Suncor Energy has agreed to acquire Petro-Canada in an all-stock deal worth about $15 billion, the companies announced Monday. The merger will combine two of the biggest players in Canada's oil sands. Under the deal, Petro-Canada shareholders will receive 1.28 shares of the expanded company for each share of Petro-Canada, while Suncor shareholders will receive new shares on a one-for-one basis. Petro-Canada shareholders will hold 40 percent of the new company, and Suncor shareholders will hold 60 percent. In a statement, the companies said the merger was expected to save about $300 million annually.