Marathon Oil (MRO) said Monday it will begin accepting bids for its North Sea businesses in the second quarter, as the energy producer moves forward with an effort to reshape its portfolio.

The Houston-based company will also expand its share buyback program by $500 million, bringing the total to $1.5 billion. Marathon has already repurchased $1 billion of its common stock.

Marathon provided the update on its 2014 plans ahead of a conference in New Orleans. The company, which first announced several initiatives in December, is coming off a year during which it surpassed its three-year target for asset sales of at least $3 billion.

In September, Marathon announced it would sell its interest in an offshore Angola block for $590 million, while buying $97 million worth of acreage at Eagle Ford in south Texas.

Marathon has made other moves to expand its shale footprint in recent years, following the 2011 spinoff of Marathon Petroleum (MPC). Shale oil production in the U.S. has lifted the company’s profits from oil and gas operations.

Chief executive Lee Tillman said Marathon has ramped up drilling in the Eagle Ford, Bakken and Oklahoma Woodford shale plays so far this year.

“We continue to have high confidence in our ability to deliver on our North America long-term production growth targets underpinned by strong resource growth through downspacing and well optimization,” Tillman explained.

The marketing of Marathon’s U.K. and Norway North Sea businesses will further concentrate its portfolio toward higher margin and growth opportunities, he added.

According to analysts at UBS, the assets could be valued at up to $3 billion.

Shares were down four cents at $34.35 in recent trading.

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