Exxon Mobil has agreed to buy natural gas powerhouse XTO Energy for $31 billion in stock. Exxon’s been long rumored to be in the market for a natural gas company, hoping to take advantage of currently low prices, the Wall Street Journal reports. In XTO it’s nabbed a fast-growing, focused company. Its production jumped 23% in the third quarter, meaning earnings fell just 4% despite lagging prices. Exxon hasn't had a major acquisition since the merger with Mobil a decade ago. (More Exxon Mobil stories.)