Marathon Oil Corp. will sell its North Sea oil-and-gas business and ramp up U.S. drilling, joining a list of exploration and production companies shedding overseas properties to focus on North America.

The planned North Sea sale—combined with a 13% increase in spending next year and a $2.5 billion share-repurchase program—will help Marathon boost output and increase shareholder returns, President and Chief Executive Lee Tillman told analysts and investors Wednesday.

The company's capital spending will rise to $5.9 billion next year. Of that, $3.6 billion is earmarked for unconventional prospects in the U.S. and Canada.

Marathon plans to invest $2.3 billion next year in the Eagle Ford Shale of South Texas, where up to 400 new wells will be drilled. The company will put $1 billion into Bakken prospects in North Dakota, and another $236 million toward work in Oklahoma's Woodford basin. Houston-based Marathon has focused on drilling for oil instead of natural gas since 2011, when it exited the refining business.

Chief Financial Officer J.R. Sult said 70% of Marathon's cash flow this year comes from North American oil and natural-gas production—a clear sign of the attractiveness of those businesses.

Spending next year is set to outpace Marathon's cash flow, but Mr. Tillman said the company will remain prudent. "We've never really viewed cash flows as a limiter, but as a healthy way to run the business," he said. "We prefer to be more opportunity driven and we're going to fund those high-quality opportunities."

The company is in the early stages of marketing its North Sea properties, which are in British and Norwegian waters, Mr. Tillman said. Marathon hasn't said how much money it hopes to raise with the sale. UBS Investment Research has said the assets could be valued at up to $3 billion.

Since 2010, Marathon has sold about $2.7 billion in assets overseas and in the U.S. A $2.1 billion sale of Angola projects is pending.

Occidental Petroleum Corp., which is based in Los Angeles, has said it would sell part of its Middle East business while increasing its presence in the Permian Basin of West Texas. Houston-based Apache Corp., which sold part of its Egyptian natural-gas business this year, also is focusing more on North America. And Houston's ConocoPhillips is trying to sell properties in Nigeria and a share in Kazakhstan oil fields as it puts more emphasis on the U.S. and Canada. Conoco already sold assets in Algeria this year.

Marathon expects oil and gas output to increase 4% next year. The estimate excludes production from Alaska and Angola fields because those properties are being sold and from Libya because of disruptions there.

Marathon's shares fell 41 cents, or 1.1%, to close at $35.69 on the New York Stock Exchange.