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Reserves Versus Depth



Consulting Geologist, Delaware Ohio


     Review of the Permian Basin was used to test the question if reservoir depth influenced the amount of recoverable reserves. One hundred and ninety-two reservoirs were present from 1,250 feet to 13,800 feet. With depths divided into intervals of 500 feet the largest number of reservoirs, 15, were found at depths of 2,500-3,000 feet, 4,000-4,500 feet and 4,500 to 5,000 feet. Recoverable reserves per reservoir varied from 10.4 million barrels to 2 billion barrels, totaled 23 billion barrels with three billion barrels, 16% of the total, at depths of 4,500-5,000 feet. Oil-column thickness varied from 47 to 1,400 feet, averaged 352 feet with average oil-column thickness of 278 feet at a depth of 4500-5,000 feet. Reservoir porosity varied from 1 to 25 %, averaged 10 %, with average porosity of 13% at a depth of 4,500-5,000 feet. Permability varied from 0 to 903 millidarcys, averaged 35 millidarcys with average permability of 17 millidarcys at a depth of 4,500-5,000 feet. Second largest reserves were at 6,500-7,000 feet with 2.6 billion barrels, average oil-column thickness of 336 feet, average porosity of 9% and average permability of 15 millidarcys. Average porosity decreased with depth and had little effect on reserves. Average oil-column thickness neither increased or decreased with depth and had little effect on reserves. Increasing with depth the most effective reservoir propertyappears to have been average permability. Results may not apply everywhere but concept in basin study is helpful.