Neptune Energy Group, a London-based private equity-backed firm, has agreed to pay $3.9bn for a majority share in the E&P assets of France’s Engie. Adding in €1.1bn in decommissioning liabilities and €90mn contingent on operational milestones, the deal is worth €4.7bn ($5.2bn). It is expected to go through in early 2018.

Engie’s E&P assets include acreage in Algeria, Egypt, Libya and Qatar as well as in the North Sea and Southeast Asia. Engie Group,...