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The Interlake Formation is a Silurian-age sequence of dolostones, which produces hydrocarbons in the Williston basin. Log analysis of numerous Interlake wells from the Nesson anticline reveals that both water-productive and hydrocarbon-productive zones commonly have calculated water saturations in excess of 60%. These high calculated water saturations, in zones that produce water-free hydrocarbons, appear to be the result of a bimodal pore system. Non-fabric selective vugular pores are the major type of porosity seen in visual examination of Interlake cores. These vugs have been interconnected by fracturing and are responsible for most of the hydrocarbon production. The matrix that separates the vugs is composed of small equant dolomite crystals and also contains large am unts of intercrystalline microporosity, which is interconnected by pore throats less than 0.5 µm across. These small pore throats result in low permeability and high capillary pressures; thus the microporosity is capable of holding 100% irreducible water, whereas the vugular pores produce water-free hydrocarbons. Because it composes up to 50% of the total porosity, this microporosity drastically reduces the resistivity of the formation.
Recognition that a formation contains a significant amount of microporosity is important not only in preventing bypassed production, but also in determining reserves and exploration economics. Determination of the percentage of effective porosity cannot be made using electric log or conventional core analysis. A combination of special core analysis and petrographic techniques is needed to better define the amount of effective porosity.
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