Exxon Mobil CEO Rex Tillerson says that a drilling method that has made vast natural gas resources accessible across the U.S. has been unable to crack some shale rock formations in Europe and China. Speaking to energy analysts, Tillerson said two attempts to tap gas-rich shale fields in Poland through hydraulic fracturing techniques have been unsuccessful despite the use of high-pressure torrents of water and sand. The drilling technique, known as fracking, involves blasting a mix of water, sand, and chemicals deep underground to shatter shale formations and free natural gas trapped within. “Some of the shales don’t respond as well to hydraulic fracturing,” Tillerson told reporters, according to Bloomberg News. “It’s going to take research and time in the lab to understand that.” Tillerson said some shale formations in the U.S. have also been impervious to fracking, and that the company is studying whether the use of different fluids or pumping techniques will make a difference. The rapid spread of the drilling technique has caused increasing concern among environmentalists and some local residents, who contend it may pollute water supplies.