By Florence Tan and Emily Chow SINGAPORE (Reuters) -Oil prices fell on Monday, giving up earlier gains, as global producers this week will likely keep output unchanged during a meeting this week and investors are cautious ahead of a U.S. Federal Reserve meeting that may spur market volatility. Brent crude futures fell 20 cents, or 0.2%, to $86.46 a barrel by 0435 GMT while U.S. West Texas Intermediate crude was at $79.57 a barrel, down 11 cents, or 0.1%. Ministers from the Organization of the Petroleum Exporting Countries (OPEC) and allies including Russia, known collectively as OPEC+, are unl…