Shell, Amoco, Exxon in $1-Billion Oil Project: The companies will jointly develop the deepest offshore oil rig ever used in the Gulf of Mexico, Shell Oil Co. said. A 41,000-ton platform will be installed in 3,218 feet of water about 125 miles southeast of New Orleans and 80 miles south of Mobile, Ala., a Shell spokeswoman said. The companies expect to extract 60,000 barrels of oil and 200 million cubic feet of natural gas daily after production starts in late 1997. Shell will provide about $380 million of the investment and operate the field where the platform will be located. Amoco and Exxon will provide about $310 million each.