BP is selling four mature producing deepwater oil and gas fields in the Gulf of Mexico for $650m (£413m) in cash just seven months after agreeing to buy them as part of a wider $7bn deal with Devon Energy.Marubeni Oil has agreed to buy the assets - a 25% interest in the Magnolia oil and gas field, a 50% interest in the Merganser gas field, 50% of the Nansen field, and 65% of the Zia field. BP only bought them in March when it took on a bundle of assets in the Gulf of Mexico, Brazil and Azerbaijan.It has been selling chunks of its portfolio for months now to help cover the cost of the explosion on the Maconda oil well that killed 11 workers and resulted in the worst oil spill in American history.BP wants to raise $30bn through asset sales by the end of 2011. Since setting the target in July, it has brought in over $11bn, including today's sale and last week's disposal of its upstream businesses in Venezuela and Vietnam to TNK-BP for $1.8bn."When BP acquired Devon's Gulf of Mexico assets it was clear that these four fields did not fit well with the rest of our business in the region. We therefore decided they would be of more value to another company than to BP," explained Andy Hopwood, BP executive vice president, Strategy and Integration.The sale of the fields, where net production is about 15,000 barrels of oil equivalent a day (boed), is expected to complete early next year.BP remains the largest producer of oil and gas in the Gulf of Mexico, with current net production totalling 400,000 boed. It's also the largest holder of leases.