LONDON — Energy companies have spent months in a state of strategic paralysis, wary of making big moves with oil prices plunging.

Now, the mind-set is shifting, as the industry giants look to capitalize on the weakness.

On Wednesday, Royal Dutch Shell agreed to buy the BG Group for $70 billion. It is the first major deal for an oil and gas producer since prices started falling last summer.

The acquisition could provide a template in the current environment, with deep-pocketed players taking advantage of their competitors’ problems to bolster their own position.

In buying BG, Shell is concentrating on the fast-growing business of producing and selling liquefied natural gas. It will also become a major player in Brazil’s offshore oil fields, where Shell’s exposure has been small.