CHICAGO--(BUSINESS WIRE)--Fitch Ratings has assigned a 'BBB+' rating to Marathon Oil Corporation's (MRO) issuance of $2 billion in senior unsecured notes. Of the net proceeds, $1 billion will be used to repay the company's 2015 0.9% notes, and the remainder will be used for general corporate purposes. Marathon's ratings reflect the company's high exposure to liquids (as calculated by Fitch, 69% of production and 81% of reserves); reasonably diverse upstream portfolio; good liquidity; trend of e