Well, maybe glut is too strong a word. After all, Thursday's closing 3-month contango was just 11 cents a barrel. Back in the depths of the Great Recession, when you couldn't give crude away, the contango was more than $15 a barrel. All this has implications for securities investors—especially those who've made allocations to oil-tracking ETFs. The big daddy among them, trading more than 16 million shares daily, is the United States Oil Fund (NYSE Arca: USO). USO tracks the front, or spot, futures contract, so it must roll its portfolio position forward monthly. That makes the ETF exquisitely sensitive to contango. And by sensitive, I mean vulnerable to pain. A roll entails selling the portfolio's existing contracts and buying the futures contract one month out. In a contango, that translates to selling the lower-priced contract and buying the higher-priced one, creating a transactional deficit.