Tesoro Corp. plans to launch an unregistered offering of $1.6 billion of senior notes to partially pay for its acquisition of Western Refining Inc.

If completed, proceeds from the offering of senior notes due 2023 and senior notes due 2026 would be augmented with cash on hand and borrowings under Tesoro's revolving credit facility to help pay for the Western Refining deal, including repayment and redemption of Western Refining's debts and other related expenses, according to a Dec. 15 news release.