LONDON/HOUSTON (Reuters) - U.S. oil and gas producer EOG Resources Inc is selling its British offshore business, a document seen by Reuters showed, as the company heightens its focus on its fast-growing U.S. shale portfolio.

A spokesman for Houston-based EOG confirmed the sale process to Reuters.

The business includes a 100 percent interest in the Conwy oilfield in the east Irish sea and a 25 percent stake in the Columbus gas project in the North Sea, according to the document. Investment bank Jefferies is running the sale process.

Together, the assets could fetch over $300 million (215.12 million pounds), according to banking sources.

Jefferies declined to comment.

"I can confirm that we have it up for sale," EOG spokeswoman Kim Ehmer told Reuters.

The Conwy field, which produced around 11,000 barrels of oil per day in February, "provides immediate positive tax-free cashflow," the document said. 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."

EOG is also withdrawing from all its remaining exploration acreage in the North Sea.

Bids for the portfolio are due by June 2018.

EOG, considered one of the most-technology savvy U.S. shale producers, has been steadily growing its U.S. portfolio in recent years and is now the largest shale oil producer in Texas as well as the entire United States.

It pumped 551,000 barrels of oil equivalent per day last year across the United States, including operations in Texas as well as North Dakota's Bakken shale formation.

(Additional reporting by Ernest Scheyder in Houston; editing by David Evans)

By Ron Bousso and Ernest Scheyder