--> Abstract: The Manetoe and Presqu'ile Dolomites: Early or Late Burial Dolomitization?, by D. W. Morrow and K. L. Aulstead; #91012 (1992).

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ABSTRACT: The Manetoe and Presqu'ile Dolomites: Early or Late Burial Dolomitization?

MORROW, D. W., Geological Survey of Canada, Calgary, Alberta, Canada, and K. L. AULSTEAD, Shell Canada, Calgary, Alberta, Canada

Coarsely crystalline white dolomite of the Manetoe Facies may be the northward continuation of the well-known Presqu'ile Dolomite, which is approximately coextensive with the Keg River Barrier at the north end of the Elk Point basin. The Manetoe Facies extends for 38,000 sq. kilometers (15,000 sq. miles) within Lower and Middle Devonian strata north of the Elk Point basin and is the host for Canada's northernmost gas fields with on-stream gas production, the Pointed Mountain, Kotaneelee, and Beaver River fields.

Early lithification and burial of fossiliferous lime wackestones and lime mudstones were followed by solution excavation of an extensive gallery system of cavernous pore space in Devonian carbonates of the Keg River Barrier and north of the Elk Point basin. Development of part of this cavernous porosity may have occurred during deposition of the deltaic Middle Devonian Watt Mountain Formation.

Petrographic evidence indicates a paragenetic sequence in which open-space precipitation of white dolomite and in situ dolomitization along with brecciation immediately followed, or was contemporaneous with, the creation of open space. Residual coatings of bitumen on the centripetally oriented crusts of white dolomite crystals lining vugs indicate that emplacement of hydrocarbons followed dolomitization. Calcite and quartz infill the centers of many vugs and clearly postdate dolomitization and probably postdate hydrocarbon emplacement. Variable amplitude stylolites and solution seams are also present throughout the Manetoe and Presqu'ile Facies. Larger amplitude stylolites clearly cross-cut dolomite fabrics of both the Manetoe and Presqu'ile Facies. In some places, larger stylolites a so appear to postdate calcite cementation.

These paragenetic relationships indicate that most, if not all, dolomite in the Manetoe and Presqu'ile Facies was pre-Cretaceous in origin and occurred during shallow rather than deep burial before emplacment of hydrocarbons and stylolite development.

This is consistent with vitrinite reflectance profiles from wells in the Beaver River gas field and other indicators of organic maturation (CAI, TAI), which imply that the Cretaceous and Tertiary were times characterized by low geothermal gradients. The high fluid inclusion homogenization temperatures (190 degrees C modal temperature) of Manetoe dolomite implies unrealistically deep burial if the Manetoe Facies developed during low heat flow in Cretaceous to Tertiary time.

 

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)