The deal is expected to see Ithaca's provable and proven reserves rise by around 20 per cent.
It is buying a further 20 per cent interest in the Cook field, where it already holds more than 41 per cent, as well as stakes of 7.48 per cent in the Pierce field and 7.43 per cent in Wytch Farm.
Les Thomas, chief executive officer, said: "I am very pleased to announce the acquisition of these three high quality, long-life assets, which represent an excellent addition to the portfolio.
"The transaction is directly in line with our strategy to further diversify and expand our producing asset portfolio.
"Moreover, each of the assets has clearly defined upsides that provide the opportunity to generate significant additional value."