Complex sentence: When Ashland bought the Permian Corporation in 1991, it merged with Scurlock to form a subsidiary known as Scurlock Permian Corporation, which was sold to Plains All American Pipeline in 1999.
Split into simpler sentences: When Ashland bought the Permian Corporation in 1991, it merged with Scurlock to form a subsidiary known as Scurlock Permian Corporation. The Scurlock Permian Corporation subsidiary was sold to Plains All American Pipeline in 1999.