(Bloomberg) -- MPLX LP agreed to buy MarkWest Energy Partners LP for about $20 billion including debt.

The acquisition will be a “unit-for-unit” tax-free transaction plus a one-time cash payment to MarkWest unit- holders, MPLX said Monday in a regulatory statement.

The proposed transaction combines the U.S.’s second-largest processor of natural gas with a growing oil and refined products logistics partnership, according to the statement.

To contact the editor responsible for this story: Amanda Jordan at ajordan11@bloomberg.net