Oil-sands investments in Western Canada that gobbled tens of billions of dollars over the past decade are proving an Achilles heel for some of the world’s biggest energy producers. Exxon Mobil Corp. slashed proved reserves the most in its modern history after removing the entire $16 billion, 3.5-billion-barrel Kearl oil-sands project from its books on Wednesday. That followed ConocoPhillips ’ announcement a day earlier that erased 1.15 billion oil-sands barrels, plunging its reserves to a 15-year low. While prolific shale plays in Texas and Oklahoma are going through an investment boom with oil above $50 a barrel, the oil sands have fallen out of favor. Current investments in the region amount mostly to long-planned expansions by large Canadian producers like Suncor Energy Inc. , while majors like Statoil ASA have sold assets. Suncor, which took over Canadian Oil Sands Ltd. less than a year ago, is down more than […]