In a speech last week, the Secretary General of the Organization for Economic Cooperation and Development (OECD) had a message for oil and gas investors: Their biggest risk isn't a spill or a blowout or a storm. And for countries deriving a large portion of revenue from oil and gas, it isn't the U.S. shale boom's competing with OPEC. Rather, it is stranded assets in a carbon-entangled world, according to OECD Secretary General Angel Gurría. It's also the biggest risk for any investor exposed to fossil fuels. The Asset Owners Disclosure Project estimates that an average 55 percent of pension fund portfolios are in high-carbon assets or sectors with major exposure to that sector.