U.S. economic growth has averaged 2.3 percent a year since the recovery started in mid-2009. That's about half the rate you might expect in a rebound from the deepest recession since the 1930s. Meanwhile, growth in China is slowing, is minimal in the euro zone and is negative in Japan. Throw in the large increase in U.S. vehicle gas mileage and other conservation measures and it's clear why global oil demand is weak and might even decline. 

At the same time, output is climbing, thanks in large part to increased U.S. production from hydraulic fracking and horizontal drilling. U.S. output rose by 15 percent in the 12 months through November from a year earlier, based on the latest data, while imports declined 4 percent. 

Something else figures in the mix: The eroding power of the OPEC cartel. Like all cartels, the Organization of Petroleum Exporting Countries is designed to ensure stable and above-market crude prices. But those high prices encourage cheating, as cartel members exceed their quotas. For the cartel to function, its leader -- in this case, Saudi Arabia -- must accommodate the cheaters by cutting its own output to keep prices from falling. But the Saudis have seen their past cutbacks result in market-share losses.