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The aim of this report is to describe a Middle Devonian dolostone reservoir in western Canada that has produced more than 57 million barrels of oil and water from moldic-pore reservoir facies with low permeability. The Slave Point Formation consists of six carbonate depositional facies, the relative proportions of which change in response to location on the basement paleotopographic surface. The most significant porosity in Slave Point dolostones is moldic porosity that formed by leaching of fossil fragments; not all Slave Point facies contain fossils. The distribution of fossiliferous carbonate facies, and moldic pores, is ultimately controlled by basement paleotopography.

The main conclusion is that there is not a good correlation between permeability and porosity in these rocks. Permeable zones are restricted to dolostones that have touching moldic pores—a constraint that has limited oil production to fossiliferous-carbonate belts that fringe Precambrian granite-basement highs. In contrast, expanses between basement knolls accumulated mostly carbonate mudstone. Mudstone lacks fossils and consequently does not contain moldic pores. The simplest description of this reservoir is that it contains pods of permeability that are surrounded by less-permeable rock, a description suited to many carbonate reservoirs. The desired impact of this work is to draw attention to methods for estimating the probability of finding more-permeable or less-permeable facies types at different locations within these types of reservoirs. This approach would lead to design of more efficient exploration and production programs in complex moldic-pore carbonate reservoirs. Both oil and water are produced from these dolostones. Understanding the effects of high fluid flow in high-permeability zones can enhance oil recovery and reduce water production from similar vuggy-dolostone reservoirs.

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