DRIVET, EVA, and ERIC MOUNTJOY, McGill University, Department Earth and Planetary Sciences, Montreal, Quebec

ABSTRACT: Porosity Variations in Upper Devonian Leduc Dolomites, Central Rimbey Meadowbrook Reef Trend, Alberta

Most of the available core in the Leduc reservoirs between townships 33 and 46 were examined. These rocks were presumably subjected to the same early complex near surface diagenetic history that can only be observed in the limestone buildups like Golden Spike and Redwater. In limestone buildups some porosity was both increased locally near surfaces of subearial exposure, and also decreased by irregularly distributed calcite cements.

Two periods of dolomitization took place, an early burial replacement dolomite (forms over 80%-90% of the reservoirs), and a later dolomite cement. In general, porosity in the replacement dolomites (6 to 10%) tends to be lower than in the limestone buildups (8 to 12%). Thus the replacement dolomites decreased the porosity slightly, although locally increased porosity as well as improved the permeability in dolomites with intercrystalline porosity.

The pore types in the replacement dolomites, in order of abundance, consist of moldic, vuggy, intercrystalline, fracture, enlarged vugs and fenestral porosity. The moldic and vuggy porosity resulted from the dissolution of Amphipora, stromatoporoids, and shells either during dolomitization and/or afterwards. In general dolomitization transformed most grainstones to intercrystalline dolomites and muddy carbonates to nonporous dolomites.

Locally these pores (molds, vugs, and some fractures) are lined with a white dolomite cement that increases downdip from 1% (twp 42) to about 5% (twps 33 to 36). Following this, both replacement dolomites and dolomite cements were subjected to solution collapse that enlarged vugs into larger (20 cm to 2 m) irregular vugs and fractured the earlier dolomites. The dolomite cements also show evidence of some solution.

Late stage anhydrite completely fills most large vugs. The amount of anhydrite increases with depth, from only traces in the Leduc buildup to about 4% in twp 42, to about 7% in twps 33 to 36. Anhydrite fills most of the vuggy and moldic porosity in the deeper wells and decreases the overall porosity by about 5%.

Thus porosity was slightly reduced by both the early replacement dolomites and the dolomite cements. Although porosity was locally increased by a later period of subsurface solution, this increase was offset by anhydrite plugging that reduced the porosity even more.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.