Canada’s efforts to curb greenhouse-gas emissions are calling into question oil majors’ ability to tap the world’s third-largest oil reserves. The new left-leaning government of the Canadian province of Alberta, which came to power in May after 44 years of conservative rule, has announced it would cap carbon emissions from its oil-sands industry , a move that threatens to strand billions of barrels of crude from supplies so vast that only Saudi Arabia and Venezuela control more. Most of these producers have taken a wait-and-see approach toward the cap, details about which remain to be worked out. Privately, executives at some oil multinationals with oil-sands assets express dismay about the sudden policy shift, and the heads of some smaller companies have voiced their concerns publicly. “The cap is a long-term issue for province and the country,” said Glen Schmidt, chief executive of Laricina Energy Ltd., an oil-sands lease owner. […]