PERMIAN BASIN <PBT> PROVEN RESERVES FALL Permian Basin Royalty Trust said that as of December 31, 1986, its estimated proved reserves totaled 18.5 mln barrels of oil and 52.9 mln MCF, or thousand cubic feet, of gas. This compares with yearend 1985 proved reserves estimates of 26.4 mln barrels of oil and 78.6 mln MCF of gas, Permian said. Permian said December 1986 future net revenues from proved reserves were 335.6 mln dlrs, down from 814.2 mln dlrs in 1985. Permian said the present value of estimated future net revenues discounted at 10 pct is 165.3 mln dlrs, compared with 374.3 mln dlrs. It said the downward revisions in both proved reserves and estimated future net revenues resulted from decreased prices for oil and gas.