ABSTRACT

The Grosmont Formation (Upper Devonian) comprises a platform carbonate sequence which extends over 100,000 sq. km in the subsurface of Alberta and contains extensive areas of calcitized ("dedolomitized") dolomite reservoir rocks. Hydrocarbon content of the Grosmont Formation is estimated to be 50 10⁹ m³ of 7 API gravity oil, with bitumen recovery rates as high as 70 percent using thermally enhanced recovery.

Environments of deposition in the Grosmont Formation range from subtidal and intertidal to supratidal. Early, stratigraphically controlled and later, cross-cutting dolomites are observed. Dissolution fabrics are generally related to the uppermost dolomitized units of the Grosmont Formation, below an unconformity which overlies Paleozoic rocks. Calcitization textures are found in limestone and dolomite units and do not bear any relationship to the unconformity.

Petrographic examination shows that calcitization of dolomite occurs in different forms. Large (0.25-0.5 mm) dolomite rhombs, observed floating in micrite or at calcitic micrite-dolomite contacts, have cores replaced by calcite. Dolomite rhombs that are not calcitized contain cloudy cores with abundant calcite inclusions. In some samples, fragments of dolomite rhombs are observed with the cores dissolved away, indicating that part of the calcitization process involves dissolution. A complete range, from dolomitic rocks that show no evidence of calcitization to calcitic rocks with the textures of sucrosic dolomites, are observed. Late calcite cements, filling the remaining available pore space, are also related to the calcitization process.

There is a strong tendency for dolomites from stratigraphic units to be grouped by their isotopic compositions. Three phases of dolomitization are interpreted in the Grosmont Formation: an early sabkha dolomite (Upper Grosmont 3); dolomites formed during reflux of hypersaline brines (Upper Grosmont 2); and dolomites related to waters expelled from shales during burial (Lower Grosmont). Further, dolomites in each unit tend to be depleted in the heavy isotopes of both carbon and oxygen with increasing degree of dissolution and/or calcitization. The geological and geochemical evidence indicates that calcitization was early and related to dolomitization, probably with a later stage that took place during uplift and the entry of meteoric water along the unconformity.