Falling oil prices may cut investment in U.S. shale oil by 10 percent next year, the International Energy Agency (IEA) said on Wednesday, slowing growth in a sector that has turned the U.S. into a major global producer.

"A well-supplied oil market in the short-term should not disguise the challenges that lie ahead, as the world is set to rely more heavily on a relatively small number of producing countries," Fatih Birol, the IEA's chief economist, stated in the agency's 2014 "World Energy Outlook" published on Wednesday.