Private equity giant EIG is gaining a bigger foothold in the world’s natural gas and oil projects after clinching an estimated $4.8 billion agreement to acquire a 25% stake in Repsol SA’s new upstream company.

Repsol Upstream, 70% weighted to natural gas, has been formed as the supermajor’s exploration and production (E&P) company. It would own and operate a global portfolio of assets in key regional hubs, with a focus on the United States. Repsol has upstream assets in the Eagle Ford and Marcellus shales, the Gulf of Mexico and the North Slope in Alaska.

EIG, based in The Woodlands north of Houston, has long been an acquisitive energy investor. The tie up with Repsol would deliver upfront capital to increase the Madrid-based company’s investments in the energy...