MADRID, Jan 26 (Reuters) - Infrastructure investment fund IFM said on Tuesday it had made a public offer for a 22.69% stake in Spanish power utility Naturgy for 5.06 billion euros. The fund is offering 23 euros per Naturgy share, a 19.7% premium over Naturgy’s closing price on Monday. It said its offer is subject to obtaining at least 17% of all Naturgy shares. (Reporting by Inti Landauro and Isla Binnie; editing by Jason Neely)