Shares of Marathon Petroleum (MPC) wrapped up Monday's trading day to the upside, by 7.8 percent. The stock is TheStreet's Move of the Day. Shares saw a boost after the oil giant announced its master limited partnership, MPLX (MPLX), will buy natural gas processor MarkWest Energy Partners (MWE) in a deal valued at roughly $15.8 billion. MarkWest focuses on dividing natural gas into other types of fuels, like propane, while Marathon handles oil transportation through its pipelines, among other specialties. Shares of Marathon reached an all-time high for the year and was the best performing stock in the S&P 500. But shares of the master limited partnership MPLX fell 14.5 percent on Monday and lost over 28 percent since the stock's high for the year back on March 2. TheStreet's Scott Gamm reports from New York.