Royal Dutch Shell plans to spend at least $1 billion a year exploiting China’s potentially vast resources of shale gas, the firm’s top China executive said, part of an aggressive strategy to expand in the world’s biggest energy market.Shell in March secured China’s first product sharing contract for shale gas, hoping that getting in early will allow it to be a big beneficiary from the sort of boom in shale that has transformed the U.S. energy market. Asked if the firm remained committed to a plan to invest $1 billion a year in China’s shale gas over the coming few years, Lim Haw Kuang, Shell’s top China executive, said in an interview: “Yes, yes and yes.”