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The main producing horizon in the Leduc field in Alberta is the D-3 zone. This is a bioherm of Upper Devonian age. This reef is about 750 feet thick but only the upper 233 feet contains oil and gas.

The average of twelve chemical analyses of the upper part of this reef indicate that the CaO/MgO ratio was 1.393 or slightly less than a true dolomite. In general the samples nearest the top of the reef were the most dolomitized.

Electric logs of adjacent wells indicate that locally definite zones can be traced.

From a set of a total of 584 large horizontal and vertical sections taken from 162 feet of continuous core four types of openings within the reef were recognized and their volumes measured. The percentage porosity of each of these types was crevices 0.26, intergranular 0.92, dense 2.92 and vugs 5.11 for a total of 9.02 per cent porosity. The average porosity of 25 small test plugs from this same overall interval was 6.8 per cent. This 162-foot section can be divided into five zones on the basis of the ratio of intergranular to vug porosity.

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