Tailwind, who partners with Mercuria on finance and marketing and oil services firm Petrofac on technology, did not disclose how much it paid.

Banking sources told Reuters in April the assets could fetch more than $300 million (234.1 million pounds).

"Tailwind will own and operate 100 percent of the producing Conwy oilfield, a 25 percent non-operated interest in the Columbus gas development project and other minor asset interests in the North Sea," Tailwind said in a statement.

Tailwind's overall output will reach around 15,000 barrels per day once the deal, which requires regulatory approval, is completed.

According to documents seen by Reuters in April, the Conwy field, which produced around 11,000 barrels of oil per day in February, "provides immediate positive tax-free cashflow." The field has around 10.9 million barrels of reserves.

The Columbus field is on track to start production "as early as 2019", representing "a robust growth opportunity albeit with minimal near-term firm commitments," according to the documents.

EOG has expanded its U.S. portfolio in recent years.

(Reporting by Shadia Nasralla; editing by Jason Neely)