions to be
either Cambrian sandstones or an influx of meteoric water during a hiatus at the end of the
Middle Ordovician. Dollar (1988) and McNutt et al. (1987) noted the strontium isotopes of
the brines and the fracture-filling precipitates to be slightly radiogenic, suggesting either a
clastic or basement influence. Prouty (1988) suggested the most logical explanation is
upward migration of hydrothermal brines which become dammed beneath the Utica Shale.
This damming resulted in the maturation of organic matter in the Utica Shale and further
upward migration through fractures into the Middle Devonian Dundee Formation.