Exxon Mobil says it will acquire XTO Energy in an all-stock transaction valued at $31 billion.

Exxon has moved quickly to pick up valuable natural gas fields and now it is snapping up XTO, which claims about 45 trillion cubic feet of natural gas.

The world's largest publicly traded oil company will issue 0.7098 common shares for each common share of XTO, representing a 25 percent premium to XTO stockholders. Exxon will also assume $10 billion in XTO debt.