West Texas Intermediate crude
Right now, futures are down 1.6 percent on the session on relatively heavy volume (click to enlarge):
Some are attributing the drop in oil to the IMF's global growth forecasts, which were revised downward today. However, that report was released at 10 AM ET, so it's unclear whether it's directly responsible for the move.
At 4:30 PM, the American Petroleum Institute will release crude oil inventory data for the week ended January 18, so it could have to do with speculation surrounding that release as well.