The $3 billion Fidelity Select Energy Fund ( FSENX) has a portfolio that includes most of the top performers in the sector, giving it a return of 14.6% this year. Exxon Mobil ( XOM), Schlumberger ( SLB), Occidental Petroleum ( OXY) and Chevron ( CVX) are among the five biggest holdings. The fourth-largest holding, at 4.8% of assets, is propelling the fund. That's from Marathon Oil ( MRO), which is up 38% this year and 65% over the past 12 months, giving it a market value of $36 billion. Marathon is an integrated energy company with exploration and production activities in the U.S., Angola, Indonesia and Norway. It operates six refineries in the U.S., has a big oil sands mining operation in Canada, and a liquefied natural gas facility in Africa. Standard & Poor's has a "hold" rating on its shares, because its current share price of $50.81 exceeds the firm's price target of $49. Marathon is in the process of reinventing itself, as it plans to split off its downstream business into a new publicly traded entity, Marathon Petroleum, which will be spun off to current shareholders in a tax-free transaction effective June 30. Analysts give Marathon Oil six "buy" ratings, seven "buy/holds," eight "holds" and one "weak/hold," according to a Standard & Poor's summary. Those same analysts' estimate is for earnings of $5.28 per share in 2011, and growth of 8% to $5.71 per share in 2012.