Crude extracted from the largest U.S. oil field weakened against the U.S. benchmark after producers lost access to Midwest markets when a 4,000-barrel spill in Louisiana forced the shutdown of a key pipeline. West Texas Intermediate in Midland, Texas, weakened by 75 cents a barrel to a discount of $7 relative to the same grade in Cushing, Oklahoma, at 11:47 a.m., according to data compiled by Bloomberg. It’s the largest discount since Oct. 1. Midland is the pricing point for the Permian Basin, which produces about 1.76 million barrels of oil a day. Sunoco Logistics Partners LP (SXL) shut a segment of its Mid-Valley Pipeline between Longview, Texas, and Mayersville, Mississippi, after it spilled as much as 4,000 barrels of crude last night, Travis Lawson, a spokesman for the Philadelphia-based company, said by phone. The Mid-Valley line delivers Permian crude from Longview to six states, including Ohio and Michigan. […]