The purchase would make Energy XXI the third-largest oil producer on the shelf, with interests in seven of the 11 largest oil fields there, the company said.
The addition of the Exxon fields would increase the company's proved plus probable reserves 72 percent, to 158.1 million barrels of oil equivalent, from 92.1 million barrels as of June 20. It would also increase its daily production more than 77 percent from its daily average as of Sept. 30.
Exxon Mobil announced in October that it intended to sell a small slice of its oil and natural gas production assets in the Gulf of Mexico, mostly in shallow waters.