Pioneer Natural Resources Co., Dallas, (NYSE: PXD) has filed an S-1 to IPO its Pioneer Southwest Energy Partners LP expecting to sell 12.5 million units at approximately $20 each for estimated gross proceeds of $250 million on the New York Stock Exchange as PSE. Pioneer Southwest will own producing properties in Spraberry Field in the Permian Basin of West Texas and will acquire producing properties in its area of operations, include onshore Texas and eight counties in southeastern New Mexico. The assets include 64% nonoperated working interest in approximately 1,100 producing wells. Production as of year-end 2006 was 4,611 barrels of oil equivalent per day. Estimated proved reserves as of year-end 2006 were 25 million barrels equivalent. The properties have a reserve life of 15 years.